What are Diving Z-Factors? Do I need them?

While teaching a class, I had a few students ask about the density in gas difference and calculating that difference in relationship to sac rate calculation.Show more

While we briefly touched on a question that was quite fair, I may have glossed over it a little too quickly with a response that was more of a statement that the difference between gasses and capacities in a cylinder was negligible at best.

The longer answer to this question is to take into account the "Z Factor" when calculating the gas capacity of a cylinder.

But, before we touch on what the Z Factor is, it is important to consider the ACTUAL capacity of the tank. IE many tanks hold as much as 10% LESS air than the size commonly used to describe them and up to 20% LESS for gasses other than air…

IE Catalina S80 Aluminum is listed as 80 cubic feet, when in fact it is 77.4. This is one of the nice things with many modern, HP Steel tanks. These tanks tend to be more accurate in describing the actual capacity. IE Faber HP 100 is actually 100 cuft.

But, the issue becomes the sheer variety of tanks available for diving.

Using the manufacturer-listed Liter Volume creates a much more concise and accurate standard of measurement that divers can more easily rely on.

Once we are able to fully appreciate the accuracy of using the published Liter size of a particular tank, we can begin to take into consideration the Z Factor for the gas that we plan to dive.

The Z Factor is used to determine how the gas will compress in a cylinder and is used to calculate its Compressibility Factor (Z).

This factor has differing values based on

Gas
Temperature
Total Pressure
So, for example, The Z factor for

air (dry) @70F - 2015 PSI = .9984 ---- IE 80 cuft alum true capacity 80.1282
air (dry) @70F - 3000 PSI = 1.0320 ---- IE 80 cuft alum true capacity 77.5193
air (dry) @70F - 3600 PSI = 1.0615 ---- IE 80 cuft alum true capacity 75.3650

Nitrox 32 @70F - 2015 PSI = .9912 ---- IE 80 cuft alum true capacity 80.7102
Nitrox 32 @70F - 3000 PSI = 1.0214 ---- IE 80 cuft alum true capacity 783238
Nitrox 32 @70F - 3600 PSI = 1.0491 ---- IE 80 cuft alum true capacity 76.2558

Trimix 10/70 @70F - 2015 PSI = 1.0833 ---- IE 80 cuft alum true capacity 73.8484
Trimix 10/70 @70F - 3000 PSI = 1.1262 ---- IE 80 cuft alum true capacity 69.1921
Trimix 10/70 @70F - 3600 PSI = 1.1529 ---- IE 80 cuft alum true capacity 69.3902

To calculate the true cylinder capacity of a gas, divide the ideal capacity by the Z Factor.

Now, with this, it is worth noting that this is not a straight line, but rather a curve and has a variance that is dependent on the amount of pressure. In the end, there is a general average that can be counted on.

It is also worth noting, that different gasses create different SAC rates for multiple reasons, and it is to your advantage to make sure that you are tracking your SAC throughout a history of diving and dividing all dives into categories... IE EAN dives (and again divided by the different levels of EAN), Air dives, Cold Water, Warm Water, Sea Level, Etc.

Also, keep in mind that factors like travel and hydration, as well of course as diving effort should be noted to be able to help determine an accurate dive SAC rate profile.

While many new divers refer to their computers and simply look at the PSI per minute SAC rate, this is far from being anything that can be considered very accurate or very useful for more complex dive planning.

This is one of the reasons it is so important to ensure that you are using the manufacturer-listed Liter capacity instead of the described cylinder size or relying on a PSI per minute based on a formula that is missing most of the important data needed.

By using the metric liter size of a tank and multiplying this by the BAR pressure, you are able to determine a more accurate picture of the actual amount of usable gas within instead of guessing based on a potentially inaccurate classification number in cubic feet. To increase the accuracy of this raw data, adding the Z Factor calculation will give the diver a more accurate picture of the amount of gas available and prepare them to be able to plan their dives accordingly.

Being able to remove as many variables from the gas calculations helps ensure a much higher potential for success and preparation for potential emergencies that may arise.

Capacity calculations also ignore the fact that it is difficult for a diver to breathe the residual gas in a tank below about 150 psi or 10 bar due to regulator flow restrictions and the fact that pressure gauges can be significantly inaccurate, especially when it comes to lower levels of gas.

With this, it becomes even more important to always plan your dives with a generous amount of reserve gas. Adopting a policy of diving the rule of thirds helps ensure that the diver is not only more prepared for an emergency that they might have but also for one that their buddy may experience as well.