Effect of Rangeability & Maximum Flow Rate on Accuracy of DP Flow meters ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Effect of Rangeability & Maximum Flow Rate on Accuracy of DP Flow meters

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If you use DP flow meters then you must read this article. It is often believed that DP flow meters have low rangeability typically 3:1. However, with continuous improvement in measurement technology especially pressure transmitters, this assertion is now a myth rather than reality. For DP flow meters, low rangeability means large errors at low flow rates since flow is proportional to square root of differential pressure. Rangeability and maximum flow rates are critical factors that should be well understood before you can accurately specify a DP flow meter or any flow meter for that matter. Having a good understanding of the two concepts can help improve accuracy and increase rangeability of a meter in a given application.
To have a thorough understanding of the effect of rangeability and maximum flow rate has on the accuracy of a DP flow meter, we need to understand the following terms;
(1) Rangeability
(2) Maximum flow
(3) Percent of flow range
Rangeability is usually defined as the ratio of maximum to minimum flow

Maximum Flow
Maximum flow can be defined in two ways:
(a) The maximum flow the flow meter can measure. This is commonly how DP flow meters manufacturers define rangeability in their product literature! Don’t be fooled and don’t fall for their trick!
(b) The maximum flow rate that occurs in a particular application. In other words, this is the maximum flow rate in the process where you require your flow meter! This is how most DP flow meters are specified and it should be the way your flow meter should be specified.
It is important you understand the difference between (a) and (b) above. Typically, the specified flow meter maximum is 2 – 3 times the maximum flow rate in a given application. So purchasing a flow meter with a rangeability in terms of maximum meter flow will result in actual meter rangeability less than the advertised meter rangeability.
To achieve the advertised meter maximum flow and rangeability in a typical application, pipe size must be reduced at the meter. This increases installation costs and permanent pressure loss, which may not be acceptable. For this reason, flow meter rangeability should be considered from application maximum, not the meter’s maximum specified flow rate.

Percent of Flow
One of the reasons we often fall for meters with high rangeability is the minimum percent of flow the meter can measure. The percent of flow is a fraction of the Maximum flow. The higher the rangeability of the flow meter, the lower the percent of maximum flow the meter can measure and for a given flow range, the more accurate the meter will be at low flow rates. Below is shown the relationship between meter rangeability and percent of maximum flow:

Relationship between Rangeability & Percent Flow Rate
As shown above, note that a flow meter with a 5:1 rangeability will measure down to 20% of maximum flow. A meter with 10:1 rangeability will measure down to 10% of maximum flow; 10% more of the flow range. A meter with 20:1 rangeability will measure down to 5% of maximum flow; only 5% more of the flow range. A meter with 40:1 rangeability will measure down to 2.5% of maximum flow. Only 2.5% of the flow range. So as rangeability increases, the difference in percent flow a meter can measure reduces. Since a meter with a high rangeability can significantly increase the cost of installation, care should be taken to specify no more rangeability than will actually be required in a particular application.

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