Dwyer PUX - Manual (Page 101)

15 - 10
D
1D/3
D
2D/3
Air bubbles
Deposit of sludge
15.4 Q & A
I. Q & A about pipes
1. How is piping setting made when piping specifications are unknown ?
Flow rate can be measured within the range of the specifications by entering the
standard value, but the accuracy cannot be guaranteed.
* Outer diameter can be confirmed by measuring the outside circumference.
* Thickness can be confirmed by using a piping thickness gauge.
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 fitted firmly to the piping, measurement 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 accuracy can
be improved by adding the lining thickness to the coating thickness and entering it prior to measure-
ment.
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 reduction of the
sectional area due to scaling will become an error.
Therefore, the flow indicated is a little larger than the actual flow.
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 flow passage cannot be mea-
sured.
Also, the flow profile is not uniform, and an accurate measurement of flow cannot be expected,
strictly speaking.
II. Q & A about fluids
1. What is homogenious fluid through which ultrasonic waves are transmitted ?
Municipal water can be measured over the range from raw water to clean water without problems.
Sewage flows can be measured up to return sludge.
If the flow contains many air bubbles, it cannot be measured. In general, the less foreign objects
(including air bubbles) the flow contains, the more easily can it be measured.
2. Is it possible to measure the flow in piping that is not full?
In horizontal piping, if the pipe is filled with liquid up to 2/3 of inside diameter D as shown below, the
flow velocity can be measured. In this case, the flow rate indicated is the assumed one under filled pipe
conditions.
Therefore, the flow indicated is larger than the actual flow.
If sludge is accumulated on the bottom of the piping, the flow velocity can be measured up to 1/3 of
inside diameter D. In this case, the flow rate indicated is the assumed one under filled pipe conditions
without any sludge.
BULLETIN F-68