Flow Meter Accuracy: Demystifying Spec Sheets

Accuracy is the very first specification listed on a mass flow meter spec sheet because measuring flow is the whole point. But flow meter accuracy specs can be confusing, and that can lead to difficulty in evaluating the equivalence of options. Alicat’s standard accuracy specification is plus or minus 0.8% of reading plus 0.2% of full scale. Add them together, and they equal 1% at full scale. Other manufacturers list only a percent of full scale, or some other variation. If they list 1% of full scale, and Alicat’s equals 1% at full scale, then they’ll perform the same, right? Not so fast.

When Alicat Scientific checks accuracy of a mass flow meter, we check it against our certified in-house calibration standards, to be sure it’s within our accuracy specification. The testing compares readings at four flow rates: 100% (also known as ‘Full Scale’), 75%, 50% and 25% of the allowable flow of the meter. Here’s a two minute video on how we calibrate accuracy and what the specification means.

In a specification, percent of full scale (FS), means a percentage of the maximum rate of the device. Percent of reading means “a fraction of whatever rate is currently displayed”. Some manufacturers call this “variation from Set Point”, or S.P., but there isn’t really a set point on meters, just on controllers, which can be a little confusing if you’re reading a combined spec sheet. The main difference in your results is that percent of reading scales down as you decrease flow on a device, while percent of full scale is a fixed amount per instrument regardless of rate, which becomes comparatively more significant as you turn down flow.

While our accuracy specification is a combination of percent of full scale plus a percent of reading, some meters use only a percent of full scale parameter. How would other accuracy specifications affect your expected results? Consider these results from the 5 SLPM meter calibration in the video above:

Accuracy test result of mass flow meter

Plotting these data points, the Device Under Test’s variation from actual, alongside the Alicat Specification and adding a “Brand B” 1% of full scale accuracy spec:
Comparison of accuracy specs and device under test actual
While both specifications start at the same uncertainty (plus or minus 1% at 100% flow), they diverge rapidly. At 25% flow, the Alicat specification comes to plus or minus 1.6% of the rate of flow…. Compare this to the ‘Brand B’ specification – at 25% flow, uncertainty is plus or minus 4% of the expected flow rate—an 8% uncertainty band. And that’s before calculating in the departure from accuracy caused by conditions that don’t match the standard temperature and pressure, or simply changing gases. On brand B that can be substantial. On an Alicat mass flow meter, the effect is small to non-existent.

Maintaining accuracy throughout a flow range is important, because how often is a meter used only at its maximum rate? With Alicat’s very high turndown ratio, it is an especially important question. If your system needs better accuracy than our standard accuracy, we offer a high accuracy calibration option, which is plus or minus 0.4% of reading plus 0.2% of full scale. And because we want you to have the very best results, we offer custom ranges for under-ranged applications, which maintain the instrument’s resolution for your specific flow system. For details check out the spec sheets, available on our mass flow meters and controllers pages.

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