19
SPEED OF SOUND COMPENSATION
The Atrato ultrasonic flowmeter is factory
calibrated on clean water and therefore the
window of calibration is based around the
measured time of flight of the ultrasonic
signal based on the speed of sound of water.
When a fluid is used in the meter that has a
significantly different speed of sound to that
of water, the stable signal may move outside
of the window causing the meter to read
incorrect and unstable time of flight signals. In
this scenario you can compensate by manually
adjusting the measurement window of the
meter in the advanced tab feature for Fluid
Speed of Sound Adjustment. A bar is seen in
the graph indicating whether a strong signal is
being measured. If a fluid has a faster speed of
sound the user can click the FASTER button
to adjust the measurement window a stronger
signal point. A RED graphic is a very poor
signal, ORANGE indicates a weak signal and
GREEN is a Good strong signal
The user then enters a simple scaling factor on the
User Settings tab, to accurately adjust the water
calibration to that of the fluid being measured.
Recorded flow
Used flow
0
0
1
1
1
1
2
2
2
2
3
2
0
3
3
3
4
4
4
4
4
4
Recorded
and
numerical order
2 3 0
0 2 3
2 is used
3 0 3
0 3 3
3 is used
0 3 4
0 3 4
3 is used
The seventh flow in the table above is zero. This could have been a small bubble passing
through the 710 meter which has a 1mm bore which could completely absorb the
ultrasound. The median filter completely ignores this reading and for the zero reading
returns the value of 3 from the results either side. This filter is particularly useful at low
flows where a small dip in value could drop a result below the internal cut off levels.
