Temperature compensation when blending scuba gases

You plan a fill to 200 bar, stop when the gauge reaches 200 bar, and return after the cylinder has cooled. The gauge now reads less than 200 bar. The gas has not disappeared. During filling, compression heated the gas and raised its pressure. Once the cylinder cooled, the same amount of gas produced a lower pressure.

Gas Blender Toolkit offers this calculation for every filling step. The blender opens the dialog, measures the cylinder temperature and enters it manually. The app then calculates the pressure that the intended amount of gas should produce at that temperature while the fill is still in progress.

The difference is not additional gas. It is the same amount of gas at a higher temperature. The value shown by the temperature-compensation dialog is the target for the pressure gauge at the measured temperature.

What “isochoric” means here

The calculation describes the gas in a rigid cylinder while two quantities remain fixed:

• the internal cylinder volume; and
• the amount of gas that should be present after the selected filling step.

Only the temperature changes. From the selected row, the app takes the complete planned cylinder state after that step: gas composition, gas amount and cylinder volume. When the blender enters the cylinder's current temperature, the app computes the pressure of that same gas at the new temperature.

The isochoric calculation does not modify the blend plan. It translates the planned gas amount into a temperature-compensated target pressure that can be used while the cylinder is warm.

How to use it during a fill

1. Before filling, measure the cylinder temperature and enter it as Initial temperature under Options or Settings. Every pressure in the result table refers to this reference temperature.
2. Start the first filling step shown in the result table.
3. Shortly before completing the step, select the thermometer button in that row, under Warming.
4. In the isochore dialog, enter the cylinder temperature measured at that moment under Actual value.
5. Read the displayed temperature-compensated target pressure and continue filling slowly until the gauge reaches that value.
6. Repeat the process for each subsequent filling step when temperature compensation is needed.

Do not change the global Initial temperature during the fill. The elevated temperature measured while filling belongs only in the isochore dialog. Changing the global value causes the entire blend to be recalculated.

Since version 2.0.34, this operating guidance is also available directly inside the isochore dialog in English, German and Spanish. A permanent reminder remains visible even if the longer help panel is dismissed.

A worked trimix example

Consider a 24-litre cylinder containing trimix 21/35. The selected fill step is intended to leave the cylinder at 200 bar when it is at the 20 °C reference temperature.

The arithmetic below uses the ideal gas law, so you can verify it yourself with a calculator. This calculation is mix-independent because it contains no gas-specific constants. The app's real-gas model — the one you should fill by — gives slightly higher values. That difference is not an error: real gases at high pressure do not follow p × V = n × R × T exactly, and how far they deviate depends on the mix.

For an ideal gas at constant volume and constant gas amount:

p2 = p1 × T2 / T1        (temperatures in kelvin)

For example:

200 bar × 313.15 K / 293.15 K = 213.6 bar

The app's Van der Waals real-gas model displays 216.3 bar for the same 21/35 mix. The blender therefore continues the step to the real-gas target of 216.3 bar. This does not mean adding another 16.3 bar after completing the planned step. The 216.3 bar reading represents the planned gas amount while that gas is warm. After the cylinder returns to 20 °C, the intended reference pressure is 200 bar.

The same comparison can be made with imperial units. A 3000 psi reference pressure at 68 °F gives 3205 psi under the ideal gas law and 3245 psi with the app's real-gas model at a measured 104 °F. The 3245 psi value was measured in the app. The 3205 psi value was calculated from the temperature ratio; 68 °F and 104 °F correspond exactly to 20 °C and 40 °C.

Why the gas model matters

Gas Blender Toolkit offers both ideal-gas and Van der Waals real-gas calculations. The ideal-gas model is useful for comparison, teaching and a calculation that every reader can reproduce regardless of the gas mix. High-pressure breathing gases, however, do not behave ideally.

The table demonstrates two important effects:

1. The deviation grows with temperature. For this example it increases from +2.7 bar at 40 °C to +4.6 bar at 55 °C and +6.0 bar at 65 °C. The hotter the cylinder becomes, the more the ideal-gas approximation would underfill it.
2. The deviation depends on the gas mix. It also grows with helium content and pressure. The real-gas figures above therefore apply specifically to this 21/35 mix. A different mix can correctly produce different values.

This is why temperature compensation should use the same real-gas model as the blend calculation. A number that differs from the hand-calculated ideal-gas value is expected behaviour, not evidence that the app is wrong.

Safety boundaries

Temperature compensation is a calculation aid, not permission to exceed the limits of the equipment.

• Never exceed the marked working pressure or other applicable pressure and temperature limits of the cylinder, valve, filling system or connected equipment.
• Gas Blender Toolkit warns from 60 °C, the temperature a filled aluminium cylinder should not reach. The app does not know which cylinder material is being filled, so the same warning is shown for steel, aluminium and composite cylinders. Treat the cylinder manufacturer's markings and instructions as authoritative.
• For mixes above 23.5% oxygen, the app uses a more conservative temperature warning beginning at 50 °C. The 23.5% boundary is also used by CGA G-4.1 and by cylinder manufacturers such as Luxfer for oxygen-service cleanliness. Luxfer explicitly does not support treating 40% oxygen as a general recreational exemption.
• These warnings do not block the calculation. A valid measured temperature is still used, because silently replacing or ignoring a measurement could produce a misleading target pressure.
• Allow the cylinder to return to the initial reference temperature before the final pressure check and before analysing the finished gas.
• Use a calibrated pressure gauge and suitable temperature measurement. The app does not measure either value.
• Neither this article nor the application replaces gas-blender knowledge, qualification, established filling procedures or analysis of the finished breathing gas.

What the calculation does not know

The app does not know the cylinder material, fill rate, compressor behaviour, ambient conditions or the temperature distribution through the cylinder wall and gas. Its temperature warnings depend only on the measured temperature and the oxygen fraction. The blender measures the cylinder temperature and enters it manually.

The temperature entered in the isochore dialog is not fed back into the blend calculation and does not change the result table. It is used only to calculate the gauge target for the fixed gas amount represented by the selected row. The temperature control accepts measured values from −20 °C to 90 °C, equivalent to −4 °F to 194 °F. The slider spans that full range, and values outside it are visibly rejected rather than silently substituted.

The practical takeaway

The result table answers: How much gas should be in the cylinder after this step at the reference temperature?

The isochore dialog answers: What pressure should that same amount of gas produce at the temperature I measure right now?

Keeping those two questions separate prevents the most important misunderstanding: the higher warm-cylinder pressure is not extra gas. It is the gauge target for completing the planned step while the same gas is at a higher temperature.

Technical references and verification

• All measurements were made in Gas Blender Toolkit v2.0.34, build 2026081402.
• The ideal-gas column was measured with the app's ideal-gas model selected and independently confirmed with p2 = p1 × T2 / T1. Both methods agree to the displayed decimal.
• The metric real-gas column was measured with the Van der Waals model selected for trimix 21/35/44 in a 24-litre cylinder, with a reference state of 200 bar at 20 °C.
• The imperial real-gas value of 3245 psi was measured in the app. The ideal-gas value of 3205 psi was calculated from the same temperature ratio and rounded to the nearest psi; it was not measured in the app.
• CGA G-4.1: Cleaning of Equipment for Oxygen Service
• Luxfer: Why does Luxfer require cleaning above 23.5% oxygen?

All operating limits must be checked against the markings and instructions for the specific cylinder and filling system in use.