Determination of tank content volume by linearised ultrasonic method

May 2001 Level Measurement & Control

A company that produces various liquids had a need for an indicator that could display, in litres, the amount of solvent in a tank. Because the solvent is corrosive, an ultrasonic level sensor was used. This type of sensor uses ultrasonic signals, bounced off of the top of the liquid, to determine distance, or depth. The customer calibrated this sensor to produce a 4 mA signal when the tank was empty, and 20 mA when the tank was full. This signal was to be measured by a 4-20 mA meter, calibrated to read in litres. Any one of Red Lion's 4-20 mA meters would have worked for this application - if the tank had parallel sides. This tank was cylindrical, with its axis horizontal to the ground.

This meant that while the signal measured was linear to depth, it was not linear to litres of liquid in the tank. (There is more volume per unit of depth half way up the tank, than there was near the top and bottom.) Because this signal is nonlinear, an IMP with the lineariser option was best suited to fit this customer's needs.

Instead of selecting a reading of 0 at 4 mA, and 9000 at 20 mA, it was possible to program 10 points along the input curve. Using single-segment scaling, it was possible to be off by as many as 900 litres. Scaling the IMP with nine segments, or 10 points, permitted the error induced by the shape of the tank to be reduced to 10 or so litres. The customer had two choices when scaling the meter; Pro 1 or Pro 2. In Pro 1, the unit measures the 4-20 mA signal at each point, and pressing the 'P' key automatically enters the input signal for the keyed-in litre amount. In Pro 2, both the gallon amounts and the signal level are keyed in, so these values have to be known prior to scaling the unit. This customer had the manufacturer of the tank furnish the values for him, so he opted to use Pro 2. The customer also wanted an alarm output to turn on a pump when the tank was almost empty. Alarm 1 was programmed to begin filling the tank when the level fell to 1130 litres, and to keep filling it until it was at 8600. This was done by programming Alarm 1 as a Lo-acting alarm with a value of 1130, and a hysteresis of 1650. After the unit was programmed, the customer locked out all the parameters to prevent anyone from entering inadvertent settings. Red Lion is locally represented by Micropower Instrumentation.

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