Gas pump compensates for temperature

Greensboro, NC--While fuel delivered to Canadian gas stations is routinely temperature-compensated to 15C, gas dispensed at the pump is not. Ten gallons pumped to a car can actually represent slightly more--or slightly less--than ten gallons, when corrected to 15C.

Automatic Temperature Compensation (ATC), built into current and future generations of The Advantage(R) fuel pumps and dispensers, solves this problem. Based on a ratiometric approach that eliminates the effect of long-term measurement drifts, the system improves a gas station's ability to track inventory and detect leaks. Long-term measurement accuracy exceeding 0.1C provides the basis for the system's high performance.

Operation involves a temperature acquisition board and the ATC controller board. The temperature acquisition board generates a free-running stream of pulses, derived from the input of as many as eight resistance/temperature detectors (RTDs) and two reference resistors. Pulse duration relates directly to the measured resistances. The RTDs, which exhibit a precise temperature-to-resistance relationship, continually monitor fuel temperature.

Using a stable clock, the ATC controller board re-ceives the pulse stream and determines pulse width. Software converts RTD re-sistance to temperature, and then determines a correction factor using an interpolation based on the RTD-generated pulse widths and those of the two reference resistors.

"Conventional fuel metering systems also translate flow to pulses per gallon," says Howard Myers, a design engineer at Gilbarco, Inc. "ATC, however, applies the correction factor to regenerate the compensated number of volume pulses. The system, based on temperature, adds to or subtracts from the normal pulser count."

Myers' ATC collects accurate remote resistance and temperature readings because there are references for interpretation. "The real beauty of the reference approach is the absence of analog adjustments, and the elimination of essentially all of the measurement errors," he explains.

Additional details...Contact Gilbarco, Inc., 7300 W. Friendly Ave., Greensboro, NC 27402, (910) 547-5000.