What is the difference between accuracy and repeatability? We frequently compare flow meters’ specifications to help select an appropriate meter for an application. Two typical specifications are often confused and sometimes thought to be the same.

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What is Accuracy?

What is the Difference between Accuracy and Repeatability? | Flow Meters
These two flow meter specs are often confused and thought to be the same.

Accuracy is the extent of how close a measurement is to the recognized true value. It is usually noted as a percent of full scale or span or percent of reading or rate.  For anyone involved in custody transfer (buying and selling of gas), accuracy is critical when selecting an instrument. If a user is controlling a chemical reaction, accuracy is also required.

The Sage thermal mass meter’s flow accuracy is +/- 0.5% of full scale and +/- 1% of reading.

What is Repeatability?

Repeatability is the measure of a meter’s ability to reproduce flow accurately. To put it another way, it is the measure of how repeatable a flow meter’s reading is at a precise flow rate.  One can determine the repeatability of a meter by ascertaining the reading at a specific flow rate, then discontinuing the flow, and allowing the meter to “zero.” Then by resuming the flow at the same rate, we can ascertain repeatability by inspecting the difference between the initial flow measurement and subsequent readings.

For combustion control and most common flow measurement applications, repeatability is a priority.  It is vital that when under the same set of conditions, a meter measures the same on Friday as Monday. The flow repeatability for the Sage meter is 0.2%.

Developing Trust in New Technology

Occasionally we have a client who challenges the accuracy of a thermal mass flow meter (TMFM). In one situation, a user trusted its longtime standby, a rotameter, over the thermal meter. To assure the client that the thermal meter was accurate, a K factor was entered on the meter to adjust the output and match the old rotameter’s readings. This satisfied the client since the TMFM now matched the expected flow rate and revealed that the thermal mass flow meter was accurate over the rotameter.

Most Accurate Thermal Mass Flow Meter

I hope this helps explain the difference and how these two parameters can help select the most accurate thermal mass flow meter, with the highest repeatability, for an application.

If you would like help identifying the best thermal flow meter for an application, do not hesitate to contact us or call Sage Metering at (866) 677-7243.

FAQ – What type of gases do thermal mass flow meters measure?