Dwyer PSX2 - Manual (Page 183)

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(3) Flow in piping is deviated
When the straight piping is short (particularly upstream side), the fl ow has become skewed
and some deviation error will occur, or fl uctuation of indicated value will occur when the
fl ow is swirling.
(4) Inside diameter different from set value due to deposits of scales inside the piping
The error is the same as noted in (1). If scales are badly deposited, receiving waves are not
available and measurement may be disabled.
(5) Change in water temperature
Sound velocity of the water is calculated in real time and change in fl uid temperature is
compensated (new sound velocity measuring system), but there is a slight error.
% of rate
10 30 50 70
н5
н4
н3
н2
н1
0
1
2
3
4
5
Flow error
Water temperature (°C)
Typical characteristics (Piping: SUS, 150A, FS: 1.3m/s)
* Example of measurement
(6) Weak received wave due to improper mounting condition and piping condition
Measurement may be possible. But, if received wave is weak, it may result in a large error
due to the effect of external noise.
(7) Output when the liquid contains air bubbles
When the amount of air bubbles contained in the liquid is lower than the allowable value
indicated before, the unit indicates volume fl ow containing air bubbles.
3. What about comparison with other fl owmeters ?
Although thermometers and pressure gauges can easily be calibrated at a site, fl owmeters are
generally very diffi cult to calibrate at a site.
Therefore, the flowmeter transmitter unit is often used for checking other fl owmeters. After checking,
the result of comparison of fl owmeters should be evaluated with care while considering to the
following points.
(1) Consideration of error of each fl owmeter
Evaluate the calibration error in consideration for accuracy indication (percentage in FS or
percentage of the rate)
(2) Study data systematically, if an error is found.
Do not compare values only at 1 point of fl ow. Draw many samples on a graph and ar-
range them systematically for clear evaluation.
(3) Thoroughly check the piping system.
If fl uid fl ows into or out of a branch pipe in the middle of a piping system, the comparison
data of each side of such a pipe-junction may not match each other.
When there is storage in the middle of piping system and it becomes a buffer for the fl ow,
the liquid level of the storage area should be taken into consideration.
(4) Comparison of 2 different sets of fl owmeters is diffi cult.
When there is a difference between 2 sets of fl owmeters, it is diffi cult to judge the correct
one. So, another judgement criteria needs to be considered.
Bulletin F-9-PUX2