2025-09
29What should be noted when converting gas mass and volume?
The flow units of gas mass flow meters are usually SCCM (mL/min) and SLPM (L/min). Sometimes we need g/s and g/min. How can they be converted between them?
How to Calculate Gas Mass by Density
We learned about the relationship between mass and volume during our middle school years:
m=ρv
Where m is mass, ρ is density, and v is volume. It seems that the conversion between mass and volume is very simple, just by converting density.
However, when using gas density, it is important to be very careful. Gas is compressible, and the corresponding gas density varies at different pressures and temperatures. Therefore, when describing gas density, we usually say: what is the density of a certain gas at a certain pressure and temperature.
The standard flow rate defined by the YIDU flowmeter refers to the volumetric flow rate of gas at 101.325kPa and 25 ℃. Therefore, to convert this standard flow rate into mass units, it is necessary to query the density of the measured gas at 101.325kPa and 25 ℃, and then use the above formula for calculation.
How to calculate if gas density cannot be queried?
Don't worry, even if we don't know the gas density, we can still convert the gas mass to volume. At this point, we need the concept of gas molar volume.
First of all, we need to know a fact: the volume of any gas at 101.325kPa and 0 ℃ is 22.4L, which is called the molar volume of the gas. By using the ideal gas state equation:
PV=nRT
We can calculate that the molar volume of a gas at 101.325kPa and 25 ℃ (the standard condition defined by a flow meter) is 24.45L. Mole is actually a unit of quantity, and 1mol of gas refers to 6.02 × 1023 gas molecules. The total mass of these molecules is called the molar mass of this gas, and the relationship between mass and molar mass is as follows:
M=nM (Formula 2)
Where m is the mass, n is the quantity of substance, and M is the molar mass. The molar mass is numerically equal to the molecular weight, which in turn is equal to the sum of the proton and neutron of the molecule. However, the molecular formula of most common gas molecules is simple, and the molecular weight can be directly observed.
The elements that make up gas molecules are usually located at the top, and their atomic weight is somewhat related to their element numbers
Knowing the molecular weight means knowing the molar mass M; knowing the standard flow rate means calculating the mass n of the substance, making it easy to obtain the gas mass flow rate
Q mass flow=Q standard condition flow/24.45 × Mr (formula 3)
Among them, Mr is the molecular weight.
Some friends may feel a bit dizzy here, it's okay. If your flowmeter is set to 101.325KPa and 25 ℃, just calculate according to the formula above and don't worry about the derivation process. Let's put it into practice and see if the mass flow rate results obtained through density calculation and molecular weight calculation are the same.
Examples of two calculation methods and comparison of results
Example: The flow meter measures a nitrogen (N2) flow rate of 100SLPM, and calculates the nitrogen flow rate in grams per second.
Answer:
Method 1: Query the density of nitrogen at 101.325KPa and 25 ℃ as ρ=1.13g/L,
Substitute into the formula
m=ρv
1.13×100÷60=1.88
The nitrogen flow rate is 1.88g/s.
Method 2: Nitrogen with a molecular weight of 28, substitute into formula 3:
100/60/24.45×28=1.9
The nitrogen flow rate is 1.9g/s.
It can be seen that the calculation results of the two methods are very close, with an error of about 1% between them, because the molar volume of the gas is not an accurate value, and the molecular weight is also omitted after the decimal point, in order to facilitate the calculation.
Advantages and disadvantages of two calculation methods
In summary, the advantage of density calculation is that it is relatively accurate, but the disadvantage is that it is necessary to know the density of the gas at 101.325KPa and 25 ℃. However, searching and querying through the internet may result in incorrect density values (most online search data are for gas density values at 101.325KPa and 0 ℃). The disadvantage of calculating molecular weight is slightly lower accuracy (1% error is still within an acceptable range), but the advantage is that it does not require querying gas density. Most common gas molecular weights can be determined by sorting each element in the periodic table.