Tokyo Keiki UFL-30, Installation & Operation Manual

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III- 46
time difference is proportional to the magnitude of the velocity of the flow and this principle is
utilized to measure flow velocities. This can be expressed by the formula:
tB - tA = t
where
t is the time difference. This time difference can be calculated precisely with a
clock pulse generated by a microcomputer to accurately measure flow.
As flow velocity
‘V’ can be determined by measuring t
､
you can calculate the volume of flow
through a pipe or open channel by multiplying t by the cross sectional area of the pipe or open
channel. In such calculations, it is necessary to employ a flow volume compensation coefficient
with the measured flow velocity
‘V’ in order to obtain average flow velocity.
1.6 Is the time difference method the only method used in ultrasonic flow measurement?
Presently there are three measurement methods in practical use.
(1) Frequency difference method
(2) Time difference method
(3) Doppler method
Methods (1) and (2) are mainly used for measurement of relatively clean water. This is because
fluctuations in propagation level of ultrasonic signals (i.e. sensitivity of ultrasonic transmissions and
reception) become problematic with dirty water which reduces signal strength.
Measurement method (3) involves detection of frequency shifts (changes) in acoustic signals. As
changes in signal propagation level is not directly a factor, this method is suitable for measurement
of dirty water such as sewage.
1.7 Does acoustic velocity vary with water temperature?
Yes. It is thus important to devise means to overcome the affect of acoustic velocity changes
caused by changes in water temperature.
1.8 Can measurement errors be caused by changes in water temperature?
The flow volume calculation formula employed by the flowmeter’s CPU automatically compensates
for the effect of water temperature changes. Therefore for all practical purposes, there are no
problems with errors caused by water temperature changes.