Dwyer PSX2 - Manual (Page 181)

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3. What happens when the liquid contains air bubbles ?
The transmitter is highly resistant to
entry of air bubbles in pipes with the
aid of the advanced ABM system as
shown below.
When liquid contains excessive air bubbles, no measurement can be made because of transmis-
sion failure of the ultrasonic waves. When air bubbles enter the liquid momentarily, the output
is retained by the self-check function, thereby causing no problems. Air bubbles easily enter
liquid in the following cases.
(1) Suction of air due to low liquid level of pump well
(2) Occurrence of cavitation
(3) Pressure in the piping becomes negative and air enters from piping connection.
P
P
Negative pressure
1.0 2.0 3.0 4.0 5.0
1
2
3
6
8
10
12 12
7.0
4.0
1.5
1.0
0.02
0.02 0.03
0.4
1.2
Advanced ABM system
Existing system
(Note) The flowmeter indicates volume flow containing air bubbles.
[vol.%]
Flow velocity [m/s]
* Example of measured data
Allowable value of the mixing amount of air bubbles
III Q & A about measuring conditions at locations
1. What about mounting the sensor on horizontal piping ?
The sensor should be mounted in the horizontal direc-
tion on the piping circumference to prevent the effects
of accumulated sludge (lower) and air bubbles (upper).
2. What about mounting the sensor on vertical piping ?
The sensor can be mounted on any external position of vertical piping.
The recommendable fl ow direction is upward to avoid the interference of bubbles.
3. When the length of straight piping is short and a pump, valve, orifi ce, etc. is
present, what is required for measurement ?
In general, the length of straight piping on upstream side should be longer than 10D, and that
on downstream side should be longer than 5D. When a pump, valve, orifi ce, etc. is present,
measurement should be made at a location greater than 30D away on the upstream side and
greater than 5D away on the downstream side.
4. How far can the sensor extension cord be extended ?
Extension cords can be connected and extended up to 100m.
(Special cable with BNC connector: 10m
u 2 or 50m u 2 available optionally)
Upper
Sensor Sensor
Lower
Bulletin F-9-PUX2