Sound Velocity; Volume Calculation - Siemens SITRANS LUT400 Operating Instructions Manual

A.4.1

Sound Velocity

The sound velocity of the transmission medium is affected by the type, temperature, and
vapor pressure of the gas or vapor present. As preset, the SITRANS LUT400 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 Siemens ultrasonic /
temperature transducer is used. If the transducer is exposed to direct sunlight, use a
sunshield or a separate TS-3 temperature sensor.
Also, if the temperature varies between the transducer face and the liquid monitored, use a
TS-3 temperature sensor in combination with an ultrasonic / temperature transducer. The
TS-3 must be installed as close to the material as possible to ensure best performance. It is
acceptable to submerge the TS-3 if necessary. Set Temperature Source (2.12.1.3.)
(Page 218) for Average of Sensors, to average the transducer and TS-3 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 Auto Sound Velocity (2.12.1.6.) (Page 219) .
The SITRANS LUT400 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
optimize 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 Sound Velocity (2.12.1.1.) (Page 218) 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
Sound Velocity (2.12.1.1.) (Page 218) may be entered directly.
A.5

Volume Calculation

The SITRANS LUT400 provides a variety of volume calculation features (see Volume (2.6.)
(Page 179)).
If the vessel does not match any of the eight preset vessel shape calculations, a Universal
Volume calculation may be used. Use the level/volume graph or chart provided by the vessel
fabricator (or create one based on the vessel dimensions). Based on the graph, choose the
Universal Volume calculation, and select the level vs. volume breakpoints to be entered (32
max). Generally, the more breakpoints entered, the greater the accuracy.
SITRANS LUT400
Operating Instructions, 07/2019, A5E33329501-AC
299