Tokyo Keiki UFP-20, Руководство по установке и эксплуатации

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3.5 FAQ
3.5.1 Measurement method
1.1 What is ultrasonic?
Ultrasonic refers to acoustic waves or vibrations of a frequency beyond the range of human
hearing (generally above 20,000 Hz).
1.2 At what frequencies do ultrasonic flowmeters operate?
The frequency generally utilized is several 100 kHz up to several MHz.
1.3 Why are such high frequencies used?
Frequencies in the normal range (i.e. in the audible range) are apt to mix with and become lost in
the ambient noise.
1.4 Is ultrasonic harmful to humans or animals?
No. Ultrasonic is used in the medical field in place of X-rays.
1.5 How does ultrasonic measure flow?
Let us illustrate this by an example of two boats, “A” and “B”, positioned at two points on a river as
shown in Fig. 9. If boats “A” and “B” approach each other at exactly the same speed, the time,
“ tA ”, it takes for boat “A” to arrive at point “D” and the time, “ tB ”, it takes for boat “B” to arrive at
point “C” will be the same.
Now what would happen if the river is flowing at velocity “V” in the direction of the arrow as
shown? What happens is that boat “A” which is proceeding downstream will arrive at point “D”
faster than boat “B” which is traveling upstream. In other words, time “ tB ” is longer in comparison
to time “ tA ” which reflects the additional time involved in opposing the flow, and thus time, “ tA ” for
boat “A” which is traveling downstream to arrive at point “D” will be faster. This 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.
A
B
Point C
Point D
V
Fig. 3.5.1-1