Thermal Mass Flow Meter Correction Factor Guide

Thermal Mass Flow Meters and thermal mass flow controllers are suitable for most gaseous media. If a gas mass flow controller or thermal mass flow meter is factory-calibrated for one gas, but you want to switch to another, then a gas correction factor (GCF) is needed.

RFQ

What is Thermal Mass Flow Meter Correction Factor?

The gas conversion factor (GCF) of a thermal mass flow meter is a coefficient used to correct for the measurement of different gas flow rates. The GCF of a thermal gas mass flow meter is mainly related to the gas’s density, specific heat at constant pressure, and molecular composition. It is usually calibrated at the factory and can be used directly by the user.

Currently, most gas mass flow controllers and mass flow meters are calibrated with nitrogen or air at the factory. If the actual gas used is different, conversion using the gas conversion factor is required, even for flow controllers and mass flow meters from other brands (such as ABB, E+H, and Kobold). This is especially true for mixed gases, which are difficult to calibrate before leaving the factory, making the calculation of the conversion factor even more crucial.

As the manufacturer, our thermal mass flow meters have the relevant gas conversion factors pre-adjusted before leaving the factory, so you can use them directly. However, if you need to switch between different gases during use, you will need to manually input the corresponding gas conversion factor.

Thermal Mass Flow Meter Correction Factor Reference

Currently, most factories cannot calibrate mass flow rates according to the actual gas used by the user. Calibration is usually performed by converting the actual gas flow rate into the flow rate of air or nitrogen.

When you use it, the output display will directly show the mass flow rate or volumetric flow rate of the actual gas used.

Conversion between different gases is done using conversion factors. The conversion factors for single-component gases can be found in a table. The table below shows the densities and relative air conversion factors for common gases applicable to our company’s thermal flow meters.

Calculation of Gas Correction Factor (GCF)

For thermal mass flow meters and flow controllers, the gas conversion factor is mainly related to the density, specific heat at constant pressure, and molecular composition of the gas (e.g., He is a monatomic gas, while N2 is a diatomic gas).

The table above shows single gases. For mixed gases, the proportion of each gas needs to be known. Then, a reasonable calculation is performed based on the gas conversion factor algorithm. If you are interested in the specific algorithm, please contact us.

Conversion for single gases is relatively simple. For the same flow meter, simply multiply the nitrogen flow rate by the corresponding gas conversion factor.

For example, if the nitrogen flow rate is 20 liters/minute and the argon conversion factor is 1.415, then the actual argon flow rate = 20 SLPM * 1.415 = 28.3 SLPM. However, most manufacturers now perform the conversion at the factory, so customers can use it directly without further conversion.

The machine is factory-calibrated for 20 SLPM of acetylene gas. If you actually want to use propane gas, the conversion method is: 20 ÷ 0.581 (acetylene gas conversion coefficient) x 0.348 (propane gas conversion coefficient) ≈ 12 liters/minute, which corresponds to the corresponding propane flow rate. It’s important to note that the full-scale setting must also be considered when converting between different gases.

Of course, we do not recommend frequent gas switching unnecessarily, as this may affect the measurement accuracy of the equipment.

Featured Thermal Mass Flow Meters and Controller

Thermal mass flow meters and controllers have a wide range of applications. They offer advantages such as fast response, wide rangeability, high accuracy, and no moving parts (low maintenance costs), requiring no additional temperature or pressure compensation.

Sino-Inst has many years of experience in gas flow measurement and control. We supply various customized solutions for high-temperature, high-pressure, and low-flow-rate measurements. Please feel free to contact our application engineers if needed!