Wednesday, June 22, 2016

Determining Accuracy in MFCs (Mass Flow Controllers)

Mass flow controller accuracy is often difficult to pin down because there is a variety of ways MFC accuracy and performance is stated.

Since manufacturers differ on how they state accuracy, having a basic understanding of the following is helpful when specifying a Mass Flow Controller. If you are in doubt, its always best to contact an applications expert for a consultation.

To properly understand MFC accuracy there are three variables to consider:
1. CMC (which stands for Calibration and Measurement Capability)
2. Repeatability
3. Linearity
CMC has a dependency on the equipment and processes used to test MFCs. Repeatability and linearity are variables inherent to the MFC.

CMC

In basic terms, CMC is a measure of how closely the calibration method represents absolute accuracy. Since no calibration equipment or method provides absolute accuracy, there is always an uncertainty greater than zero associated with CMC. CMC represents both the inaccuracy of calibration system components and any statistical variation during its use.

Repeatability

Repeatability is the MFC’s ability to repeat a flow measurement under the same conditions in a short period of time. If an MFC was used to create a specific flow rate repeatedly and in rapid succession without changing conditions, the resulting distribution of the flow measurement data points (beyond the variation in the CMC) represents the repeatability of the MFC.

Linearity

A linearity specification is required because all Mass Flow Controllers are nonlinear to some degree. To compensate, a curve-fit correction is applied to the MFC by collecting multiple data points during a calibration process and then calculating the curve fit equation. The linearity specification is a statement to the degree the curve-fit correction compensated for the MFC’s inherent non-linearity.

Each of these three variables contribute uncertainty to the accuracy of an MFC. The sum of these uncertainties represent the MFC accuracy.

Accuracy = CMC + Linearity + Repeatability

It’s important to understand that these three variables relate to the accuracy of the MFC itself. Other variables, namely long-term stability, temperature/pressure coefficients, process conditions vs. calibration conditions, and conversion factors, relate to, and affect, the actual process accuracy.