Many mass flow controller users have experienced costly problems using MFCs that were selected by the so called generic MFC approach.

A "generic" MFC is an MFC calibrated and labeled for one gas but used to control another gas. As an example, assume you want to flow 30 sccm of Boron Trichloride (BCl3) through a 100 sccm Nitrogen (N2) MFC. If the manufacturer’s conversion factor for BCl3 with respect to N2 is 0.41, this corresponds to an equivalent Nitrogen flow rate of 73.17 sccm, +/- 4 percent. (This error can be 8% or higher for other gases.)

Using a conversion factor does not take into consideration the problems associated with:

• Gas density
• Correct elastomer
• Controller response

Gas Density

Gas Density is one of the factors that determines how the MFC control valve should be adjusted during the MFC assembly. Density, discussed in another paper, also effects the sizing of the mass flow controller valve orifice. Both of these factors will determine how well the controller operates over its entire control range.

In the above example, the relative density of N2 is 1.25 g/l @ 0 degree C. The relative density of BCl3 is 5.227 g/l 0 degree C. Thus, BCl3 is approximately 4 times more dense than N2. BCl3 gas will require a larger opening inside the control valve to accommodate its full control range. Therefore, the control valve on a BCl3 MFC should be adjusted to provide a larger opening than would the control valve on a N2 MFC with similar differential pressures.

Another density problem occurs when N2 gas is used to check the calibration of MFCs calibrated for very low density gases such as Hydrogen and Helium. The relative densities of Hydrogen and Helium @ 0 degree C. are 0.0899 g/l and 0.1786 g/l, respectively. Nitrogen is approximately 14 times more dense than Hydrogen and 7 times more dense than Helium. If the control valves are properly adjusted for controlling these two relatively low density gases, the MFCs often will not allow full flow of the higher density Nitrogen. It will appear that the MFCs are defective because they will not flow the expected amount of gas, when actually the MFCs are adjusted correctly for Hydrogen or Helium.

In the above examples both situations are analogous to attempting to flow honey, a high density fluid, and alcohol, a low density fluid, through the same small orifice!

The density problems are more pronounced in thermal valve type mass flow controllers. The electro-mechanical type valves used with newer type mass flow controllers can also exhibit problems when the MFC is used with a gas which is chemically very different from the gas for which it is adjusted and calibrated.

Correct Elastomer

Viton® is typically the recommended elastomer for non-reactive gas service. Reactive gases require a more durable elastomer, such as Kalrez®, for safe and proper operation. If a mass flow controller equipped with seals made of Viton® is used to flow a reactive gas, very serious problems can occur.

In field tests of BCl3 MFCs equipped with elastomers made of Viton® and heated to 35 degrees C., the average lifetime of the MFC was approximately six to ten weeks. The Viton® elastomers would harden and no longer seal correctly. This produced reactive gas leaks resulting in:

• Safety problems
• Manufacturing delays
• Destruction of valuable equipment
• Production of scrap product
• Increased repair costs

MFCs equipped with Kalrez®, a more expensive compound, had an average lifetime between six to twelve months under the same conditions.

Controller Response

The response of a mass flow controller is dependent upon:

• Gas properties
• Valves & electrical components
• Mechanical tolerances
• Flow rates

Using the generic approach prevents proper response adjustments being made for the unknown actual gas used.

Conclusion

Since we started servicing mass flow controllers (in 1980) we have found that the above problems continually plague users who attempt to employ the "generic" MFC approach.

If you must use generic mass flow controllers, at the very least, inform your service organization of the actual gas you will flow through the MFC. This will allow them to properly adjust the MFC for the actual gas used, thus, reducing your mass flow controller related problems. This information is requested on our Repair Tags.

Our recommendation is: Do not use generic mass flow controllers.

Viton® and Kalrez® are registered trademarks of DuPont Dow Elastomers.