How important is digit count when considering a digital multimeter? What about half digits? Do I need a 3-and-a-half or a 4-and-a-half-digit display?
A: The specifications important with a DMM are resolution and full-scale voltage – and you want to be sure a DMM has resolution for the measurements you make. Installers who are looking for a new digital multimeter may run across the term ‘half-digit’ in specifications literature. This half-digit is actually a display digit that’s only able to be 0 or 1 – it can’t adopt the full 0-9 digit range. Half-digits are always the left-hand digit, so at full stretch a complete 3-and-a-half-digit display is going to be 0-1999.
Most important to bear in mind is that half-digits serve the purpose of allowing low readings on measurements that nearly overload one range, making it necessary to step up a range. If you had a 3-digit display for example, it would be impossible to measure 1.042 volts on the 0.999 range – you’d wind up with an overload. Why not just switch to the 9.99 read range? Because you’ll wind up with less resolution – the instrument will read 1.04 volts. Given most digital multimeters have accuracies that are no better than plus or minus a digit, it’s possible that this reading will be 1.03 volts or even 1.05 volts.
When you’re choosing a DMM, a 3-and-a-half-digit unit will be more than adequate if you can save some money over purchasing a 4-digit instrument. Certainly, a 4-and-half-digit DMM will give you accuracy of 0.05 per cent but for most security installers the 0.5-1 per cent accuracy variation offered by a three-and-a-half-digit display are more than enough.
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