Iii q & a about measuring conditions at locations, Bulletin f-68 – Dwyer PUX User Manual
Page 102
15 - 11
3.
What happens when the liquid contains air bubbles ?
The PUX unit 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 transmission
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 direction on
the piping circumference to prevent the effects of accumu-
lated 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 flow direction is upward to avoid the interference of bubbles.
3.
When the length of straight piping is short and a pump, valve, orifice, 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 down-
stream side should be longer than 5D. When a pump, valve, orifice, 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
×
2 or 50m
×
2 available optionally)
Upper
Sensor
Sensor
Lower
BULLETIN F-68