Basic knowledge

Q: The customer has provided feedback on some issues with the flow meter of YIDU Micro Flow: even though the gas source has been turned off, the flow meter still reads; The reading cannot stabilize for a long time after the bypass inflation is closed during the airtightness test. Why did this situation occur?

Answer: YIDU has a flow meter with a full range of 0.5 SCCM, which can measure a minimum flow rate of 0.005 SCCM, and even customize models with smaller flow rates. Such precision instruments sometimes do not perform well in the system, largely due to insufficient consideration of the impact of volumetric effects.

What is volumetric effect

Definition and Explanation

We all know that gases undergo thermal expansion and contraction. A certain amount of gas, when the temperature changes, will also cause changes in pressure or volume, which is the volumetric effect. In general, this does not have any impact on gas measurement, and the flow meter itself is integrated with a highly sensitive temperature sensor, which can obtain accurate standard flow rate through temperature compensation. However, when measuring extremely low flow rates, the influence of gas volumetric effect is amplified.

Case Explanation

If the inner diameter of the back-end pipeline of a 0.5SCCM flowmeter is 6mm and the length is 1m, then the volume of this pipeline is approximately 113mL. If the entire system is under standard conditions (101.325kPa, 25 ℃), the temperature of the back-end pipeline will decrease by 0.1 ℃. According to the ideal gas state equation, the pressure will become 101.291kPa. To achieve pressure balance between the front and rear ends of the flowmeter, a volume of 0.038mL of gas needs to flow through it. At this time, the differential pressure between the front and rear ends is very small (less than 40Pa), and the flow rate will also be very small (about 0.005-0.01SCCM). It takes 4-8 minutes for 0.038mL of gas to flow through! And this is only caused by a temperature change of 0.1 ℃ in this small pipeline!

        Reduce the impact of volumetric effect

How can we reduce the impact of volumetric effect?

 If the customer chooses a small and micro flow meter, it means that the measured flow is basically in the microliter level, and the requirements for the system are also very high.

Firstly, the corresponding pipeline design standards should also be improved, aiming to make the pipeline thinner and shorter, reduce the number of valves, and make the pipeline volume smaller;

Secondly, the gas source should be stable. For small and micro flow meters, slight fluctuations in the gas source can cause significant changes in the flow rate;

The third is to keep the ambient temperature as constant as possible and reduce system temperature changes through sealing, isolation, water bath, and other methods.

 What is the concept of a small pipeline temperature change of 0.1 ℃? Um... it's probably caused by your body being close to the pipeline (without touching it) for a while.