Linearising to obtain volume from level measurement

February 2004 Level Measurement & Control DSP, Micros & Memory

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. The customer calibrated this sensor to produce 4 mA of current when the tank was empty, and 20 mA of current when the tank was full. This signal was to be measured by a 4 to 20 mA meter, calibrated to read in litres. Any one of Red Lion's 4 to 20 mA meters would have worked for this application, if the tank had parallel sides, but this tank was cylindrical, and laying on its side. 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 cm of depth in the middle of the tank, compared to very little volume per cm of depth near the top and bottom.)

Because this signal is non-linear, 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 2000 at 20 mA, 10 points along the input curve were entered. Where the measurement could have been out by as much as 200 litres with single-segment scaling, nine-segment scaling (or 10 points) makes it possible to reduce the error induced by the shape of the tank down to a few 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 gallon 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 250 litres, and to keep filling it until it was at 1900 litres. This was done by programming Alarm 1 as a Lo-acting alarm with a value of 250, and a hysteresis of 1650. After the unit was programmed, the customer locked out all the parameters to prevent anyone from entering inadvertent settings.

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