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Combined factor

When filling a cylinder, the purpose is to fill the rigt amount
of gas in. When measuring the amount of gas by pressure,
some error exist due to compressibility and heat. 

Both T and Z are part of the equation

PV = nZRT      (real gas)

Because the purpose is to reach specific n number by measuring
parameter P, equation is 

n= PV / (ZRT)

Z is scheaduled factor, but also function of P. Also in a fill
T is fucntion of P (when pressure icreases, also temperature
increases).

Say, we fill helium 100 bar in a cylinder, and it heats 13 C
(heating factor is case specific), we get a correction factor

Z * T2 / T1 = 1.0470 * 303 K / 290 K = 9.4 %

Say, that we fill helium 125 bar in the same cylinder, and it 
heats 16 C (in this case), we get a correction factor

Z * T2 / T1 = 1.0590 * 306 K / 290 K = 11.7 %

These factors are of course against constant temperature ideal
gas.

When mixing, one problem is when you stop the gas flow, the
gas and cylinder starts to cool. During first minits the change
of pressure is fastest due to stabilisation of the system - gas 
shares energy with the cylinder. After this short period the 
temperature measured from surface of cylinder, and the pressure 
inside cylinder start having linear correlation. (PV=nRT)

In case of fast mixing, that is done in precense of time / heat 
factor, the factor can be conrolled by making several test fills. 
During these fills temperature is measured and deltaT factors defined.
To avoid error from stabilization, it may be well argued to
implement 10 min pause between different gases.

This text is purely information about scientific base only. 


Pekka Raty

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