Most of today’s flow technologies (paddle wheel, turbines, orifice plates, variable area, pitot tube, vortex, and magnetic) are volumetric, meaning that these devices measure velocity and then calculate volume by using a known cross section area (Velocity (feet/second) x Area (Ft2) = Volumetric flow rate (ft3/s)). In all cases, the higher the velocity the easier it is for the device to measure– and measure accurately. The reasons for this vary depending on the technology, but without exception the higher the velocity the better.
In the past, with pumps on or off, one could expect a typical flow system’s velocity to range from 5 ft/s to 20 ft/s. However, with all of the environmental worries that have emerged, the days of “pump on/pump off” have practically ceased. In an effort to save water and energy, flow systems are adjusted to have a velocity range of essentially 0-20 ft/s. To make matters worse, as the energy measurements become available the next logical step is to improve the efficiency and reduce losses at the point of use. This will result in an even lower maximum flow rate as peak demand will decrease as efficiency is optimized. This unfortunately creates a sizeable problem for volumetric flow meters when considering their functionality as it relates to accuracy.
As a result, many manufacturers will try to write accuracy specifications which suggest performance that is actually not possible in order to meet these new low flow requirements. This is called poor “specsmanship.” Accuracy is usually expressed in one of three ways: as a percentage of rate or reading, as a combination of percentage of rate with fixed inaccuracy as a function of velocity, or as a percentage of the meter operating span. This article will examine the mathematics used to find each expression of accuracy, and reveal how these rates can be skewed when velocity is turned down to a minimum.
1. The most straightforward and understandable way to state true accuracy is as percentage of rate or reading, which is what we like to use. 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. A more deceptive way of representing the accuracy specification is as a combination of percentage of rate with fixed inaccuracy as a function of velocity. 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%.
3. Another less-transparent representation of accuracy is as a percentage of the meter operating span. This is where you would see an accuracy statement such as “+/-1.0% of full scale.” In this case you need to find the full operating scale of velocity, which is typically 1ft/s to 20ft/s, or a span of 19ft/s. Take that 1% and multiply it by the 19 ft/s and you get 0.19 ft/s, the accuracy variance at any given flow reading. When applied to maximum velocity of 20 ft/s, the +/- 1% is actually true ((0.19 ft/s / 20 ft/s) x 100% = +/- 0.95%.) However, when applied to minimum velocity, this statement of accuracy is untrue. ((0.19 ft/s / 1.0 ft/s) x 100% = +/- 19.0%) This would mean that when the meter reads 1 ft/s, the velocity could actually be anywhere from 0.81 to 1.19 ft/s.
With all that said, it is easy to understand how misleading accuracy specifications can waste energy and cost you money. Choosing Cadillac Meter means you can expect the most accurate meters in the industry and truly straightforward specsmanship.
Stay tuned for Truth in Specsmanship Part 2, where accuracy statements as they relate to turndown capability get even more suspicious…
