INFICON Composer Elite Gas Concentration Monitor - Manual (Page 62)

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Composer Elite Operating Manual
sound. The resonant method does not lose accuracy because of uncertainty of
guessing when the center of a low energy acoustic pulse packet precisely leaves
the Sender and reaches the Receiver. See
Figure 4-4 .
For a TOF instrument to achieve resolution equivalent to Composer Elite
(1 part in 50,000), it must achieve time measurement precision of 3 nanoseconds.
This resolution is achievable for sophisticated instruments measuring photons or
particles, but is approximately forty times better than the resolution possible on the
poorly defined 15 kHz ultrasonic wave pulse in hydrogen. One wavelength of a
15 kHz tone is 0.0856 m, almost one half the length of a 0.2 m path length
instrument.
4.1.8.1 Calculation Example
The time interval for an acoustic pulse to travel 0.2 meters in hydrogen is computed
as follows (see
equation [1] and Figure 4-4 )
NOTE: The speed of sound in hydrogen is 1284 meters/second.
[1]
In order to achieve equivalent resolution to the Composer Elite resonant technique,
the TOF instrument’s ability to measure the pulse must be 1/50,000 of the transit
time, or: 1.56 E-4 (1/50,000) = 3.12 E-9 seconds
Figure 4-4 Where does the pulse begin or end?
0.2 m
1284 m/s
---------------------- 1.56 E-4 s transit time =
Where is the pulse?
Wavelength of 15 KHz ultrasonic in H
2
(1284 m/sec x 1/15 KHz) = 0.0856 m
0.2 m
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Ultrasonic
Sender
Ultrasonic
Receiver