# Flow Measurement & Control

FAQ's & Application Notes

**Frequently Asked Questions:**

How do I select and develop a **DeviceNet™ network**?

Where can I find a **Gas Conversion Chart** of gas correction factors for thermal-based flow?

How do you calculate a **gas correction factor** if you flow a mixture of gases?

What **gcf** do I use if flowing a gas that is different than the gas that the MFC was calibrated for?

What do the most **common fittings** on MKS Instruments products look like?

**How do you calculate a gas correction factor if you flow a mixture of gases?**

For gas mixtures, the calculated mass flow controller (MFC) gas correction factor (GCF) is *not* simply the weighted average of each component's GCF. Instead, the GCF (relative to nitrogen) is calculated by the following equation (for n gases):

Here, a_{1} through a_{n} are the fractional flows of gas 1 through n; s_{1} through s_{n} are the molecular structure factors; d_{1} through d_{n} are the standard densities; and c_{p1} through c_{pn} are the specific temperatures. Values for s, d, and c_{p} are available for most gases; many are listed in our MFC manuals. The values for a_{1} through a_{n} will depend upon the application. (Remember that a_{1} through a_{n} must add up to 1.0.)

Molecular Structure correction factor(s):

- 1.030 for monoatomic gases
- 1.000 for diatomic gases
- .941 for triatomic gases
- .880 for polyatomic gases (four or more atoms)

*Example:*

Gas 1 = argon at 150 sccm

Gas 2 = nitrogen at 50 sccm

**GCF=1.302**

**What is an sccm?**

Sccm stands for Standard Cubic Centimeters per Minute. Where "Standard" means referenced to 0 degrees Celsius and 760 Torr.

**What gcf do I use if flowing a gas that is different than the gas that the MFC was calibrated for?**

Take the gcf of the gas that you are flowing and divide that by the gcf of the gas that the MFC was calibrated for. Example: the MFC is calibrated for 100 sccm of Argon (gcf 1.39) and you are flowing CO2 (gcf = 0.7) the resulting effective gcf would be 0.7/1.39 = 0.50. Therefore the 100 sccm Argon instrument will be flowing 50 sccm of CO2 at full scale.

**What should the gcf (gas correction factor) be if I am flowing the gas that the instrument is calibrated for?**

The gcf would be 1.

**Need help?**

Contact an Applications Specialist by **sending an email to MKS Flow Measurement & Control**, or call **978-645-5500**.