One of the key factors typically considered when selecting a measurement device, such as a mass flow controller (MFC), is accuracy. Anyone who has researched Mass Flow Controller accuracy likely knows that there is a wide variety – both in how accuracy is stated and the device performance.
So how does anyone make sense of this variety of accuracy statements? Let’s start by looking at the three basic building blocks of accuracy:
- Calibration and Measurement Capability (commonly referred to as CMC)
- Repeatability, and
The first element, CMC, relates to the equipment and process used to test devices, while repeatability and linearity are related to the device itself.
In short, CMC is a measure of how closely the calibration method represents “truth” or absolute accuracy. No calibration equipment or method can perfectly reflect “truth”; therefore, the uncertainty associated with CMC is always >0. CMC captures both the inaccuracy of the components of the calibration system and the statistical variation during its use.
For more information on CMC, visit: http://www.isobudgets.com/know-cmc-uncertainty/
This represents the device’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 over and over again in rapid succession without changing conditions, the distribution of the flow measurement data points (in excess of the variation in the CMC) would indicate the repeatability of the MFC.
This element is needed because all Mass Flow Controllers are inherently nonlinear to some degree. To account for this, a curve-fit correction is applied to the devices. This is accomplished by collecting multiple data points during a calibration process and determining a curve fit equation. Linearity indicates how well the curve-fit correction worked.
Each of these building blocks contributes some amount of uncertainty to the accuracy of an MFC. The sum of those uncertainties equate to the device accuracy.
Accuracy = CMC + Linearity + Repeatability
All of these factors impact the accuracy that you see on the spec sheet for an MFC or other measurement device. Other factors, such as long-term stability, conversion factors, temperature/pressure coefficients and process conditions vs. calibration conditions impact the actual process accuracy.
To get an apples-to-apples comparison of MFC accuracy, it is important to understand the above elements of accuracy and how they relate to the device specifications. Some MFC manufacturers include all three elements in their accuracy statement, some do not. For assistance selecting the most accurate MFC for your application, please contact our Applications Engineering team or your local Brooks Instrument representative.