AQ M-Tech AB Ultrasound Controller Manual 37
minimize the risk of electric interference it is important that sources of interference like frequency
inverters should use shielded cables especially between inverter and motor.
Measuring Gel
Gel media used in chromatography systems
consists of small particles. These particles
scatters ultrasound and this causes attenuation
of the ultrasound echo. Measuring ultrasound
attenuation is a reliable method to measure the
gel concentration. Ultrasound attenuation is
proportional to gel concentration as long as gel
concentration is fairly low. The amount of
attenuation also depends on the ultrasound
frequency and gel type.
In the calibration process the internal amplifiers
are adjusted so that the relative echo strength
(Relativ Signal) becomes 100% and relative
attenuation (RelativAtt) then becomes 0dB/m,
and gelconcentration (Gelconc) becomes 0%.
Calibration should be done with liquid without
gel. Then with increasing gel concentration, the
relative echo strength (Relativ Signal) drops and
attenuation (RelativAtt) increases and (Gelconc)
increases.
The gel concentration is available on the relay outputs as an indication that gel concentration is
above or below a threshold (GEL-THRESHOLD). But gel concentration is also available as an
analogue value either via 4-20mA outputs or via Profibus.
Just as gel causes ultrasound-attenuation, unsolved dense matter and also tiny bubbles causes
attenuation. GEL-THRESHOLD should be set high enough so that wrong detection of gel is avoided
but not so high that the desired detection of gel is missed. You can look at Gelconc % or RelativAtt
or Relativ Signal with your different liquids in the container to check how much the liquid affect
attenuation. Relativ Signal can become over 100% but Gelconc % and RelativAtt can not become
negative.
Two not so well mixed iquids with different sound velocities can cause refraction of the ultrasound
and thereby attenuation. This happens for example if a liquid is suddenly switched from one type
of liquid to another.
Air (low liquid level) will cause very high attenuation and cannot be distinguished from high gel
concentration. However by measuring the presence of air with the WR-technique detection of gel
can be prevented when there is air.
In SHOW DATA all measurement data can be viewed.
Measurement technique
Gel concentration is determined by measuring ultrasound echo attenuation using echo-technique.
Air or liquid can also be measured by the same sensor using the WR technique and this is
enabled by setting Liquid Sensor SET WR THRESHOLD to other than “disabled”.
The echo is bouncing off the reflecting surface, either the opposite container wall or the pin on
GF90. Sound pulses are constantly being transmitted and the returning echo is constantly being
measured.
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