The importance of the temperature and
maintaining the isotherme
At the end of the filling process, the heat of compression increases the temperature of the mixed-gas cylinders giving a false indication of the pressure in the cylinder.
The calculation requires the final pressure to be taken at the same temperature as initial pressure.
Even though the software can easily manage the effects of the temperature on the pressure readings, and consequently correctly calculate the density of gas, it is unrealistic to establish the inner tank gas temperature (and not the metal wall of the cylinder) at the moment of the pressure reading.
The gas temperature could be calculated in an inductive way (see Tools, Pressure and Temperature) by looking at the pressure drop between ‘hot’ and ‘cool’; nevertheless, there are many variable factors that make the game unreliable:
Just to add to complications, during the filling, some heat energy is used to warm up the metal of the cylinder, so the gas temperature changes continuously.
That's why Tech Gas Blender in all mixing calculations assumes isothermal conditions throughout the filling period, with equal initial and final temperatures.
Implementing the software with the temperature function combined with the pressure reading, a new risk of mistake could take place: what happens if for any reason, the filling is paused and the gas temperature falls ?
For best results, the key is to allow the mix to cool, then test the dropped pressure and re-adjust for a small, “no-temperature effect” pressure adjustment.
Waiting a little is a good investment: after a few hours, the mixture is at ambient temperature and another possible cause of mistakes can be eliminated.
When productivity is an issue, like for diving shops, the usual water cooling method, and possibly reducing the flow rate by filling more cylinders with the same mixture at the same time, are the only possibilities to mitigate this effect.
In order to get an idea of what the impact could be, you can use the "What if - Ideal Gas and Real Gas" and “Pressure and Temperature”.
• The composition of the gases
• The type of the filled gas/mixture
• The dimension of the tank
• The ambient temperature
• The compressor size/flow rate
• etc. etc.