A brief introduction to gas mass flow and volume flow

## Background

Gas refers to a special fluid with no shape and volume that can be compressed and expanded. Like liquids, they can flow and deform. Unlike liquids, gases have large distances between gas molecules and can be compressed and expanded. If there is no restriction (container or force field), the gas can expand to an unlimited volume.

## Ideal Gas Law

Gases are divided into real gases and ideal gases. In middle school chemistry, we learned the ideal gas equation, which includes the concept of an ideal gas: an ideal gas is an ideal model established by people to simplify the actual gas. An ideal gas has two characteristics:

①The molecule itself does not occupy volume;

② There is no interaction between molecules. Actual gases are different. The conclusions obtained from the discussion of ideal gases are only applicable to actual gases whose pressure is not high and the temperature is not low.

The ideal gas equation of state can be expressed by PV=nRT, where: P is the pressure, V is the gas volume, T is the temperature, n is the amount of substance in the gas, and R is the molar gas constant.

## The Definition and Difference Between Volume Flow and Mass Flow

Gas flow represents the amount of gas passing through a certain cross-sectional area in unit time. When the amount of fluid is expressed in terms of volume, it is called volumetric flow; when the amount of fluid is expressed in terms of mass, it is called gas mass flow. Gas volume flow refers to the gas volume flowing through the pipeline per unit time, and the unit is generally L/min or mL/min. The volume of gas can be divided into the volume under working condition and the volume under standard condition.

They all refer to the volume under a certain pressure and a certain temperature. Hilin Technology mini pumps are all under standard conditions (temperature is 25℃, standard Atmospheric pressure 101kPa) measured no-load flow value. Gas mass flow refers to the mass of gas passing through the transportation pipeline per unit time. Unit: Kg/min.

The difference between the two is that gas mass flow measures the number of molecules in a flowing gas, while volume flow measures the space occupied by those molecules. Because gases are compressible, the volumetric flow rate changes significantly when pressure or temperature changes.

Select gas mass flow when the focus of the flow measurement is the gas itself. Volumetric flow is selected when the focus of flow measurement or control is within the gas volume. Applications such as those in industrial hygiene and ambient air monitoring work best in volumetric flow measurement, since the goal is to quantify the number of particles within a volume of air under actual measurement conditions.

## Volume Flow Meter VS. Mass Flow Meter

There are many types of flow meters on the market, which can be roughly divided into gas mass flow meters and volume flow meters. Volume flowmeters include glass rotor flowmeters, soap film flowmeters, etc.; there are even more types of gas mass flowmeters, which can be divided into direct, indirect, thermal, differential pressure and Coriolis based on the measurement principle.

Mass flow meters can directly measure gas mass flow and dynamic detection of density without compensation, and can also directly measure volume flow. The volume flow meter can only directly measure the volume flow rate. If you want to measure the gas mass flow rate, you need to give a fixed density value, so you cannot dynamically detect the density, and the accuracy is poor. Mass flow meters are expensive and more accurate in measurement, while volume flow meters are cheaper, but they can be converted between the two.

## Conversion of Volume Flow and Gas Mass Flow

The conversion formula between mass flow rate and volume flow rate: Qm=Qv×ρ, where: Qm is the mass flow rate, ρ is the fluid density, and Qv is the volume flow rate. If you know the gas density and one of the flow values, you can directly convert the other. However, gas is compressible, and gas density is affected by actual pressure and temperature and changes greatly.

1. Ideal gas equation: PV=nRT, where: P is the pressure, V is the gas volume, T is the temperature, n is the amount of material in the gas, and R is the molar gas constant.

2. Gas density formula: ρ=m/V, where ρ is density, V is gas volume, and m is gas mass.

3. Gas molar mass: M=m/n, where: M is the molar mass of the gas, n is the amount of material in the gas, and m is the mass of the gas.

According to the above three formulas, we can get: ρ=MP/RT, where: ρ is the density, M is the molar mass of the gas, P is the pressure, R is the molar gas constant, and T is the temperature. If it is a gas with a single component, for a certain pure substance, its molar mass is fixed, while the mass of the substance changes with the amount of the substance. If the gas composition is known, the molar mass of the gas can be known. Under a certain pressure and temperature, we can obtain the density of the gas based on this formula to convert mass flow and volume flow.