20 Ultrasound Controller Manual
Level Switch Type and Technique
There are two different measuring techniques: Echo and WR (Wall Reverberate). The different
Level Switches are optimized for each technique. Level Switch RS works well with the WR-
technique and do not work well with Echo-technique. Level Switch MK has a soft silicone surface
adapting to the container surface and works well with Echo-technique and does not work well with
the WR-technique. Two Level Switch MK also works well with the dual-technique for improved
detection when the container is small. Level Switch KS and LS works well with the Echo-
technique and on plastic containers they also works well with the WR-technique.
Echo Technique
The echo-technique uses echo from a reflecting surface inside the container (usually the opposite
container wall) to determine if there is liquid or not inside. All types of Level Switch except Level
Switch RS can be used.
The echo-technique transmits short sounds and then measures the echo bouncing on the
opposite reflecting surface. With liquid inside, there is an echo and with no liquid there is no echo.
If the liquid attenuates the sound too much the echo may disappear. Small bubbles and particles
in the liquid can cause attenuation and unmixed liquids having different sound velocities can
cause refraction of the sound and thereby the echo disappear resulting in wrong indication of low
level. It is important that the sound-beam from the sensor is reflected, back to the sensor and not
diverted in the wrong direction.
It is desirable that the ultrasound passes as easy as possible through the container wall. How well
it passes depend on the wall material and thickness. Steel or glass wall should be in the range
1,2mm - 15 mm and plastic wall <15mm (PP<10mm). Plastic with fiberglass can be troublesome.
Testing on the actual container is recommended. Any welding or other unevenness in the wall
should be avoided as it can refract (bend) the sound-beam in an unwanted direction.
The ultrasound beam behaves similar to a light-beam, the direction of the echo depend on from
what angle it hits the reflecting surface. Obstructing object in the path between sensor and the
reflecting surface should be avoided. The sound beam is approximately 1cm in diameter. A tube
in the center of the container can give an echo but it is weak. In case there are obstructing objects
inside the container, perhaps the Level Switch RS with the WR-technique is a better choice.
On small containers, disturbing background echoes becomes stronger in relation to the echo.
Level Switch LS and KS should therefore not be used on metal and glass containers smaller than
44mm diameter.
WR Technique
The WR technique measures vibrations in the container wall to determine if there is liquid or not
behind the wall. Level Switch RS should be used. However, on plastic walls Level Switch KS or
LS can also be used.
The WR-technique transmits sound and then measures the remaining vibrations in the container
wall. Vibrations decay more quickly with liquid inside due to the dampening effect of the liquid.
The WR-technique works with most liquids since the sound does not have to travel through the
liquid. But the WR-technique is dependent on the material and thickness of the wall. Steel or glass
wall 1,2mm - 15 mm or plastic wall < 15mm (PP<10mm) should be ok. Fiberglass-plastic would be
troublesome and testing on a container is recommended. WR technique can have quite high
temperature dependence, depending on wall material, and should not be used if temperature
differ more than 15ºC between temperatures during operation and calibration.
When using the WR-technique on plastic walls Level Switch KS can be used instead of Level
Switch RS.
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