When is a Scuba Cylinder Full?

… or, “why does my cylinder pressure change so much?”

Let’s face it, we’ve all been there. We go to a shop, we get our cylinders filled, we check the Pressure, we’re happy that they’re full, we go diving, and as soon as we hit the water, we notice that there’s less Pressure in our cylinder. Why is this, and where did that Pressure go?

To explain this, we have to start off with a basic understanding of gas laws and how they relate to diving. From our Open Water course, we all remember Boyle’s Law. Boyle’s Law spoke to us about Pressure, Volume, and their relationship; specifically, that Pressure and Volume have an inverse relationship.


As you recall, that means that as Pressure increases, Volume decreases. But, there are two assumptions associated with Boyle’s law: The first is we’re talking about a flexible container. If the container isn’t flexible, the volume can’t actually change. The second is we’re talking about a relationship at constant Temperature, and this is the part that people forget.

Dive physics and gas laws are based upon what is known as the Ideal Gas Law. The Ideal Gas Law relates Pressure, Volume, and Temperature together to describe how gases behave. And each of the specific laws that we speak about in scuba diving are based upon holding one of those variables constant.


To understand scuba cylinders, we need to understand Charles’ Law. Charles’ Law states that, “If the Pressure of a gas is constant then the Volume of the gas varies directly with Temperature. If the Volume of a gas is kept constant, then the Pressure of the gas varies directly with Temperature.” And this latter statement applies directly to scuba cylinders.


For scuba cylinders, the Volume is fixed. You can feel your cylinder – it’s got rigid walls. The Volume of it cannot change. Therefore, Volume is held constant and Pressure is related to Temperature. The difference here is that in Charles’ Law, this is a direct relationship. In other words, as Pressure increases, Temperature also increases, and vice versa. And this is what happens to the PSI in your cylinder when filling, draining, cooling, or heating that cylinder.

When a cylinder is filled, Pressure is increasing. The cylinder is going from low Pressure to high Pressure. And as that happens, per Charles’ law, Temperature increases as well. On a fast fill, we’ve all felt that our cylinders are hot to the touch afterwards, and that’s why we want our dive shops to take their time in filling them – so that the cylinder Temperature remains low during the filling process, minimizing Pressure variance on cooling. Once that cylinder is full at a specific Temperature, that Pressure is going to hold, as long as the Temperature remains constant.

Therefore, when filling or draining a cylinder, the Pressure is being changed, which also changes the Temperature of the cylinder.  And when the Temperature of the cylinder increases or decreases, there will be a commensurate change in Pressure.

Now let’s say we take a cylinder that is filled to 3000 PSI at 90°F. We take that cylinder and hop in the water, which is 60°F. What’s going to happen to the Pressure in that cylinder? To understand, we can utilize Charles’ law.

Before we get started, remember that in physics, all Temperatures have to be put into absolute Temperatures, and Fahrenheit has to be converted to degrees Rankine by adding 460 degrees.

In the equation below, we see that a cylinder filled to 3000 PSI (P1) that is taken from 90°F (T1) to 60°F (T2) will end up with a Pressure of 2836 PSI (P2) – a loss of 164 PSI just by changing 30 degrees in Temperature.


In addition, prior to diving, we’ve also gone through some type of test on our system. We test our regulators by breathing off of them. We inflate our BCD to ensure its functionality. And all of that is consuming air from the cylinder, dropping PSI. Additionally, remember that the gauges that we’re utilizing on our scuba systems are rarely calibrated. Therefore, there could be variation between what they read and what the shop’s gauges may be reading. And all of these factors combine together to change the Pressure in our cylinder from time of fill to time of dive.

Finally, remember that it is illegal to over-fill cylinders. Pressure losses due to expected Temperature decreases can’t be compensated for by asking our dive shop to add a couple of hundred PSI to our cylinders. Cylinders are regulated by the Department of Transportation. The effects of overfilling can be dangerous. Overfilling puts additional stress on the cylinder and may change its structural integrity, leading to catastrophic failure. In addition, since cylinders have to be hydrostatically tested every five years, a cylinder that has been consistently overfilled is in danger of failing hydrostatic test, resulting in the cylinder being condemned and requiring replacement. Overfilling, in the long run, can cost us more money (and potential injury) than the couple of hundred PSI that we lose due to Temperature change is worth to us.

So, when is a scuba cylinder full? By definition, a scuba cylinder is full when the Pressure in the cylinder matches the working Pressure of the cylinder, at 70°F. Therefore, your results may vary, and that is to be expected.

Author: mikecisdivers

Owner of Channel Islands Scuba, Thousand Oaks, CA.

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