This installment of “Truth In Specsmanship” will examine the insertion magmeter technology as it relates to turndown. Turndown is the operating range of a given meter, based on the maximum operating velocity, divided by the turndown factor. For example, suppose a meter has a maximum operating range of 30 ft/s and a turndown of 100:1. To find the minimum operating velocity, you would take the maximum velocity (30ft/s) and divide by the turndown factor (100), which would give you 0.3ft/s So, if a meter that had a 30 ft/s maximum operating velocity and a 100:1 turndown, the meter would have an operating velocity range of 0.3 ft/s to 30 ft/s.
In some metering technologies (turbines, paddle wheels, vortex, etc) this minimum velocity is where the meter ceases to function, even though there may still be fluid moving through it. In other technologies, this may be where the accuracy starts to drastically change and therefore no accuracy is stated or given. This is the point where accuracy statements and turndown factors become relevant.
Let’s examine some accuracy specifications associated with insertion magmeter technology. A manufacturer (without naming names) states that its insertion magmeter is accurate to +/- 1.0% of rate or reading from 2.0 ft/s to 20.0 ft/s, and that at or below 2.0 ft/s the meter accuracy is +/- 0.02 ft/s. Then they state that the meter has a turndown of 200:1.
The manufacturer fails to reference any accuracy associated with the turndown capability, and here’s why:
The meter will operate from 20 ft/s down to 0.1 ft/s, which defines the turndown of 200:1. From 20 ft/s to 2 ft/s, which is a 10:1 turndown, the meter has a +/- 1.0% of rate accuracy. At or below 0.2 ft/s the meter has an accuracy of +/- 0.02 ft/s. That 0.02 doesn’t seem all that bad, however, if they were to state this as a percentage of rate accuracy ((0.02 ft/s/0.1 ft/s) x 100%) like they do in all other accuracy statements, at the meters minimum flow velocity, the meter would be accurate to +/- 20% of rate. This is obviously not a specification that a manufacturer would want to present, so they have mixed and matched their measurements in a way that presents their specifications better. This is another example of what I call “poor specsmanship.”
In the next installment of this series I will examine another technology, which will be the single turbine insertion style meter, so stay tuned.