Appendix E: Technical References; Principles Of Operation; Blanking Distance; Tvt (Time Varying Threshold) Curves - Siemens sitrans PROBE LU Instruction Manual

Appendix E: Technical References

Principles of operation

The transducer emits a series of ultrasonic pulses: each pulse is reflected as an echo
from the material and sensed by the transducer. The echo is processed by
SITRANS Probe LU, using Siemens Milltronics' proven Sonic Intelligence techniques.
Filtering is applied to help discriminate between the true echo from the material, and
false echoes from acoustic and electrical noises and agitator blades in motion.
The time for the pulse to travel to the material and back is temperature compensated and
then converted into distance for display and mA output.

Blanking Distance

The crystal which produces the transmit pulse has to stop vibrating before it can receive
an echo. The blanking
measurement is not possible, because within that distance, the echo would return before
the vibration had ceased.
The reference point for measuring blanking distance is the transducer face. The minimum
recommended blanking value is 0.25 m (10"), but this can be increased in order to extend
the blanking.

TVT (Time Varying Threshold) curves

A TVT curve describes a threshold below which any echoes will be ignored. The default
TVT curve is used, until P837 and P838 are used to create a new 'learned TVT curve' .
Auto False-Echo Suppression
False echoes can be caused by an obstruction in the beam path (pipes, ladders, chains,
and such). Such false echoes may rise above the default TVT curve.
P838 allows you to set a distance, and P837 then instructs the Probe LU to 'learn' where
the obstructions/false echoes are within that distance. The new TVT curve is set above
the false echoes, screening them out.
1.
Also referred to as "Near Blanking".
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distance is the space in front of the transducer face where level
SITRANS Probe LU – INSTRUCTION MANUAL 7ML19985HT01