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. Assume the manufacturer’s gas conversion factor (CF) for BCl3 with respect to N2 is 0.41. This corresponds to an equivalent Nitrogen flow rate of 73.17 sccm
30 sccm BCl3 ¸ 0.41 N2/BCl3= 73.17 sccm N2
Note: The error introduced by using a CF can as high as 10% per the manufacturer’s CF list.
Using a conversion factor does not take into consideration the problems associated with:
- Gas density
- Correct elastomer seal material
- Controller response
- Mounting Position
Gas Density is one of the factors that determine how the MFC control valve should be adjusted during the MFC assembly. Density, also discussed in the pressure tutorial, also affects the sizing of the mass flow controller valve orifice depending on the model. 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 degrees C. Thus, BCl3 is approximately 4 times denser 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 denser than Hydrogen and 7 times denser 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!
Correct Elastomer Seal
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 some reactive gases, very serious problems can occur. Other gases, such as Ammonia, require a different seal altogether and most manufacturers recommend Neoprene.
In tests of BCl3 MFCs equipped with seals made of Viton® and heated to 35 degrees C., the average lifetime of the MFC was approximately six to ten weeks. The Viton® seals would harden and lose seal integrity.
MFCs equipped with Kalrez®, a more expensive compound, had an average lifetime between six to twelve months under the same conditions.
See tutorial on Pressure.
See the tutorial on mounting position.
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.
Viton® and Kalrez® are registered trademarks of DuPont Dow Elastomers.