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. If Operation (P001) is set
for any value other than "tripper", the result (round trip distance) is divided by 2.
Distance =
Sound Velocity x Time
2
The Reading Value displayed is the result of performing any additional modification to the calculated distance
(Operation P001, Units P-005, Volume Conversion, P-050 to P-054, Reading Value, P060 to P063.)
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 DPL+ 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(s) are exposed to direct sunlight, TS-3 temperature sensor(s) should be used.
Also, if the temperature varies between the transducer face and the object monitored, a TS-3 temperature
sensor, mounted near the material surface (solids) or submerged (liquids), should be used in combination with
the ultrasonic/temperature transducer. Then when Temp Source, (P660) is set for "both", the transducer and
TS-3 temperature measurements are averaged.
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 DPL+ 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, frequent
Sound Velocity Calibrations may be required to maintain optimum measurement accuracy.
Sound Velocity calibration frequency may be determined with experience. If the sound velocity in two vessels
is always similar, future calibrations may be performed on one of the vessels and the resultant Velocity (P653)
entered directly for the other vessel.
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.
SCANNING
When echo processing is complete, (if more than 1 vessel is monitored) the scanning relay changes state to
supply the transmit pulse to the transducer of the other vessel after the Scan Delay (P727).
Scan Delay is automatically set by Measurement Response (P003). When high speed scanning is required
(sometimes the case for equipment position monitoring), the Scan Delay may be reduced. Reduce the Scan
Delay only as required, otherwise premature scanning relay fatigue could occur.
PL-508 78
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