Tank Level Control – For Distributor's Inventory

A distributor in Indiana used USR6 short-range ultrasonic sensors on his bulk liquid tanks to gauge the volume of product left in his tank before he needed to call for a delivery to replenish the inventory. Since this application was not in a hazardous area or used for custody transfer, this was an ideal use of the ultrasonic sensor in a simple linear tank level application. The USR6 provided a low-cost solution for continuous level indication. Configured with a 0-5 V output, they were interfaced to a PLC so that each bulk storage tank could be monitored from the office. This prevented stock out situations and added delivery costs as the planned reordering of multiple liquids could now be accomplished.

For small tanks, if the level is needed right up to the top of the tank, a stand pipe can be used so that the deadband region is effectively located above the top of the tank. The stand pipe needs to be wide enough and short enough that the sound wave does not hit the pipe wall and give a false echo. This will provide better monitoring of the small tank’s maximum capacity or full level point. This type of application already numbers in the hundreds.

Note: If the tank’s liquid is under high turbulence (due to continuous mixing, etc.), an engineered sensor can be configured to increase the signal sampling, thus avoiding a still pipe. This acts like a stilling well, allowing 15-20 seconds of averaging that cancels out level variations created by turbulence.

Avoiding false signals
All sensors have a time right after the transmit pulse where a return echo cannot be detected. This is called the deadband because the sensor is blanked from detecting a signal this close to the sensor. What often happens is that the signal is so strong that a second echo is detected but gives a false level indication. For example, a level 4” from the sensor might read as 8” (2nd echo).

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