Digital multimeters are invaluable tools for anyone working with electrical installations and equipment. The multimeter is a crucial testing tool that combines, in a single unit, the ability to measure multiple electrical values including voltage, current, and resistance. Multimeters can also be used to measure more advanced functions such as frequency and capacitance. A dial on the multimeter allows the user to choose which function to measure.
The multimeter is connected to the electrical apparatus that is being tested by a pair of test leads connected to input jacks (red for positive, black for negative). Some multimeters are portable and are suitable for use by electricians working on domestic call-outs, for instance. Others are bench equipment intended for use in the laboratory or at major installations.
Multimeters are particularly useful for measuring continuity order to check whether two points are connected. This is exceptionally useful in troubleshooting broken systems or finding breaks in the wiring.
Analogue multimeters display these readings using a moving needle that indicates the value against a scale, roughly similar to a speedometer. Digital multimeters display the reading in numbers. While analogue may be more intuitive, and can still be useful where a rapidly varying value is being monitored, the digital multimeter is both more accurate and able to provide a wider range of functions. For instance, it can log data over a period of time to show how electrical power in an installation fluctuates. That can be useful if you want to check for irregularities. Many digital multimeters can also provide a graphical as well as a numerical readout.
Factors you need to consider when choosing a multimeter
One issue that a purchaser needs to consider is the resolution, that is, the smallest change in the input signal that produces a change in the output signal – or to put it more simply, the number of digits that can be displayed on the multimeter. Most commonly available digital multimeters have resolutions of plus/minus 1,999 or 19,999 (equivalent to three or four digits).
Not all multimeters can measure non-linear loads, such as are commonly found in electronic controls. ‘True RMS’ multimeters can, but ‘average responding’ multimeters are not able to cope with non-sinusoidal waveforms. Most industrial and commercial uses, such as checking variable speed motor drives or computer equipment, require true RMS to provide an accurate reading.
Some multimeters require the user to specify the measurement range; this can help lock into a specific range to take repeated readings, so it’s useful where a similar or identical task will be carried out using the multimeter. On the other hand, automatic ranging multimeters take some of the work out of setting up the instrument since they automatically choose the correct measurement range for the job in hand. They are more useful in situations where a diversity of different functions are required or where the user will be visiting different sites and carrying out different tasks from day to day.
It’s important to choose a multimeter with adequate voltage and current ratings. These need to be higher than the main voltage and current of the equipment that is being tested. For instance, a multimeter that is adequate for domestic use will not be able to cope with industrial systems and is likely to melt or combust if its power ratings are exceeded. It’s also important to allow a margin of error so that the multimeter can cope with high-voltage spikes. Only CAT III and CAT IV rated multimeters are likely to be able to cope with industrial use (three-phase) safely; CAT III is recommended for commercial lighting as well as for three-phase equipment.
Data logging and other advanced functions
Higher specification digital multimeters provide the ability to log data over time. A record of maximum and minimum values, for instance, is useful in assessing whether the variability of electrical supply is a problem – the multimeter can be left connected and will provide these values for a given period. Min/max functions can also be useful for measuring the initial flow of current when equipment is switched on since the maximum value reached will remain displayed on the screen.
A multimeter is as vital to the electrical engineer as a measuring tape is a builder. It’s a basic tool of the trade, but it’s also a highly flexible one that can provide all the data required to analyse and troubleshoot a complex electrical system.