Dwyer PSX2 - Manual (Page 180)

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16.4 Q & A
I. Q & A about pipes
1. How is piping setting made when piping specifi cations are unknown ?
Flow rate can be measured within the range of the specifi cations of transmitter by enter-
ing the standard value, but the accuracy cannot be guaranteed.
* Outer diameter can be confi rmed by measuring the outside circumference.
* Thickness can be confi rmed by using a piping thickness gauge available optionally.
2. What is the effect of coating outside the piping ?
In general, when the outside wall of the piping is rusted and contaminated with deposits of
foreign objects, coating materials, etc., so the sensor is not fi tted fi rmly to the piping, measure-
ment cannot be made if there is an air gap which prevents the passage of ultrasonic waves.
In this case, the sensor should be mounted after removing the contamination.
Measurement at a point with uniform coating can be made without problems.
There are no problems with a thick coating (more than several mm), but the measurement ac-
curacy can be improved by adding the lining thickness to the coating thickness and entering it
prior to measurement.
When wrapped with jute, the jute should be removed before measurement.
3. What is the effect of scales in the piping ?
Measurement can be made even when there are scales in the piping, but the amount of reduc-
tion of the sectional area due to scaling will become an error.
Therefore, the fl ow indicated is a little larger than the actual fl ow.
When the scale thickness is known, it can be compensated by adding it to the lining thickness
and entering it for measurement. In general, the state of deposit of scales in old piping is not
uniform, and shows an uneven surface. Therefore, an accurate cross-sectional area of fl ow
passage cannot be measured.
Also, the fl ow profi le is not uniform, and an accurate measurement of fl ow cannot be expected,
strictly speaking.
II. Q & A about fl uids
1. What is homogeneous fl uid through which ultrasonic waves are transmitted ?
Municipal water can be measured over the range from raw water to clean water without prob-
lems. Sewage fl ows can be measured up to return sludge.
If the fl ow contains many air bubbles, it cannot be measured. In general, the less foreign ob-
jects (including air bubbles) the fl ow contains, the more easily can it be measured.
2. Is it possible to measure the fl ow in piping that is not full?
In horizontal piping, if the pipe is fi lled with liquid up to 2/3 of inside diameter D as shown
below, the fl ow velocity can be measured. In this case, the fl ow rate indicated is the assumed
one under fi lled pipe conditions.
Therefore, the fl ow indicated is larger than the actual fl ow.
If sludge is accumulated on the bottom of the piping, the fl ow velocity can be measured up to
1/3 of inside diameter D. In this case, the fl ow rate indicated is the assumed one under fi lled
pipe conditions without any sludge.
D
1D/3
D
2D/3
Deposit of sludge
Air bubbles
Bulletin F-9-PUX2