how can we measure the pressure in underground cooling
water pipeline by using pressure transmitter?what about the
tapping position?
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Answer Posted / carl_ellis
If the transmitter is not directly mounted on the
underground pipe, which is not unusual unless there is
vault access, then an impulse tube/pipe has to come up out
of the ground. The pressure measurement will be in error,
low, by the difference in elevation between the pipe's
elevation and the pressure transmitter's elevation.
Suppose the pipe is 2.1m below the grade level and the
pressure transmitter is 1.6m above grade. The elevation
difference is 3.7m.
The measurement is 3.7m or 0.362 bar low.
It is not unusual to have water supplied at pressure
between at 4-8 bar. At 4 bar pressure, an error of 0.36m
water column is a 9% error, a vary large value for an
insturment capable of 0.25% or better accuracy.
The commissioning of the unit should include the
appropriate zero correction to compensate for such a large
error.
Carl Ellis, Measure First
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