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ACCURACY - SPAN vs. RATE When comparing instrument accuracy, is it very important to identify whether an accuracy specification is a percent of rate* or a percent of span (table 1). It is also important to identify the relationship of the measurement made to the measurement desired, such as the pressure being a square relationship to flow. (*"Rate" is often referred to as "reading")
It is difficult to compare the accuracy specifications of different instruments. We can look at various types of flow measurements for an example of this difficulty. Below are listed the specifications for steam flow using a vortex shedding flow meter and using a velocity averaging flow bar sensor with a differential pressure transmitter (table 2). If we compare a 1000 lb./hr steam line over its flow range we can better understand the accuracy relations. A differential pressure type system also must consider the fact that the differential pressure increases at a rate of the square of the flow. The flow bar and differential pressure transmitter have good accuracy over the upper range but as the flow decreases the effect of the differential pressure accuracy becomes large because it is a percent of the full span pressure. The pressure changes very little for large changes of flow at the low end of the range so that the fixed error of pressure span makes very large changes in the flow reading. Finally, analyze the application and see if the accuracy meets the requirements. Many applications do not require such high accuracy at low flows. Perhaps in totalizing energy, the amount of energy used at low flow is very small compared to the amount of energy used at high flow, so the lower accuracy in the low flow range is significant. Note: according to the ISA reference manual on flow measurement, if your specifications do not state whether they are percent of span or percent of rate, you should assume they are percent of span.
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