Accuracy, repeatability, reliability, installation ease, and price – these are some of the basic criteria on which flowmeters are selected. And while it may be preferred to have the best flowmeter that money can buy, it is not always necessary to buy that most expensive one. For example, you may not need the most accurate flowmeter available, whereas, the same is not necessarily true of repeatability – especially if you use your flowmeter to control a process or a batching operation. But even in such a case, you may not need to buy the most repeatable flowmeter available.
Reliability varies as well from flowmeter to flowmeter, between different flowmeter technologies and even within similar technologies. In fact, reliability may vary within a single manufacturer’s product line. And face it, reliability costs money: The more reliable the flowmeter is expected to be, the more costly its purchase price is anticipated to be.
In some cases, you may be able to purchase several “less reliable” flowmeters for the same price that you would pay for a single unit of the most reliable flowmeter. And sometimes, but not always, installation ease of a flowmeter costs more. Keep in mind that installation ease is not correlated with accuracy or reliability; sometimes you have to give up accuracy or even reliability to get easy installation.
Applied Cost Analysis Price is a fairly good indicator of a flowmeter’s limitations. But that doesn’t mean that it is a good indicator of “how good” the flowmeter actually is. “How good” is really a function of how well matched the flowmeter is for your application.
As applications get more difficult, the number of flowmeters that work will decrease and the price of the flowmeter that can perform well in that application will increase. Conversely, as applications become easier, the number of flowmeters that can perform well will increase, whereas the price of those flowmeters will drop. This is an important point. Many flowmeter users immediately purchase the kind of flowmeter they are familiar with or the kind they believe to be the most accurate or reliable without actually thinking through the application. In many cases, you can save time and money by evaluating the application first and then selecting the flowmeter based on what will actually fit the application. Indeed, you can often plan the application so that you can use an inexpensive flowmeter if you start early enough in the design stage.
For example, if you have a conductive liquid at ambient temperature and moderate pressure and you have provided sufficient straight run both up– and downstream from the meter and you have sized the meter to produce approximately 60 percent of signal at the application’s average flow rate, then you can use any flowmeter you want with the appropriate materials of construction. Or if you have a gas flow at reasonable temperature and pressure and you’ve provided an adequate piping configuration, then you are free to use many different types of gas-flow devices.
In another example, if you are simply totalizing flow over a daily, weekly or monthly period, a variety of flowmeters will be adequate. The longer the baseline over which you are totalizing, the more accurate your total will be, regardless of what flowmeter you use. The flow totals from sewer-flow data loggers that take data every five minutes or so are known to be as good as continuous measurements totalized in the same location.
As a final example, if you are controlling a metering pump or other chemical feed device, you need only have a flowmeter in which the accuracy is better than that of the chemical feed device itself. It isn’t necessary to use a terrifically accurate flowmeter with a chlorine gas feeder, which is accurate to +/-4 percent of full scale.
So how do you figure out if you can use an inexpensive flowmeter? Simply study your application. If the application parameters can be done with the inexpensive flowmeter to the desired accuracy, repeatability, reliability and cost effectiveness, then use the cheapest flowmeter you can find.
If you really want to make sure that you are getting the least expensive flowmeter that will work adequately in your application, reverse the field. Start by setting a cost target for the flowmeter. Set a low one. Examine all the flowmeters under that cost target against your application. If none fits, raise the cost target and redo from the start. Keep raising the cost target bar slightly; doing so will permit you to work with, as well as evaluate, different flowmeter types and those more costly flowmeters that were over the bar previously. Eventually you’ll hit a flowmeter that will perform adequately. Buy that one.
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About the Author Walt Boyes is a senior member of ISA and current vice president of ISA’s Publications Department. He is a writer and consultant who has delivered numerous technical papers. He has more than 25 years in the practice of flow control.