Calibration Tutorial

Voltage is measured in Volts, Current in Amps and Resistance in Ohms.

Examples are:

By using this method of abbreviation, large numbers are removed which makes the figures easier to read.

Multimeters have various functions. The most common will measure A.C. and D.C. voltage and current plus resistance.

Most multimeters have separate input terminals for a high current range e.g. 2A and 10A. Some even have separate terminals for all current ranges. Many have internal fuses to protect the current ranges.

Continuity or diode checking may also be included on some multimeters. This may just sound a beep or indicate a value on the display. A capacitance range might also be included which uses a charge/discharge technique.

A.C. is measured in one of two ways. Some meters may measure true R.M.S. (Root Mean Square) and thus the average value. The true R.M.S. value is the same as a D.C. value which dissipates the same amount of power in a fixed-value resistor. Many low cost multimeters take an average of an A.C. input and apply a scaling factor to give the R.M.S. value. It should be noted that measurement errors occur with this technique if the signal waveforms are irregular or distorted.

A good check for true R.M.S. meters is to input a sine wave of a known value and see if the reading is the same with a square wave. A multimeter using an averaging method shows different results.

When calibrating A.C. ranges, take care with the test frequency. Occasionally the display will “beat” with the mains frequency if the frequency of the test has been set to be the same as the mains. If the reading tends to wander up and down, change the test frequency. It is best to choose frequencies which are not multiples of your mains frequencies to avoid potential errors due to “beating”.

Many multimeters are prone to interference when measuring high resistance values (>1MOhm) to their full spec accuracy. The cause is electrical pick up on the test leads. It is important to keep the leads (between the calibrator and multimeter) as short as possible and screen them. If it is not possible to eliminate the noise completely, take a mean value as the result e.g.:

If for a 1MOhms test the reading wanders between 0.997MOhms and 1.005M i.e. wanders by 0.008MOhms, the centre reading would therefore be 1.001MOhms.