DISTANCE CALCULATION
To calculate the transducer to material level (object) distance, the
transmission
medium
(atmosphere) sound velocity (P653) is multiplied by the acoustic transmission
to reception time period. This result is divided by 2 to calculate the "one way" distance.
Distance =
Sound Velocity x Time
2
The Reading displayed is the result of performing any additional modification to the
calculated distance (as determined by Operation P001, Units P005, Volume
Conversion, P050 to P054, Reading, P060 to P063, OCM, P600 to P611, and/or
Totalizer P622 to P633 parameters).
SOUND VELOCITY
The sound velocity of the transmission medium is affected by the type, temperature,
and vapour pressure of the gas or vapour present. As preset, the HYDRO+ assumes
the vessel atmosphere is air at 20 °C (68 °F). Unless altered, the sound velocity used
for the distance calculation is 344.1 m / s (1129 ft / s).
Variable air temperature is automatically compensated when a Milltronics ultrasonic /
temperature transducer is used. If the transducer is exposed to direct sunlight, use a
sunshield or better yet, a seperate TS-3 temperature sensor.
Also, if the temperature varies between the transducer face and the liquid monitored,
use a TS-3 temperature sensor, (submerged in the liquid) in combination with an
ultrasonic / temperature transducer. Set Temp Source (P660) for "both", to average
the transducer and TS-3 temperature measurements.
Atmosphere composition other than air can pose a challenge for ultrasonic level
measurement. However, excellent results may be obtained if the atmosphere is
homogeneous
(well mixed), at a fixed temperature, and consistent vapour pressure,
by performing a Sound Velocity Calibration (P651).
The HYDRO+ automatic temperature compensation is based on the sound velocity /
temperature characteristics of "air" and may not be suitable for the atmosphere
present. If the atmosphere temperature is variable, perform frequent Sound Velocity
Calibrations to maintain optimum measurement accuracy.
Sound Velocity calibration frequency may be determined with experience. If the sound
velocity in two or more vessels is always similar, future calibrations may be performed
on one vessel and the resultant Velocity (P653) entered directly for the other vessel(s).
If the sound velocity of a vessel atmosphere is found to be repeatable at specific
temperatures, a chart or curve may be developed. Then, rather than performing a
Sound Velocity Calibration each time the vessel temperature changes significantly,
the anticipated Velocity (P-653) may be entered directly.
PL-507 150
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