Practical knowledge

 A certain customer has a thermal flow meter with nitrogen as the working fluid. The YIDU laminar differential pressure flow meter is set to nitrogen as the working fluid, and the two flow meters are compared in series. When nitrogen is introduced, the readings of the two flow meters are consistent. But when helium gas was introduced, it was found that the readings of the two flow meters differed greatly. Is it true that the customer thinks at least one flow meter is problematic?

Let's calculate the displayed flow rate of the nitrogen flowmeter when helium gas is introduced. For ease of calculation, if there is an accurate helium flow controller at the front end of two flow meters connected in series, controlling the helium flow rate to 100SLPM.

First, let's take a look at the thermal flowmeter. When different gases are introduced, a conversion coefficient must be used. The conversion coefficient of a certain model of thermal flowmeter from a certain manufacturer to helium is about 1.4 (the conversion coefficient of different brands and models of thermal flowmeters is also different). Therefore, the current reading of this thermal flowmeter should be 100 ÷ 1.4=71SLPM.

Looking at the laminar differential pressure flowmeter again, it distinguishes different types of gases based on gas viscosity. According to Hagen Poiseuille's law, the calculated flow rate is inversely proportional to the viscosity. At this time, helium gas is introduced but the viscosity value of nitrogen gas is used for calculation. The query shows that the viscosity ratio of helium gas to nitrogen gas is about 1.11. Therefore, the current reading of this laminar differential pressure flowmeter should be 100 ÷ 1.11=90SLPM.

It can be seen that although both of these flow meters are nitrogen flow meters and are filled with 100SLPM helium gas, there is a significant difference in numerical values! This is mainly due to different measurement principles.

From the above calculation process, it can be seen that in order to accurately measure flow rate, the working fluid of the flowmeter should be consistent with the actual gas type. Of course, if using an EasyFlow laminar differential pressure flowmeter, it is easy to switch the working fluid and achieve the goal of accurately measuring multiple gases with one flowmeter.