Quiz Corner: The effect of analog input accuracy on flow measurement

Question 1: What is the approximate combined flow measurement accuracy at 10 percent flow of a flowmeter with 0.50 percent of flow rate accuracy when using an analog input module with an accuracy of 0.30 percent of span?

Question 2: What is the approximate combined accuracy using a different analog input module with 0.03 percent of span accuracy?

1. 0.50 percent of flow rate
2. 0.75 percent of flow rate
3. 1.00 percent of flow rate
4. 2.00 percent of flow rate
5. 3.00 percent of flow rate

The accuracies of these analog input modules are representative of programmable logic controllers (PLC) and distributed control systems (DCS), respectively. They may appear to be acceptable at first glance, but they are quite different. Combining the accuracies of the flowmeter with its analog input module involves squaring their accuracies, adding them and then taking the square root of that sum.

Answer 1: At 10 percent flow, the low accuracy of the input module dominates the calculated combined accuracy of 3.04 percent of flow rate, which is close to Answer E.

Answer 2: The higher accuracy input module accuracy of 0.03 percent flow rate at 10 percent flow yields a combined accuracy of 0.58 percent of flow rate, where the flowmeter accuracy dominates. The closest response is Answer A.

Additional complicating factors

These calculations show that significant degradation in accuracy can occur when using a relatively low accuracy analog input. The accuracy of analog input modules is important and should be carefully considered to maintain flow measurement system accuracy.