Dwyer PUX - Manual (Page 104)

15 - 13
(3) Flow in piping is deviated
When the straight piping is short (particularly upstream side), the flow has become skewed and
some deviation error will occur, or fluctuation of indicated value will occur when the flow 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 avail-
able and measurement may be disabled.
(5) Change in water temperature
Sound velocity of the water is calculated in real time and change in fluid temperature is compen-
sated (new sound velocity measuring system), but there is a slight error.
(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 measurement indicates volume flow containing air bubbles.
3. What about comparison with other flowmeters ?
Although thermometers and pressure gauges can easily be calibrated at a site, flowmeters are generally
very difficult to calibrate at a site.
Therefore, the meter is often used for checking other flowmeters. After checking, the
result of comparison of flowmeters should be evaluated with care while considering to the
following points.
(1) Consideration of error of each flowmeter
Evaluate the calibration error in consideration for accuracy indication (percentage in FS or percent-
age of the rate)
(2) Study data systematically, if an error is found.
Do not compare values only at 1 point of flow. Draw many samples on a graph and arrange them
systematically for clear evaluation.
(3) Thoroughly check the piping system.
If fluid flows 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 flow, the liquid
level of the storage area should be taken into consideration.
(4) Comparison of 2 different sets of flowmeters is difficult.
When there is a difference between 2 sets of flowmeters, it is difficult to judge the correct one. So,
another judgement criteria needs to be considered.
% of rate
10 30 50 70
-5
-4
-3
-2
-1
0
1
2
3
4
5
Flow error
Water temperature ( ° C)
Typical characteristics (Piping: SUS, 150A, FS: 1.3m/s)
* Example of measurement
BULLETIN F-68