The quantity of a gas can be indicated by way of its mass or its weight in the units of measure normally used for mass or weight.
Quantity of gas (pV value), (mbar ⋅ l)
The quantity of a gas can be indicated by way of its mass or its weight in the units of measure normally used for mass or weight. In practice, however, the product of p · V is often more interesting in vacuum technology than the mass or weight of a quantity of gas. The value embraces an energy dimension and is specified in millibar · liters (mbar · l):
p·V=m/M·R·T
pV = nRT
N=m/M
M is the molar mass of the gas
n is the number of moles of the gas
m is the mass of the gas
R is the Universal Gas Constant
Where the nature of the gas and its temperature are known, it is possible to use the equation below to calculate the mass m for the quantity of gas on the basis of the product of p · V:
m=(p·V·M)/R·T
Although it is not absolutely correct, reference is often made in practice to the “quantity of gas”
p · V for a certain gas. This specification is incomplete; the temperature of the gas T, usually room temperature (293 K), is normally implicitly assumed to be known.
Example: The mass of 100 mbar · l of nitrogen (N2) at room temperature (approx. 300 K) is:
Analogous to this, at T=300 K:
1 mbar.l O2=1.28·(10³gO2)
70 mbar·l Ar=1.31·(10³gAr)
The quantity of gas flowing through a piping element during a unit of time – in accordance with the two concepts for gas quantity described above – can be indicated in either as Mass flow or pV flow.
Mass flow q_m (kg/h, g/s), this is the quantity of a gas which flows through a piping element, referenced to time q_m = m/t or as
pV flow qpV (mbar · l · s^(–1)).
pV flow is the product of the pressure and volume of a quantity of gas flowing through a piping element, divided by time, i.e.:
q(_pv) = (p · V)/t = d(p · V)/dt
pV flow is a measure of the mass flow of the gas; the temperature to be
indicated here.