Basic knowledge

 Student A asked:

Both laminar differential pressure flow meters and thermal flow meters have a unified name in the field of microfluidics: mass flow meters. Have you ever thought that the flow unit is clearly SCCM (mL/min) or SLPM (L/min), both of which are changes in volume over time, but why is it called a mass flow meter instead of a volume flow meter?

Answer from YIDU pre-sales engineer:

This is simple. Let's take the example of 'how traditional household gas cylinders measure the amount of gas they contain'. Firstly, place the empty jar on the scale and read it. After filling it with gas, read it again and calculate the gas quality based on the difference in mass before and after. There are many industrial scenarios where the quality of gas can be determined by weighing, but once the gas is connected to the system and fills various pipelines and valves, it is impossible to determine the mass of gas passing through a certain pipeline interface. Therefore, it is necessary to calculate mass by measuring other quantities, which gave rise to various types of gas flow meters, including mass flow meters.

       Gas tank weighing diagram

The measurement result of a mass flowmeter is generally referred to as standard condition flow rate, which refers to a standard atmospheric pressure (101.325KPa) and a state of 0 ℃ (standard condition temperature may vary in different industries). The mass of gas is concentrated on gas molecules, and the diameter of gas molecules is much smaller than the distance between molecules. Temperature and pressure are macroscopic manifestations of molecular motion, so when temperature and pressure are determined, the distance between gas molecules is also determined. After countless experiments, it has been verified that the volume of any gas (with a molecular number of 6.02 × 1023) in 1mol at 101.325Kpa and 0 ℃ is about 22.4L, and its mass is only related to the molecular weight of the gas. For example, if the molecular weight of hydrogen (H2) is 2, then the mass of 22.4L of hydrogen under standard conditions is 2g. The macroscopic description of this conclusion is that the density of a specific gas under standard conditions is a constant value, so knowing the standard flow rate also means knowing the mass flow rate.

Student A asked:

Why can a mass flow meter measure standard flow rates when the actual measured gas is not under standard conditions?

Answer from YIDU pre-sales engineer:

Taking the laminar differential pressure flow meter as an example, the laminar differential pressure flow meter obtains the volumetric flow rate of gas at the current temperature and pressure by measuring the differential pressure, and then converts the flow rate under standard conditions through the ideal gas state equation.

In practical applications, customers often care about the amount of gas rather than the volume it occupies. For example, to accurately mix two gases in proportion, it is necessary to control and mix the two gases in a certain mass ratio. Flow meters are commonly used in this situation and are habitually referred to as mass flow meters.