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Take the basic non-compressible fluid DP flow calculation:

Q = C’ x sq. root of DP

If DP is 10″ H2O, taking sq. root twice:

sq. rt. of 10 = 3.16″
sq. rt. of 3.16″ = 1.77″

The result would be a false LOW reading.

Now, let’s look at a DP of .9″:

sq. rt. of .9 = .948″
sq. rt. of .948″ = .974″

In this case, the result of taking sq. rt. twice would be a false HIGH reading.

Therefore, correct answer is C) Could be either.

Taking a square root of a fraction (below 1″ or 25.4mm H2O) obviously yields a higher number whereas sq. rt. of whole number yields a lower number.

The same relationship exists in gas and steam calculations because hw is in the numerator portion of the equation.

Question and Answer provided by:

John Good, Flow Measurement and Instrumentation Consultant 


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