The majority of today’s flow technologies are volumetric, meaning the devices measure velocity and calculate volume by using a pipe’s inner cross-sectional area (Velocity (feet/second) x Area (Ft2) = Volumetric flow rate (ft3/s)). In every case without exception, the higher the velocity the more accurate the device’s measurements will be.
Traditionally, with pumps on or off, a typical flow system’s velocity would be expected to range from 5 ft/s to 20 ft/s. However, with all the environmental concerns that have risen, the days of “pump on/pump off” are nearly over. In an effort to better manage consumption, many flow systems have been adjusted to a velocity range of essentially 0–20 ft/s. As these new flow measurements become available, the next step is to optimize efficiency and reduce losses at the point of use, resulting in an even lower flowrate as peak demand decreases. All of this creates a definite problem for volumetric flowmeters when you consider their functionality as it relates to accuracy.
Because of all these industry changes, and in order to meet new low flow requirements, many manufacturers have written accuracy specifications suggesting performance that is not actually possible. We’ll call this poor “specsmanship.” Flowmeter accuracy is generally expressed in one of three ways: as a percentage of rate or reading, as a combination of percentage of rate and a fixed inaccuracy as a function of velocity, or as a percentage of the meter’s operating range. Cadillac Meter’s blog post about “Truth In Specsmanship” discusses the mathematics involved in each expression of accuracy, and reveals how these fudged ratings become unfeasible when velocity is turned down to a minimum. The analysis of two different expressions of accuracy are detailed below. The third and most deceptive specification can be found in Truth In Specsmanship, Part 1.
“1. The most straightforward and understandable way to state true accuracy is as percentage of rate or reading. This means that the accuracy statement is tied directly to what the meter reads (gal/min) or measures (ft/s). For example, let’s say a meter has an accuracy of +/-1.0% of rate or reading. This would mean that if the system is flowing at a maximum of 20 ft/s, you can calculate the likely reading error: 20 x 0.01 = +/- 0.2 ft/s. This would mean that when the meter measures a velocity of 20 ft/s, it could be anywhere from 19.8 to 20.2 ft/s. Adjusting those calculations for minimum velocity (1 x 0.01 = +/- 0.01 ft/s) would mean that when the meter measures 1 ft/s, it could actually be anywhere from 0.99 to 1.01 ft/s.
2. Accuracy can also be expressed as a combination of percentage of rate with fixed inaccuracy as a function of velocity, which is a bit more deceptive. A manufacturer may claim +/- 1% of rate in combination with +/- 1mm/s. That 1 millimeter may not seem like a lot until you do the math. First we must convert mm/s to ft/s. (1 mm/s x 1 in/25.4 mm x 1ft/12in) = 0.0033 ft/s. At a higher velocity this factor has much less influence (20 ft/s x 0.01) = 0.2 ft/s + 0.0033 ft/s = 0.2033 ft/s. accuracy is (0.2033 ft/s/ 20 ft/s) x 100% = +/- 1.02%. However, at minimum velocity this statement is less true. +/- 0.01 ft/s + 0.0033 = 0.0133. Accuracy would be (0.0133 ft/s / 1.0 ft/s) x 100% = +/- 1.33%. This means that when the meter reads 1 ft/s velocity, it could actually be anywhere from 0.9867 to 1.0133 ft/s. This is even more of a concern with the low end velocities we see with VFD pumping systems, which can easily be required to measure in the 0.05 ft/s range, which can turn accuracy rates up to +/- 7.6%.”
This blog post was written for Flow Control by Cadillac Meter, www.cadillacmeter.com, which offers engineered, off-the-shelf and customized solutions for today’s flow and energy measurement challenges.