Offsetting & Calibrating Your Digital Panel Meter

Whenever you’re dealing with some type of physical process — for example, you’ve installed a solar panel or you’re filtering the seawater in your aquarium — you will most likely want to monitor that process in order to monitor what’s happening and troubleshoot any issues that may arise. A digital panel meter enables you to measure current, temperature, salinity, and much more. Because you need to ensure the readings are accurate, you will need to calibrate your panel meter.

If you have a digital temperature display or thermometer that reads 100°C but the water isn’t boiling, the measurement doesn’t match the actual value, which is to say the temperature display or thermometer isn’t properly calibrated. Calibration is the process and the result of correlating your measurements with the actual value. The first step in calibrating your panel meter is to determine the actual value, which can be accomplished in three ways: using a calibrated variable source, an established physical value, or a panel meter that’s already been calibrated.

When you use a calibrated source or an established physical value you are adding a known value to your monitoring system. To use our boiling water example again, you might have a calibrated stove and set the temperature to 100°C. You could also set the actual value by boiling water and relying on the fact that water boils at 100°C in normal circumstances and then record the measured value displayed by your digital panel meter.

You won’t always have a source to provide an accurate value, in which case you’ll need another device to provide reliable measurements. You need to be confidant in the readings provided by your panel meter, so you’ll need a panel meter whose readings you can trust.

You need to generate two or more pairs of actual and measured data points to calibrate your digital panel meter. Using the previous example, when you want to ensure your digital temperature display is providing accurate measurements, you’ll need to calibrate for a range of roughly 20° to 75°C, meaning you would take one pair of readings at 20° and another at 75°. Record this as actual1, measured1, actual2, and measured2 so you can generate your equation.

Proper calibration will give you an equation with the actual measurement as a function of your panel meter reading: actual = slope x measured + offset. You calculate the slope with the equation (actual2 – actual1) / (measured2 – measured1). Use (actual1 + actual2) / 2 – slope x (measured1 + measured2) / 2 to find the offset. If your panel meter reads 23.2 at 20°C and 81.6 at 75°C the slope and offset would be 0.942 and -1.86 respectively. To calibrate the reading of your panel meter you would then use the equation actual = 0.942 x measured – 1.86.

George

George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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