David W. Spitzer, P.E.

Despite claims that may seem to the contrary, not all flowmeters are equal. In particular, not all flowmeters of the same technology are equal. Further, not all flowmeter models of the same technology made by the same manufacturer are equal. There are both subjective and objective differences between flowmeters. Subjective differences include ease of configuration, poor factory service, familiarity with the salesperson, and the like. Objective differences include flowmeter performance, flowmeter geometry issues, cost, and the like.

Flowmeters are purchased and installed to produce data that provides measurements of the flowrate of the fluid over time. Flowmeter performance specifications describe how well the data reflects the actual flowrate. Comparing the flowmeter performance specifications of different flowmeters can be like comparing apples and oranges.

Flowmeter performance specifications are typically expressed as a percentage of flowrate, a percentage of full scale, a percentage of meter capacity, or a percentage of calibrated span. These terms mean completely different things and cannot be directly compared with one another. For example, a flowmeter with 1 percent of rate performance will measure 100 +/- 1, 50 +/- 0.5, 25 +/- 0.25, and 10 +/- 0.1 flow units. Assuming a range of zero to 100 flow units, a flowmeter with 1 percent of full-scale performance will measure 100 +/- 1, 50 +/- 1, 25 +/- 1, and 10 +/- 1 flow units. Note that the errors are completely different, even though both performances are 1 percent. Similar calculations can be performed for the percentage of meter capacity and the percentage of calibrated span.

Fortunately, with sufficient information, these specifications can be converted from one to another for comparison purposes. However, for proper analysis, all of the performance specifications should be converted to the same type of error for comparison purposes. The percentage of flowrate error is almost always used for comparisons between flowmeters. In other words, if a performance specification is expressed as a percentage of any other value, it is converted to a percentage of flowrate performance to provide a fair basis for comparison.

For example, at 100, 50, 25, and 10 flow units the first flowmeter in the previous example exhibits 1, 1, 1, and 1 percent of rate performance. At 100, 50, 25 and 10 flow units the second flowmeter exhibits 1, 2, 4, and 10 percent of rate performance. Putting the error into common units allows a valid comparison of apples and apples. It can be clearly seen that the performance of the first flowmeter is superior to that of the second flowmeter (all else being equal). Note that the percentage of rate statement is different at different flowrates. This means one percentage of flowrate number does not necessarily represent the performance of the flowmeter.

These comparisons are not always this easy. Some flowmeters have different performance statements when calibrated and operated at different flowrates. Other flowmeters have performance statements that are the sum of two or more types of statements. Each should be carefully converted to a percentage of flowrate statement for comparison purposes. In addition, some flowmeters have more than one component that should be considered in the calculation. An example of this type of flowmeter is an orifice plate and differential pressure transmitter, where each component has a different performance statement and different statement type.

Setting up the calculations to compare performance statements can involve a significant amount of time, even when using a spreadsheet. Having done this for Coriolis mass flowmeters, differential pressure flow transmitters, magnetic flowmeters, ultrasonic flowmeters, and vortex shedding flowmeters, I can guarantee that you will find significant differences between flowmeters. Not all flowmeters are equal and, further, not all flowmeters within a given technology are equal.

About the Author
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 25 years of experience in specifying, building, installing, start-up, and troubleshooting process control instrumentation. He has developed and taught seminars for almost 20 years and is a member of ISA and belongs to ASME, MFC, and ISO TC30 committees. Mr. Spitzer has published a number of books concerning the application and use of fluid handling technology, including the popular The Consumer Guide to… series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at spitzer@spitzerandboyes.com or 845 623-1830.

For More Information: www.spitzerandboyes.com