# Understanding Enriched Air Nitrox

*Let me start the article with stating what you are about to read is NOT a substitute for training from a certified Enriched Air Instructor. What you read here compliments that instruction. It may be used to prep before you participate in instruction or as a refresher. Enriched Air Certification is earned only through a certified Enriched Air Instructor. * *Do Not attempt to dive Nitrox without being certified.*

If you are interested in becoming Enriched Air certified, known as Nitrox throughout this document feel free to reach out to Larry Davis or your local dive organization.

There will be a vast variety of experiences reading this article. The intent of the article is for someone wanting to know more about Nitrox. Since the age, level of experience, knowledge of the individual reading is not known the article is written to allow the least knowledged and experience person to learn. This may prove too basic for your level of understanding so please keep this in mind when reading.

You may be here because you are curious and you want to learn. When you complete the article, you will be more knowledgeable and not at a superficial level. If your intention is to take a course after going through this article, I am confident that when you understand the concepts, know the formulas and have ingrained images that reinforce the Why’s, How’s, When, and What’s you will be more prepared for the course and it’s quizzes as well as having become a more knowledgeable and safer diver.

The way the article is written is to make one think. Concept are introduced with the intent to implant a thought. Questions may be asked that may read a little off. You may find questions being asked with no immediate answer provided. This too is intended to introduce concepts and terms. When you come across these types of questions, make note of them. We will be returning to the point later in the article. It may just be too soon for the details.

As the article progresses acronyms, concepts and terms introduced earlier will begin to be defined and be used more often. You may feel a bit overwhelmed at times if a subject is something new to you. My advise, read it again, perhaps take it in smaller chunks and if necessary to clarify an understanding, reach out to the author. Larry is an excellent instructor and will help when he can. The intent is to introduce and reinforce concepts and points, not to make someone feel incompetent.

We all know reading something without having access to immediate clarification may be a bit taunting. I understand there may be those who will pick and point out deviations and challenge them. For example.. throughout the article we will refer to the percentage of Oxygen and Nitrogen in a scuba cylinder filled with air as 21% and 79% respectively. We know this not to be true. Oxygen and Nitrogen are approximately 20.8xy% and 78.6xy% respectively. For simplicity Oxygen and Nitrogen have been rounded to 21% and 79%. We’re not looking at sending someone to Mars, just out of this world diving.

Before you begin. I recommend you take the Nitrox Quiz. After reading the article, take the quiz again and compare the two quizzes to see your knowledge gained.

Click HERE to take the quiz

**What is Nitrox? **

The air you breath is primarily comprised of Oxygen and Nitrogen along with minuscule amounts of other gasses such as argon, carbon dioxide, neon, methane, helium, krypton, hydrogen, xenon, ozone, nitrogen dioxide, iodine, carbon monoxide, and ammonia. For the purpose of this discussion those other minuscule amounts of others gasses will be forgotten as we focus on the two primary gasses, Oxygen and Nitrogen.

It is simplest terms, Nitrox is the addition of extra Oxygen added to your scuba cylinder as a substitute for Nitrogen. The result is a tank that has a higher percentage of Oxygen and a lower percentage of Nitrogen when compared to a tank filled with air. Recreational Nitrox cylinders may be filled with Oxygen between 21.01% and 40%.

**N***itrox is simply substituting a percentage of the Nitrogen with Oxygen*.

The two scuba cylinders below have one filled with air which has 21% Oxygen and 79% nitrogen and a Nitrox cylinder filled with 32% Oxygen will have 68% Nitrogen. The sum of the percentage of gasses in a bottle must equal 100%. Another way of saying this is the Fraction of Oxygen, FO in the tank to the left is .21 and the FO in the tank to the right is .32. Make note of the term Fraction of Oxygen, FO. We will be using it later.

Would it be fair to say the total pressure exerted by the two gasses, part of that total pressure comes from Oxygen and the other Nitrogen? Yes, of course it would. We could elaborate the total pressure is Partially from the FO in the cylinder and partially from the Fraction of Nitrogen, FN for a combined pressure of 100%.

**Daltonâ€™s Law**

The total pressure exerted by a mixture of gases is the sum of the pressures that would be exerted by each gas if it were alone and occupying the entire space.

P_{total} = P_{1} + P_{2} + P_{3} + … + P_{n}

We will be referencing PPO later when we dig deeper into Enriched air.

**Why Use Nitrox? **

*Extended bottom time*. When the percentage of Oxygen in a scuba cylinder is increased, the Percentage of Nitrogen is decreased.

When a scuba cylinder is filled with air, that cylinder will comprise of 21% Oxygen and 79% Nitrogen. If you are a certified diver, you should be familiar with the bottoms times allowed while diving with air. For the benefit of those who may not, diving on air to 60 feet, the No Decompression Limit is 55 minutes. Fill that same cylinder with 32% enriched air and the No Decompression Limit increases to 90 minutes. This of course is for the first dive of the day.

If understanding dive tables is something you need to learn or brush up on, this would be a good time to do so. Visit an article by clicking on **Understanding the Recreational Dive Planner, RDP** before continuing. Awhile later we will go over Enriched Air dive tables but only briefly since the EAN tables work the same way as the air tables.

To assist in making the purpose for dive tables a little clearer, Dive Tables calculate time. Dive tables are all about “TIME.” Time at depth, time on the surface, time between repetitive dives, No Fly Time, etc.

**Question**: Food for thought. Is the extended bottom time afforded by enriched air primarily due to the increase of Oxygen or the decrease of Nitrogen in the cylinder?

Answer: Due to the reduction of Nitrogen in the cylinder the allotted bottom time increases.

The table in Blue is an AIR Table. The Yellow is 32% Nitrox Table. When you compare the numbers in each of the small squares on the Nitrox table to the Air table, you will find they are greater. The Black box found at the bottom of a column (circled) is the No Decompression Limit, NDL. NDL is the maximum time for a given depth a diver may remain before they go into decompression. We want to avoid going into Decompression.

Follow the 60 foot column on both tables down to the NDL, the AIR table (in blue) shows a 55 minute MAX bottom time while the 32% Nitrox table (Yellow) shows 90 minutes MAX bottom time. That is an increase of 35 minutes of bottom time at the same depth when using Enriched Air Nitrox, EAN of 32% over that of air. That is the benefit of Nitrox, *Extended bottom time*.

There are other benefits and with benefits come dangers. We will cover those in more detail later. For now, let’s just stick with time as the main benefit.

**When is Nitrox right for me?**

There are many factors which play a roll in improving your dive time such as diving skills, buoyancy, breathing, equipment, comfort, and other factors are examples. As your experience and comfort progresses so does your gas management. You don’t run out of gas as quickly as you did when you first started.

Most when they first start diving, run out of gas in their cylinders before their bottom time runs out. As you progress in your habits, you eventually cross a line where your bottom time is now the limiting factor. When you get to a point where the majority of your dives have you returning because you are reaching the maximum bottom time, while you still have plenty of gas in your cylinder, Nitrox may be your solution.

The image above depicts three divers. The New diver consumes gas much faster and begins surfacing at around 30 minutes into a dive to 60 feet. The Comfortable divers gas management is better and begins surfacing at around 50 minutes into a dive to 60 feet where the Seasoned diver has enough gas to support the dive to 60 feet and could begin surfacing at around 75 minutes because they are diving Nitrox. The comfortable diver is a candidate for Nitrox because Nitrox will extend their bottom time.

Diver Mary, the Comfortable Diver decides to take a Nitrox course. Mary figures she is close enough to managing her gas that with more diving, she will get better and when she does, the Nitrox will be an available option to compliment that.

Does this mean you must wait for your gas management to get better before acquiring your Nitrox Certification? No, of course not. You may earn your Nitrox certification during your Open Water Training.

**Another Benefit**: Reduced Nitrogen Load

When some percentage of Nitrogen from the cylinder has been removed, you do not accumulate as much Nitrogen compared to someone diving on air. The advantages here are increased bottom time, reduced surface interval times and longer repetitive dive times. You gain these benefits because your body has absorbed and stored less nitrogen over time.

The reduction of Nitrogen loading is a reason why a new diver would consider a Nitrox Certification.

At this point, you learned What Is Nitrox, Why Use Nitrox and When is Nitrox an option for you. You were introduced to terms and acronyms such as Partial Pressure of Oxygen (PPO), as well as Fraction of Oxygen (FO). Let’s now look at Managing dive profiles for planning purposes.

Wait! Computers are being pushed today. We can stop here and you can just go out, buy a Nitrox compatible dive computer and trust your computer after getting your certification. You may close this window now OR you may continue to read and learn the physics and physiology on the use of Nitrox as well as formulas in hopes your computer doesn’t malfunction (3 times for this author) and never have to hold a Nitrox conversation with anyone.

**DANGERS of using Nitrox**

We can all agree we need Oxygen to survive. We also agree the percentage of Oxygen in the air is 21%.

Before we dive into the depths on the dangers, let’s get a level set on our base leading up to the dangers. Having the knowledge of how and what under belt will help with the why.

**Question**: At w*hat depth are you when you reach 2 atmospheres absolute in sea water? *

Your answer should be 33 feet.

**Question**: What is an Atmosphere?

An atmosphere is the total pressure exerted by a column of air between sea level and space above or a specific depth of water, in our case. That column has weight.

**Question**: Now much does one atmospheres of air weight?

One atmosphere of air weights approx 14.7psi. That is 14.7 pounds per square inch, psi.

**Question**: How many feet of sea water, fsw makes up one atmosphere?

1 column of salt water, 1″x1″x12″ weights approx. 0.445lbs.

If 14.7psi is the weight of one atmosphere we can divide 14.7 by 0.445 and we will get 33ft.

**Question**: How many atmospheres is 44.1psi?

44.1 / 14.7 = 3atm

Every 33fsw is 1atm and every 1atm is 14.7psi then every 33fsw is 14.7psi

**Question**: How much pressure is exerted on us at 33fsw?

Your answer should be: 29.4psi the amount of pressure exerted on us at 33fsw.

Every 33fsw exerts 14.7psi. The amount of pressure exerted by the column of water at 33fsw is 14.7psi. Add to that pressure of water the pressure from 1atm of air and you end up with 2ata. At a depth of 33fsw, you have (2×14.7) 29.4psi bearing down on you. 1atm of air and 1atm of water for a total of 2ata.

**Question**: What is Atmosphere Absolute?

Atmosphere Absolute is the pressure exerted upon us, in our case by the column of water above plus the atmospheric pressure of air upon that. It is denoted by the characters, *ata *(atmosphere absolute). In the previous question *ata *was used at the end of the last sentence.

**Question**: How much does one atmosphere of water weight?

One atmosphere of water weights 14.7psi

Where the depth for one atmosphere of fresh water is 34 feet, and salt water is 33 feet.

**Question**: *How do you calculate Atmospheres Absolute, ATA?*

The formula to get your current ATA in salt water is:

ATA = ( Depth / 33 ) + 1

ATA= ( 33 / 33 ) + 1

2 = 1 + 1

The quotient from the two numbers found between the () is the number of atmospheres of water above you. The +1 is the atmosphere of air. The sum of the two is the total atmospheres exerting pressure above you. Your ATA.

**Question**: What is your ATA at a depth of 85fsw?

At 85 feet you are at approximately 3.57 atmospheres absolute. That is 2.57atm of water + 1atm of air.

What is this Absolute? Putting it simply it is the addition of the atmosphere of air at sea level added to the atmosphere in water.

At 66 feet you are under 2 atmospheres of water plus 1 atmosphere of air for a total of 3 atmospheres absolute. If you do not include the 1 atmosphere of air, then you would denote atmospheres with (atm). Include the 1 atmosphere of air it then becomes (ata).

**Question**: How many psi is exerted at 3 ATA?

( 3 x 14.7 ) = 44.1psi

you could write the equation as (( Depth / 33 ) + 1 ) * 14.7

Perform the following calculation. Do not look at the answers following them. When done calculating, look at the answers to see if yours are the same.

**Question**: **a) How many atmospheres absolute are you at a depth of 99 feet sea water and b) how many psi of pressure is exerted at this depth?**

Answers: a) (99 / 33 ) + 1 = 4ata b) ( 4 * 14.7 ) = 58.8psi -or- ((( 99 / 33 ) + 1) * 14.7)

**Equivalent Percentage**. What is it and what does it mean? Equivalent is resembling something without being that something. In the context of Oxygen Equivalence it is to take the FO of Oxygen in the cylinder and multiply it by the ATA to determine the equivalent percent oxygen you are breathing at depth. This becomes the PPO at depth.

Examining the contents of a Nitrox tank you determine the FO to be .32. When you are at 2ata, the PPO you inhale is .64 or 64%. We say you are breathing the equivalent of 64% Oxygen at 2ata. We say this because the percentage of oxygen in the tank remains at 32% but the Oxygen you breath at 2ata is 64%.

The formula to calculate PPO is:

PPO=( ATA * FO )

PPO=((( Depth / 33 ) + 1) * FO)

**Question**: *What is the equivalent percentage of Oxygen breathed at 3 atmospheres?*

Your answer should be 3 x %age (3 times percentage) or thrice that on the surface. If you were at 4ATA, then the answer would be 4 times that on the surface.

To Calculate your Equivalent Oxygen at depth simply take the PPO and convert it to percent.

For example if your is PPO=.64 your Equivalent Oxygen is 64%.

**Question**: Why do we need to know the equivalent percentage of Oxygen?

The short and direct answer, You are strongly advised not to breath more than a 1.4 PPO. a PPO of 1.4 equates to 140%. When you know the FO, you can determine the maximum depth you may dive on that mix. This Maximum depth becomes known as Maximum Operating Depth, or MOD.

Question: Why is knowing the equivalent percentage of Oxygen important?

It is important because the use of nitrox accelerates and compounds the physiological effects as a result of Oxygen being at higher PPO on the body. The higher and longer your body is exposed to increased PPO the more change occurs within the body. The more change, the greater the dangers.

Another one of the conditions of diving Nitrox is it limits the depth which you may safely dive. If diving on Nitrox provides extended bottom time, it also limits/restricts the depth which you safely may go.

*The higher the percentage of Oxygen in your cylinder* is, the longer your bottom time yet* the shallower you must dive.*

Another factor to consider, monitor, and manage is the time exposed to higher PPO.

You will be monitoring **Oxygen Exposure Time**. Sometimes referred to as **Oxygen Toxicity Load**. Just as you have been monitoring your Nitrogen exposure, you will now do the same for Oxygen exposure. Now that you will be diving Nitrox you will be monitoring both Nitrogen and Oxygen load. Your Nitrox dive computer may report your Oxygen Exposure as **CNS**.

The image above shows a computer set to 32% Nitrox. The MOD for a 32% mix is 111fsw. Apparently the diver has not performed a dive yet, and their CNS shows 0%. After performing the dive, the CNS value will be a positive integer representing a percentage of O^{2 }exposure on the body. Oxygen Exposure will be covered in more detail later.

The table below shows that with a fixed gas percentage (%), the partial pressures (pp) will change in open circuit diving.

Depth | Pressure | % O | PPO | % N | PPN | Total Pressure | PSI |

Sea Level | 1 bar/ata | 21 | .21 | 79 | .79 | 1 bar/ata | 14.7 |

10m/33ft | 2 bar/ata | 21 | .42 | 79 | 1.58 | 2 bar/ata | 29.4 |

20m/66ft | 3 bar/ata | 21 | .63 | 79 | 2.37 | 3 bar/ata | 44.1 |

30m/99ft | 4 bar/ata | 21 | .84 | 79 | 3.16 | 4 bar/ata | 58.8 |

40m/132ft | 5 bar/ata | 21 | 1.05 | 79 | 3.95 | 5 bar/ata | 73.5 |

*A VERY important concept. As the Ambient pressure increases so does the percent equivalent of a given gas. As the ambient pressure decreases, so does the equivalent percentage. As you rise and fall along the water column, so does the percent equivalent of a gas. This is one concept you must know and understand. Your life depends on it*.

Where is the danger in that? Fair enough question…

As Oxygen is placed under increased *pressure*, the higher the percentage the body receives on a given breath. A scuba cylinder filled with 32% Oxygen will on the surface have you breathing 32% but take that same cylinder down to 2 ATA and you are breathing the equivalent of 64%. At 100 feet you are breathing the equivalent of 128% Oxygen. The danger is the higher PPO and longer the exposure the more danger you subject yourself to.

**Question**: How can we increase the PPO we are breathing, while at sea level without increasing the percentage in the cylinder?

The answer may be found in the article above. It may not be spelled out in black and white, but it is there… Got to think.

**Answer**: *Pressure*. Increasing or decreasing pressure will increase or decrease the PPO. Consider how a ‘recompression’ chamber works.

You just learned Pressure has a direct effect on a gasses. The deeper you go, the more pressure is exerted, the higher the equivalent percentage goes.

At this point you have been subjected to the term, Equivalent Percentage. What is meant by Equivalent Percentage? It is the percentage in your cylinder multiplied by the ATA. Since the percentage in the cylinder remains constant, the factor that changes the percentage is pressure. When you apply pressure to a gas, the equivalent percentage increases. At the surface a Nitrox cylinder may be filled with 32% Oxygen, you are breathing 32% Oxygen. Take that same cylinder down to 33fsw and you are now breathing the equivalent of 64% Oxygen. Your PPO increased by a factor of 2. This subject will be covered in more detail later and why it is important to grasp the concept.

**Question**: How does breathing higher concentrations of Oxygen affect the body?

Everyone is physiologically different. Everyone is affected differently and to different degrees. No one denies that breathing higher percentage of Oxygen does not adversely affect the body. The longer one is exposed to *Partial Pressure of Oxygen, PPO* greater than that found at sea level, the more severe the complications.

Delving into the chemistry of exposing the body to higher PPO over extended periods of time is beyond the scope of this article. For the purpose of knowing, exposure to higher percentages over an extended period of time causes stresses on the body’s tissues and Central Nervous System. Something we must monitor and minimize at all times.

This is why it is important to minimize the time spent as well as the percentage over a period of time? Divers want to spend as much time as they can diving and do so as safely as possible. There must be a balance where the scales tip more towards safely than time. This can not be more true when it comes to diving Nitrox.

Convulsion: Grand Mal seizure, Usually without warning |

Twitching: Usually facial muscles. most frequent symptoms |

Ears: Ringing in the ears or any other change |

Nausea: Mild to severe. Continuous or intermittent |

Vision: Tunnel vision or any other change |

Irritability: Behavior or personality changes |

Dizziness: Vertigo or disorientation |

Above are some of the warning and dangers of diving on higher PPO. The more repetitive dives on Nitrox you make, the more physiological irritation your body must endure. Just like Nitrogen accumulations over repeated dives, the physiological effects of Oxygen on the body also accumulate. You must give your body rest from this irritation by not constantly subjecting your body to the irritant. This may be done by reducing the PPO or eliminate it by taking a break from diving Nitrox or diving for a day. (go do a land tour, sit around the pool)

This author has been Nitrox certified for almost 20 years. Though I have experienced physiological anomalies due to Nitrox, lucky enough I am still here to write about it. Some of my fiends are not. Let me add a disclaimer on what I just wrote. Diver Error was the cause for all my friends mishaps. Perhaps it was their comfort that came with diving over a long period of time and that comfort brought complacency. Do not let yourself become complacent. Do not challenge the dangers of Nitrox diving. On land that’s one thing, underwater that could lead to injury and/or drowning.

The unknown variable in Nitrox diving is the diver. No one can predict how any one diver will perform while on Nitrox. The above is not intended to frighten you away from Nitrox, just to make you more informed.

You just learned as the dive depth increases, so does the pressure. This increase in pressure affects the equivalent percentage of any given gas. As the percentage increases, so does the risk and the primary benefit is increased bottom time. You were introduced to PPO and how pressure affects the PPO as well as the pros and cons of that increase. You also learned what Equivalent Pressure is and how it factors in with the PPO.

**Let’s review all the things mentioned so far**.

We learn that Nitrox is the addition of Oxygen and a substitution of Nitrogen in your scuba cylinder. We also learned the reason for using Nitrox which is to extend bottom time. Also when is it the right time to acquire a Nitrox certification. We also learned that the Partial pressure of a given gas rises and falls as we descend and ascend the water column and that exposing our bodies to an increase in PPO for an extended period of time is an irritant on the bodies tissues.

**Lets Dive Deeper on the subject.**

Knowing the primary reason for Nitrox is extending bottom time, How about depth?

**How deep may you go on Nitrox?**

To say to gain one thing we have to give something else up. There must be balance and the thing you give up once you begin diving Nitrox is Depth. The higher the percentage of Oxygen in the cylinder, the shallower you must dive.

The industry rule teaches us to remain below a PPO of 1.4 with contingency for a PPO of 1.6. To get contingency out of the way it means do not plan for a dive with a PPO greater than 1.4. Should you ‘need’ to break past this floor of 1.4, try not to exceed a PPO of 1.4 for any extended period of time. For example, if the sandy bottom is say 10 feet below you and you are at a PPO of 1.39, you drop your spear, reel, knife, etc.. descending down 10 additional feet to a PPO of 1.52 to recover the item should be done long enough to recover the item then ascend to a safer PPO level.

Let us look at the subject of depth and how the PPO plays a role. We need to understand this for later you will use it to identify your * Maximum Operating Depth,* MOD.

Earlier you were introduced to PPO. You learned how to calculate ATA and apply that value to determine your PPO at a given depth. Let us now dive deeper into how PPO is used to determine your MOD while diving Nitrox.

Partial Pressure of Oxygen, PPO at depth is the percentage of Oxygen multiplied by the amount of pressure exerted upon it. In the case of diving as we descend, pressure from the column of water presses down on us. It compress gasses forming tighter and tighter groups. For example, at 1ata (sea level) let say there are 1 million molecules of Oxygen per square inch. Head to 2ata and that same square inch now has approx twice the number of molecules occupying it, ~ 2 million. This means on every inhalation, at 2ata you intake twice the number of molecules of Oxygen as you did at 1ata.. If you were at 3ata, the count would be 3 times the cylinders percentage of Oxygen.

**As pressure is exerted on a given gas the gas form tighter groups. The density will increase while the volume decreases.**

To help put a picture to how pressure affects a gas, the image below depicts a gas bubble and it size when pressure is increased and decreased.

The the image above, you find a representation of a bubble. To the left of the vertical line is the bubble under increasing pressure, where on the right is a bubble under decreasing pressure. To help explain and clarify the depiction, the bubble from the surface taken down to 2ata is 1/2 its original size. The formula is 1/ATA. A bubble formed at 2ata taken to the surface has doubled its size and is depicted as ATA/1

Another example. Let’s say you went down to 99fsw for an extended time. The gas that was captured would begin to expand on your ascent. Upon reaching the surface, that bubble would have grown approx 400% in size. That is one reason for ascending slowly, say 30’/min vs the older mythology of 60’/min. Add to that ascent the newer mythology of half stops. Stopping 1/2 the distance from your deepest point for no less than 1 minute, then resuming your ascent towards the surface. For example, if the deepest point of your dive was 100′, you would stop when you reached 50′ for no less than 1 minute to help facilitate off gassing.

**Question**: Assuming no decompression obligation, using the new ascent mythologies how long, at a minimum will it take you to break the surface from a depth of 90 feet?

Your Answer: (My answer may be found at the end of this article)

**Determining the percentage of Oxygen in the cylinder?**

You request the dive shop your desire to have your cylinder filled to 32%. When you return to pick up your tank, you will analyze the Oxygen content. You do this through the use of an analyzer. You do this so you know what percentage of Oxygen is in your cylinder. When you know the percentage of oxygen is in your cylinder, you know the PPO and the PPO is used to calculate **Maximum Operating Depth, MOD **for that blend. Sound like rocket science? Really, it’s not.

Below are just a few pictures of Oxygen analyzers you may come across. The details of how an analyzer works is beyond the scope of this article. In simplest terms let’s say it counts the number of oxygen molecules passing over its membrane and provides a numeric readout representing percentage.

Here are a few analyzers so you may get an idea what one may look like. As you may see on some of the displays on the analyzer the number 20.9. This represents the percentage of Oxygen in the air.

This is your starting point. You must calibrate the analyzer to start at 20.9% or 21% before you begin analyzing the percentage of Oxygen in the cylinder. One way to do this is to calibrate it a know gas, an AIR Cylinder. You then hold the opening of the analyzer up to the opening of the cylinders valve and open the valve ever so slightly, so that just enough gas is able to escape the cylinder and into the intake of the analyzer.

You will see the numbers of the display rise and eventually stop at or near your requested percent filled. Once the readout has stabilized that number should be documented on the tank along with the MOD. Using a piece of painters tape will do.

Below is an example of marking the tank. You see the percent is 36.2 (PPO=.36), the pressure is 3600psi, the MOD of 95′ and the date filled.

Mentioned earlier to gain time, we loose depth. Once you know the percentage of Oxygen in the cylinder, you can figure out your Maximum Depth, what we’ve been calling MOD, your Maximum Operating Depth. Above on the marking is written 95′ for MOD. How did they come up with the MOD?

**Question**: How do you calculate MOD once you know the percentage of Oxygen in the cylinder?

* MOD is based off of the highest PPO you should not exceed, that PPO being 1.4, *with a contingency of 1.6.

Studies have show that exposing your body to high levels of Oxygen over extended and repeated periods of time is unfavorable no matter how slight. Exposure to high levels of PPO regardless of time could be severely detrimental. The dive industry dictates we keep our PPO below 1.4.

You have all the numbers needed to determine MOD. How is it calculated?

One way is to use a chart. Charts similar to the one seen here are usually found near the analyzer. On the chart below **the far left column is PPO**. **The top row is FO**, the percent O^{2} in your cylinder.

Click on the image to enlarge it

To locate the MOD for the mix you analyzed, locate the intersecting row and column, (PPO and FO) to come to the MOD for that mix of gas. Since you are looking for the MAX depth, you search down the left column, the PPO column for the number that comes closes to 1.4 (without exceeding 1.4), scan to the right for the box where the PPO and FO meet. The number in the intersecting box represents your MOD.

**Question**: Using the chart above, what depth is the MOD for a cylinder analyzed with 32% O^{2}?

Your answer should be 110.7fsw (or 111fsw)

If you do not have a chart readily available, you may use the tried and true.. The Calculation for MOD is:

*MOD = (( PPO / FO ) – 1 ) * 33 *

MOD = (( 1.4 / .32 ) – 1 ) * 33

MOD = (4.375 – 1 ) * 33

MOD = 3.375 * 33

MOD = 111 fsw

Imagine being on a boat and someone asks, What is the MOD for a nitrox mix of 36%?

You pull out your pad and paper, whip through the formula to tell them the MOD at 1.4 PPO on 36% Oxygen is 95 feet.

Question: What is the Percent Equivalent of oxygen at a PPO of 1.4?

Answer: Convert the 1.4 to a percentage. Take the decimal, move it two places to the right and replace it with a % character. At 95 feet with an FO of 36% you are breathing the equivalent of 140% Oxygenâ€¦

**Question**: If contingency is up to 1.6 PPO, what is the depth, on a cylinder with an FO of 32% you would need to reach for a PPO of 1.6?

Answer: ** Contingency Depth = ((( PPO / FO ) – 1 ) * 33** ) It is the exact same formula for MOD, just with a different PPO, in this case 1.6. You should have come out with approx 132 feet.

Contingency Depth = ((( 1.6 / .32 ) – 1 ) * 33 )

Do not plan dives to contingency depths. *Plan dives no deeper than MOD*. MOD simply tells you the depth to the Nitrox floor. You do not want to go through the floor and if you must, try and keep it to no more than the contingency and as short a period as possible.

Calculate the following on your own.

You know the formula to calculate MOD, here is the formula to calculate PPO:

**PPO=(((Depth / 33 ) + 1) * FO)**

Presume a MAX PPO of 1.4 for the following questions. Answers will follow after the last question. Try not to peak before answering.

**Question 1**: What is the maximum depth for a 40% Nitrox mix?

MOD=

**Question 2**: What is the PPO at 90fsw on a 40% Nitrox mix?

PPO=

**Question 3**: Is it safe to plan a dive to a depth of 90fsw on a mix of 40%

( Yes / No ) and Why?

**Question 4**: What is the Oxygen Equivalent on a tank filled with 30% Oxygen at a depth of 72 feet?

PPO=

ANSWERS:

- 82.5fsw. You could round up or down.
- For a 40% Nitrox mix at 90fsw the PPO is 1.49
- Your answer should be No. The MOD for 40% Nitrox is 82.5fsw at a PPO of 1.4. 90fsw exceeds the 1.4 PPO
- PPO=95 The formula is: PPO=(ATA * FO) ; ((Depth / 33 ) + 1) * FO ); hence the formula is ((72 / 33) + 1 ) * .30 )

The image above depicts three separate blends. 21%, 32% and 36%. A diver on a cylinder filled with air will have a NDL of 55 minutes at 60 feet. The diver on EAN32 will have 90 minutes, while a diver on EAN36 will have an NDL of approx. 121 minutes depicted by the column on the left.

The MOD for each of the three gasses are depicted by the depth of the plain for the respective gas. EAN36 has an MOD of approx. 95fsw, EAN32 111fsw and air is off the charts (approx 187fsw).

Calculate out the MOD for the three gasses (.21, .32, and .36)

MOD=(((PPO / FO) – 1) * 33)

MOD=

MOD=

MOD=

The chart shows the higher the EAN level, the more time you gain yet the shallower the dive must be.

If the three gasses above represented three divers which will dive together, *all three would begin their ascent before the diver on air reached their NDL*. You dive as a team. You do not let one diver go alone to the surface because their Bottom time has run low. This was poor planning. Divers on a team should be diving similar mixes amongst other things.. This is a separate subject all together. The point above being the one on air will have to surface before the other two.

I would like to note that we have been discussing MOD. MOD does does not mean you must dive the MOD. MOD is for you to know where the Nitrox floor sits. In fact, you should not be diving your max PPO on every dive. I would encourage you plan dives at a lower PPO.

This bring up the next **question**: What mix should you be requesting for your dives?

If for example, you plan on performing 4 dives in a day and you want to dive Nitrox on all four dives, performing the deepest dive first, second deepest on your second dive and so on, what mix or blends would you request for your four cylinders?

Let presume the deepest dive will not be deeper than 110 feet. There is a really cool wreck there and you always wanted to check her out. Your second dive is no deeper than 80′, the third 60′ and the last 50′.

The wrong answers are where your PPO exceed 1.4 or a dive hovering close to 1.4 for a prolong period of time.

I can’t provide you with an answer because there are so many with so many conditions and reasons. The thought behind the question is to have you think about what you are planning to dive and how you should be managing it. For example you may ask: How much Oxygen Toxicity will I incur? How much nitrogen will I load? What will be my Residual Nitrogen Times, RNT? What Surface Interval times will I need? What are the depths at the sites we plan to dive?

These are things you may consider when you are in control of all aspects of the dive. When you leave it up to an operator, ie. when you travel and a dive operator decides the sites, the dive times are limited to say 50 minutes, surface intervals are no less than 1 hour and depths are no deeper than 100fsw. You may choose the PPO for your cylinder but most everything else is managed for you.

At this point, you have been introduced to quite a bit of information. Some new, some old yet everything you have been taught led you to this point. The point where you can now begin planning dives using Nitrox.

From here we will be looking at a number of tables provided to help you calculate a dive. Some calculations will require you to use the formulas mentioned earlier and yes, some new formulas because there are no tables for some calculations. That’s on you.

Are you ready?

Here are a few tables you may find divers using. There are many more and these were arbitrarily picked. Since we have been referencing the tables sanctioned by PADI we will continue with those.

There are three cards, available. Two cards are Recreational Dive Planner, RDP for 32% and 36% Nitrox, the two most common mixes.

The third card contains tables for calculating mixes other than 32% and 36% and your Oxygen Exposure. Guess what? There are formulas you may use as well… Oh goodie….

If understanding dive tables is something you need to learn or brush up on, visit an article by clicking on **Understanding the Recreational Dive Planner, RDP** before continuing.

From this point on it will be presumed you know how to use the air tables. The two cards for 32% and 36% work exactly the same way but there is one card that may look new. It is this one

This card contains on one side, the Equivalent Air Depth, EAD

The EAD is used to Assist in calculating your NDL for a given percent of Oxygen in your cyinder as well as your PPO at depth. You may also figure your MOD from this table. This table exists because there are no printed cards for every single blend of Nitrox. Just two of the most common mixes, 32% and 36%. If you have a different mix, this is the card and table you will use.

Flip the card over and you get your Oxygen Exposure Level.

The Oxygen Exposure Table keeps tract of your Oxygen load. It will also be used to plan your dives and verify you are where you’re suppose to be after performing your dives. To assure you are within your limits and warn you when you come to close to those limits.

One of the first thing you will need to know is what are the planned depths for your upcoming dives. To allow you to work the tables on above card let us presume the depths for the following:

4 dives will be, **100fsw, 80fsw, 60fsw and 40fsw.**

Now that you know the approx depths

Question: What percentage of oxygen should you have your cylinders filled to support the 4 dives? Will a 32% mix in all four tanks work?

**Question**: What will the PPO on a 32% mix be at 100fsw?

Calculate the PPO for a depth of 100fsw: ______________________

**Question**: How much time do you plan to spend on each dive?

**Question**: What is your NDL for your first of a series of dives to 100fsw on a 32% mix?

What is the NDL?: ______________________

There are more questions to ask but we’re not here reading this article to learn about managing dives. I posed the few questions above to plant a seed. Thinks to consider when planning.

#### How to use the the EAD and Oxygen Exposure table

You are aware there are three Recreational Dive Planners on the market published for PADI. They are the air, 32% and 36% Nitrox planners shown below.

What do you use when your mix of Nitrox gas is not 32% and 36%? How do you calculate your Max Depth, Nitrogen Load, Surface Intervals, and everything you can do with a planner?

**Turn to the Equivalent Air Depth tables, EAD. **

Because there are no RDP’s for every blend of Oxygen mix, the above tables were created. They help you to identify what your EAD, and your PPO for the planned depth of your dive as well at the MOD for the mix you are diving. You will be using both the EAD chart along with the Air RDP. You will move back and forth between the two cards to calculate depth and time.

The EAD tables assist in converting a percent mix of Nitrox to the air equivalent. Once that conversion has been performed, you use you Air Planner to calculate your depth, Nitrogen load, Surface Interval and so on.

Let’s take a look at the table to identify how it is organized.

There are 11 distinct blocks of information. You can spot two right off the bat. They are the two surrounded by the black lines box, the 32% and the 36%. Surrounding those two boxes are 9 blocks on information.

Next, the very top row highlighted in Red, look to the far left you find the word **Oxygen **and to it’s right a series of percentage starting with 30% to the right of that, 31% ending that row with 35%. About 1/2 down the card, on the left you find the word Oxygen and the percent Oxygen continues to the right starting with 36% up to 40%.

The row just below the work Oxygen you find the word **Nitrogen** and to the right of Nitrogen a percent value representing the Nitrogen in the cylinder, highlighted in Cyan.

The third row contains a series of values for each block of information. Those value are **Depth**, **EAD,** and **O2 p.p.** (The O2 falls in the row for Nitrogen highlighted in Yellow. They needed the space above p.p. for the designation for Oxygen so O2 really goes with the column for p.p. and is noted as O2 p.p. for Oxygen Partial Pressure.

Let’s look at one block for what a block contains. Let’s choose the 32% since it is conveniently highlighted for us. Later we will work with other blocks.

The three columns which fall under every block are, Depth, EAD and p.p. (O2 p.p.) where Depth is the Depth you plan to dive, the EAD is the equivalent depth as if you were on a cylinder of Air and the p.p. is the PPO at the depth found under the depth column.

The way it works is as follows. Let presume you analyzed your Nitrox fill to contain a 30% mix of Oxygen. 30% is being used in the example because if we were to use 32% we could just the the RDP for Nitrox 32.

Looking at the above chart for a 30% Nitrox mix you read it as follows: Scan down until you find your planned depth. We will use 60fsw. Scan across to the next column and you find 49fsw. When diving to a depth of 60fsw on a 30% Nitrox mix, you are diving the EAD of 49fsw. 60fsw on 30% is equivalent to 49fsw on 21%. The last column, O2 p.p. is your PPO at 60fsw on 30% that shows .85.

You now know the PPO for a dive to 60fsw on 30% is .85. You also learned 1) The PPO does not exceed 1.4. You do not want to exceed a PPO of 1.4. In order for you to exceed a PPO of 1.4 you would need to dive deeper than 120fsw. (See circled)

What you also learn is the EAD. On a 30% mix to 60fsw the EAD is 49 feet. Now that you have the EAD, you may turn to the AIR Recreational Dive Planner and manage your dives from there. Once you get the EAD and move to the RDP for air, you work the RDP like you would if you were on air. In the above example for an EAD of 49fsw, you would work off the 50fsw column on the air RDP.

**Question**: What is missing from the table above? Time

**Question**:What does the RDP calculate? Time

Now that you know the EAD (49fsw), Move to the AIR RDP to calculate time at depth.

**Question**: What is your NDL at 50fsw on the Air RDP?

Answer: 80 minutes. 80 minutes becomes your NDL for a 30% mix at 60fsw, presuming this is your first dive.

Let’s say you performed the dive to 60fsw on 30% Nitrox for only 50 minutes.

**Question**: What will your Pressure Group be at the end of the 50 minute dive?

Answer: P

After a 1 hour surface interval, your Pressure Group becomes: E

You enter your second dive in Pressure Group E.

See, nothing new here.. Once you find the EAD from the EAD Table for the Depth and mix of Oxygen you will be diving, it’s onto the Air RDP to calculate the time.

#### Let’s run through the first scenario of your scheduled four dives. As a reminder they are to the depths:

**(1st) 100fsw, (2nd) 80fsw, (3rd) 60fsw and finally (4th) 40fsw.**

Let us presume you have 32% Nitrox in all four of your cylinders. I understand there is a table made specifically for 32%. For the purpose of learning the EAD Table, we will use the EAD table.

Looking at your EAD Chart to find your EAD for the mix and depth of your first dive.

**Question**: What is the EAD for 32% to 100fsw?

Answer: 81fsw

**Question**: What is the PPO?

Answer: PPO=1.29

*As a thought.. In my planning I would calculate for 90 feet as well. Just in case you did not quite reach 100fsw, you would be credited with some time and that credit may prove handy*.

You use the EAD tables to determine your EAD. Once you have your EAD, you move over to the AIR tables.

**Question**: What is your NDL?

Answer: 25 minutes

Where did you get 25 minutes from? The AIR RDP under the 90fsw column. The black box at the bottom of that column reads 25.

**Question**: Why did you use the 90fsw column on the AIR RDP and not the 80fsw?

Answer: Your EAD on the EAD Chart showed a dive to 100fsw on EAN32 is equivalent to a depth of 81fsw on air. Since you round UP, 81 becomes 90.

If we were to take a detour for just a second. Loot at the NDL for 32% on the Nitrox RDP for 32%, what is the NDL?

Answer: 30 minutes

You see, if you were to use the 80fsw column on the AIR RDP vs the 90fsw, you would have 30 minutes. See why I mentioned above why planning the next lower depth? What if you never quite reached 100fsw, your AIR depth would have you using the 80fsw column instead of the 90fsw and you would have most likely gained 5 minutes to your NDL.

Let’s stick with our 100fsw plan for now and continue the example.

Now that you know the NDL is 25 minutes, How many minutes would you like to spend at depth?

Let’s plan for 15 minutes.

**Question**: What Pressure group do you end your dive to 100fsw for 15 minutes on EAN32?

Answer: H

Let’s plan to spend 1 hour on the surface interval

**Question**: What pressure group are you in after a 1 hr surface interval?

Answer: B

You will enter the water for your second dive to 80fsw as a Pressure Group B.

Turning the AIR RDP over to table 3 the Repetitive Dive Timetable, let’s calculate the amount of time we have to spend at our next depth of 80fsw on 32% EAN.

**Question**: What is the EAD to 80fsw on EAN 32%?

Answer: 64fsw

We obtained the answer by going back to the EAD table and locating the EAD for a dive depth of 80fsw under the 32% section. Now that you have the EAD, move back to the AIR table and calculate your RNT for a depth to 64fsw with a Pressure Group of B.

**Questions**: What is your RNT and your ABT values for the second dive to 80fsw on EAN32? (hint: found in the Air RDP Table #3)

Answers: 9 and 31 respectively.

You may now plan a second dive to 80fsw on EAN 32 to be no longer than an ABT of 31 minutes…

See how this is performed.. You continue through the series of steps until the fourth dive.

You take over from here..

Let me summarize so far so the information we learned into one area. You take over the remaining calculations starting at Dive 2. I will provide you with some information so that we may remain consistent on our results.

Answer each question as they are presented before moving on. The answers will follow at the end of this exercise.

Previously provided info in Blue Additional Info I am providing in Red

**(1st) 100fsw, (2nd) 80fsw, (3rd) 60fsw and finally (4th) 40fsw.**

Dive 1) 100fsw on EAN32 for 15 minutes. Exit the water in Pressure Group: H. A 1 hr surface interval brought you to Pressure Group: B.

Dive 2) 80fsw on EAN32 Dive time: 20 minutes

**Question**: What is the EAD for 80fsw on EAN32?

Your Answer: 64fsw

**Question**: What is your Pressure Group at the end of your 20 minute dive?

Answer: **N**

**Question**: What is your Pressure Group after a 1 hr Surface Interval?

Your Answer:

Dive 3) 60fsw on EAN32 Dive time: 45 minutes

**Question**: What is the EAD for 60fsw on EAN32?

Your Answer:

**Question**: What is your Pressure Group at the end of dive 3?

Your Answer:

**Question**: What is your Pressure Group after a 1 hr Surface Interval?

Your Answer:

Dive 4) 40fsw on EAN32 Dive time: 60 minutes

**Question**: What is the EAD for 40fsw on EAN32?

Your Answer:

**Question**: What is your Pressure Group at the end of dive 4?

Your Answer:

#### The Answers to the above questions. Don’t peek until you answered the questions just above.

Dive 2) 80fsw on EAN32 Dive time: 20 minutes

**Question**: What is your Pressure Group after a 1 hr Surface Interval?

Answer: D

Dive 3) 60fsw on EAN32 Dive time: 45 minutes

**Question**: What is the EAD for 60fsw on EAN32?

Answer: 47fsw

**Question**: What is your Pressure Group at the end of dive 3?

Answer: U

**Question**: What is your Pressure Group after a 1 hr Surface Interval?

Answer: G

Dive 4) 40fsw on EAN32 Dive time: 60 minutes

**Question**: What is the EAD for 40fsw on EAN32?

Answer: 30fsw

**Question**: What is your Pressure Group at the end of dive 4?

Your Answer: R

BUT WAIT! There’s more!

WHAT? More? Remember it was mentioned earlier in addition to keeping track of your Nitrogen load, which is what you just performed switching between the EAD table and your Air RDP you must also keep track of your Oxygen load.

Let’s walk through that now. You would normally keep track of both your Nitrogen and Oxygen at the same time. The two have separate to focus on one aspect at a time.

Turn your EAD Table over to find the Oxygen Exposure Table

Go back to the first dive to 100fsw where your bottom time was 15 minutes.

**Question**: How much Oxygen Toxicity or CNS load have you incurred?

For this table you will need the PPO for the depth of the dive and the time spent on the dive.

Going back to the Equivalent Air Depth table, you look up the PPO for the depth of the dive.

For a dive to 100fsw on EAN32, your PPO is 1.9. Moving back to the Oxygen Exposure Table the row across the top is PPO. You do not find a 1.29 but you do find a 1.3. Now scan down the column below 1.3. You first see a 9, then an 18. 15 minutes falls between 9 and 18 so you use the 18 minutes. NOTE: You always round UP.

To the far right is 18 you see a percentage. That percentage should read 10%. That is your Oxygen Toxicity Load for this dive.

For a 15 minute dive to 100fsw on EAN32 your Oxygen Toxicity load is 10%.

Do the same for the second dive to 80fsw. That second dive was for 20 minutes. The PPO at 80fsw on EAN 32 is 1.10

Looking at the OET for 1.10 on the top row, scan down until you find 20 minutes or the next highest number. You should have found 24 minutes and to the far right an Oxygen load of 10%..

You body Oxygen load for the first dive was 10%, your second dive was 10% for a total of 20% (10% + 10%).. The total load is the sum of all your dives for the day. It is accumulative and you do not earn any credit for your surface interval.

So far you are up to 20%…

The third dive was to 60fsw on EAN32 for 45 minutes

Ansswer the following questions by performing the steps on your own.

**Question**: What is your Percent Oxygen Exposure for that dive?

Answer: 15%

**Question**: What is your total Oxygen Exposure at this point?

Answer: 35% (10%+10%+15%)

The fourth and final dive was to 40fsw on EAN32 for 60 minutes

**Question**: What is your Percent Oxygen Exposure for that dive?

Answer: 15%

**Question**: What is your total Oxygen Exposure at this point?

Answer: 50% (10%+10%+15%+15%)

I hope the article made sense and it proves to be helpful. Should you feel the need to reach out to Larry Davis. you may do so by filling out the contact form below.

**Answers to questions** Previously Asked

The question: Assuming no decompression obligation, using the new ascent mythologies how long, at a minimum will it take you to break the surface from a depth of 90 feet?

My answer: A minimum of 8 minutes… at 30’/minute

**Question**: What will the PPO on a 32% mix be at 100fsw?

PPO=(((Depth / 33 ) + 1) * FO) ; PPO=((( 100 / 33 ) + 1) * .32)

Answer: **PPO=1.28**

**Question**: What is your NDL for your first of a series of dives to 100fsw on a 32% mix?

Answer: 30 Minutes

Take the Nitrox Quiz. If you took the Nitrox Quiz prior to reading this article, take the quiz again and compare your results. See if your knowledge improved after reading the article

Click HERE to take the quiz

### Gas Monitoring and **Tracking Gas Consumption** (Bonus Section)

Regarding your air consumption rate, you should consider keeping track of your consumption rate. With the use of SAC, Surface Air Consumption along with the rate you document your consumption at depth may be used to calculate time at depth before you must ascend for future dives. I add this recommendation here because it is relative. We will not go over consumption rate in this article. I thought you should be introduced to it for your own knowledge. I had added some detail for SAC at the end of this article for your reference.

Your buoyancy, trim, weights, breathing technique, currents, equipment, health, environment, your competitiveness with matching or beating your fellow divers rate, experience, and other considerations all factor into your gas consumption rate. Your log book should have most of those details so you may reference them at a later date.

Most recreational divers know over time whether they are coming up short on bottom time or gas volume. You may hear people around the dive site asking their friends and buddy’s, How much air did you come up with? Usually to show off if they came up with more or kept their mouth closed if less usually in good fun and to tease. Do not ever let someone convince your you are performing bad on gas management into making poor decisions. You will get better over time and with more experience.

Tracking your gas consumption can be burdensome for some and the attention to detail for others. If knowing your gas consumption is something that you will consider, there is no time like the present to begin. If you are unsure, I suggest you begin. It will add to your knowledge base, assist you in conversations, provide input when calculating gas needed at depth, provide clarity and much more.

You could collect just the starting and ending pressure and the difference between the two extremes. What you will get is a very general SAC value or you can go further in your data collection and collect numbers for the amount of time and gas consumed at each of the specific depths during the course of the dive.

I am not here to tell you which way is best for you. Provided below is a basic review of what to track and how to track information. It is up to you decide how much to use.

To keep track of your SAC, you will need to collect some basic information and perform a little math. A spreadsheet or a program found on the Internet can help but a pen and paper will for sure.

The resulting figures are the average rate of gas consumed per minute based on an Aluminum 80cf cylinder filled to 3000psi and having consumed 2000psi during the course of the specific dive. Change the tank size, the working pressure, the exertion, equipment, etc. for example and your SAC rate changes. Makes you wonder if you should be separating your SAC rates based on all the variables listed above.

**Surface Air Consumption, SAC**

To get the total cubic feet of gas consumed, to the cylinder size used, use the following formula, For example on an 80cf cylinder filled to 3000psi:

**((80 cubic feet x 2000 psi) / 3000 psi) = 53.3 cubic feet of gas consumed**

To get the cubic feet of gas consumed per minute, use the following formula**(((80 cubic feet x 2000 psi) / 3000 psi)**

**/ dive time)**

**53.3 cubic feet / 20 minutes = 2.6 cubic feet / minute **

*Note the above and below examples are the Average over the course of the 20 minute dive.. Most likely the diver did not spend the whole 20 minutes at one specific depth. realistically, you are at various depths during the course of a dive. *

If it is the **psi** vs. **cf **the formula would be:

((*Air consumed during dive/dive time) / ATA) = SAC psi per minute*

**( (psi_consumed / DiveTime) / ATA) = psi per minute **

**(( 2000psi/20 minutes) / 3) = 33.33 psi per minute **SAC

**(( 2000psi/20 minutes) / 2) = 50 psi per minute **SAC

As you can tell, the lower the SAC rate the better.

Take this one step further to calculate your **Respiratory Minute Volume**, RMV. The main difference is the RMV rate is independent of tank size. You may use RVM to calculate volume based on variable sizes of tanks. If one dive is on AL80 and another on LP95, the volume of the two cylinders are not the same. Knowing your RMV, you can apply it to the specific volume of cylinder you plan to use.

Let me provide an example. If you are able to drive 200 miles on a gas tank with a volume of 20 gallons, changing the volume of the tank changes the number of miles you may travel. Using a 30 gallon tank, you would expect to go 300 miles. 1/3 farther than on a 20 gallon. The quantity of gas used per mile didn’t change, just the size of the tank.

To turn your SAC into RMV, simply factor in the size of the tank you were using by adding a tank conversion factor.

**Imperial**: Divide the tank volume, in cubic feet with the tank operating pressure then multiply it with your SAC.

( 80cf / 3000 ) * SAC

Using the above SAC rate of 33.33 with an 80cf Aluminum and working pressure of 3000psi:

RMV=( 80 / 3000 ) * 33

RMV=0.88cf/min

For a steel 95cf cylinder with a 2440 working pressure:

RMV= (95 / 2440) * 33

RMV=1.28cf/min

Sounds a bit daunting, all these numbers and different ways to calculate to have to learn. Just keep learning. Perform some internet searches and read more about it. Practice until you feel comfortable and by then you may find it is something you will perform and log.

Stop here and watch the video. Harry does a nice job of explaining with examples what was taught up above. Hearing and seeing the SAC and RVM calculated along with audio descriptions will compliment your comprehension on the subject.

I found this RMV/SAC calculator. Perform the steps above and compare your results to that from the calculator. Do not be concerned about the rounding difference, ie. if you came up with .88 and the calculator .89. Your numbers are comparably close.

That should cover SAC and RMV.

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