Equations for Offsetting & Calibrating the Digital Panel Meter

Calibration

• Calibration is both the process and the result of correlating a measured value with the ̶0;actual̶1; value. For example, if your thermometer reads 100 degrees Celsius, but your water isn̵7;t boiling, then the measured value does not correspond to the actual value. So the first step in calibration is to generate ̶0;actual̶1; values. There are three ways to do this: a calibrated variable source, a known physical value and another calibrated meter.

Known Values

• If you use a calibrated source or a known physical value, you are putting a known actual value into your measurement system. Using the temperature example again, you could have an oven you trust and set the value to 100 degrees Celsius. On the other hand, you could boil water and depend on the fact that water boils at 100 degrees Celsius -- under normal conditions -- to set the actual value. You would then record the measured value from your panel reading.

Calibrated Meter

• If you don̵7;t have a source that you trust to provide an accurate value, you need to have another measurement device whose readings you trust. The most accurate meters can trace their calibration back to the National Institute of Standards and Technology, which maintains ultimate physical standards in the United States, but you don̵7;t need that level of ultimate traceability if you just want to be confident in your panel readings. You just need a meter you trust. For example, you can put an accurate thermometer in the oven and record those readings as actual at the same time as you read your panel meter for a measured value.

Gather the Data

• To calibrate your panel meter, you need to generate at least two pairs of actual and measured data points. For example, if you want to be sure your digital panel thermometer will give accurate readings for your cheesemaking, you want to calibrate over the range of about 20 to 75 degrees Celsius. So you̵7;d take one pair of readings around 20 degrees and one around 75 degrees. For purposes of generating your equation, record this as actual1 and measured1, and actual2 and measured2.

The Equation

• You want to end up with an equation that gives you the actual measurement as a function of your panel reading, in the form of actual = slope*measured + offset. The slope is given by: (actual2 - actual1)/(measured2 - measured1). The offset is given by (actual1 + actual2)/2 - slope*(measured1 + measured2)/2.

  For example, if your panel meter reads 23.2 at 20 degrees and reads 81.6 at 75 degrees, the slope and offset would be: slope = (75.0 - 20.0)/(81.6 - 23.2) = 0.942; offset = (75.0 + 20.0)/2 - 0.942 x (81.6 + 23.2)/2 = -1.86. So to get a calibrated reading from your panel meter, you would use the equation actual = 0.942 x measured - 1.86.