The purpose of installing a flowmeter system is to measure flow accurately and reliably. In specifying a flowmeter, issues dealing with physical properties, process parameters, electronic features and interconnections often are considered extensively, while the expected measurement quality of the installed flowmeter is virtually neglected. Relatively little emphasis typically is placed on how well the flowmeter will perform its intended purpose. Adding to the confusion are the different ways in which performance is expressed by vendors and the incomplete nature of the available information. Given these discrepancies, how can engineers and plant managers accurately compare various flowmeters, much less choose the best instrument for their application?
Accuracy StatementsThe performance of a flowmeter is quantified by its accuracy statements. The person comparing the literature on each device must understand not only which parameter is being described, but also how the statement is expressed. In flow measurement, parameters commonly are described in terms of a percentage of the (actual flow) rate, a percentage of the full scale (flow rate) or a percentage of the meter capacity. These terms are mathematically related, so it is possible to convert one to another. (Note that other terminology might be used to express these concepts. When this situation occurs, confirm exactly what the other terms mean so as to correctly relate them to the terms used in this article.)
A percentage of rate statement describes a parameter that is in error by a constant percentage of the actual measurement. In the measurement of 0 to 100 units measured with an accuracy of 1 percent of rate, the absolute error can be calculated as shown in figure 1.
In contrast, a percentage of full scale statement describes a parameter that is in error by a constant amount, regardless of the flow rate. In the measurement of 0 to 100 units measured with an accuracy of 1 percent of full scale, the absolute error can be calculated as shown in figure 2.
Note that the absolute error decreases for a percentage of rate statement, yet it remains constant for the percentage of full scale statement. Therefore, what might appear to be the same - a statement of 1 percent accuracy - actually can be different. For this reason, it is imperative to insist on completely defined statements and not simply a number.
Further confusing the issue is the percentage of meter capacity statement. This statement is similar to a percentage of full scale statement in that its absolute error is constant, but the meter capacity is related to the maximum capacity of the flowmeter. In the measurement of 0 to 100 units measured with an accuracy of 1 percent of meter capacity, where the meter can be adjusted as high as 0 to 400 units, the absolute error can be calculated as shown in figure 3.
Note that an accuracy of 1 percent of rate, 1 percent of full scale, and 1 percent of meter capacity are significantly different statements.
Calculating Rate StatementsTo compare the performance of flowmeters fairly, the same type of accuracy statement should be used for each device. The best measure of performance is usually a percentage of rate statement. Therefore, in most cases, statements should be expressed in or converted to a percentage of rate statement before using the information for comparison purposes. The conversion factors are shown in figure 4.
Using these relationships, the rate accuracy for flowmeters with percentage of full scale and percentage of meter capacity statements can be calculated. The resulting figures should provide a true indication of each flowmeter’s performance and help guide the final decision.