EC Instruments - precision flow meters

Volume flow vs. Mass flow

The ReciFlow measures volume flow. Other types of flow meters that measures mass flow often present the flow rate in volumetric units such as ml/min. What is the story behind this and what is the relationship between volume flow and mass flow?

First some basic laws of thermodynamics.

Avogrados law

Two given samples of an ideal gas, at the same temperature, pressure and volume, contain the same number of molecules.

Thus, the number of molecules or atoms in a specific volume of gas is independent of their size or the molar mass of the gas.

The total weight of a sample of gas is dependent of how many molecules or atoms there are (which in turn depends on the volume, pressure and temperature) and their individual weight.

For example, the weight of one litre of air at 0°C and 1 bar is 1.293 gram.

Boyle's law

For a fixed amount of an ideal gas kept at a fixed temperature, the product of pressure [p] and volume [V] is constant.

p x V= const.

p1 x V1 = p2 x V2

This means that if you if you would compress the one litre in the example above to halv the volume, 0.5 litre, the pressure would double to 2 bar. The weight would however be the same, 1.293 gram.

Charles's law (also Gay-Lussacs)

Charles's law describes how gases tend to expand when heated.

V/T = const.

V1xT2 = V2xT1

At constant pressure, the volume of a given mass of an ideal gas increases or decreases by the same factor as its temperature on the absolute temperature scale (Kelvin).

The ideal gas law

p x V = m x T x R

p = pressure

V = volume

m = mass of the gas

T = temperature

The constant R is depending on the specific gas at hand according to

R=RM / M

RM = the universal gas constant, which is empirically measured.

M = the molecular weight of the gas

This law is a combination of the laws above together with some empirical measurements to get the value of the ideal gas constant.

So if the type of gas is known, i.e. the molecular weight, calculations of one of either pressure, volume, temperature or mass can be made if three of the four are known.

Conversion of flow rate to normal conditions

If volume flow rate is measured it can be converted to a reference condition regarding pressure and temperature. As alternative one would always have to present all three of flow rate, the actual temperature and pressure.

Since the ReciFlow measures both pressure and temperature conversions into a flow rate at a normal condition can be made.

There are more than 10 different combinations a temperature and pressure used by different organization as the "normal" condition (see Wikipedia for an example of the confusion). An understanding of what is really meant by "normal condition" is essential. If the normal temperature is mistaken for 25°C instead of 0°C it would lead to a flow rate error of 10%!

The normal condition used by the ReciFlow instrument is default set to 0°C and 1.01325 bar.

The temperature and pressure used as normal condition can also be set by the user if some other standard is preferred.

Mass flow and volume flow

Mass flow meters often present the flow rate in a volumetric unit, such as ml/min. The gas type must be know for such a conversion. Remember Avogrados law above; a certain number of gas molecules have a known volume (at certain pressure and temperature). To know how many molecules the mass of a sample corresponds to you have to know the weight of the molecules at hand.

The ReciFlow however is independent of the type of gas as it is measuring volume flow.