BUYING A DIGITAL MULTIMETER (DMM)
There’s so many digital multi meters out there. Which one is right for you depends on what you’re going to use it for, and how accurate you need it to be. While you might think “aren’t they all the same?”, there’s actually many subtle differences between them. We’re not going to worry too much about individual features for meters here, as you can research what these are yourself. And likely if you don’t know what HFe testing Dwell Angles are, you’re probably not going to require those features. So outside of just what it measures, what makes these meters different?
All meters are only as accurate as the electrical circuits within them allow. Essentially all meters transform current, capacitance, resistances, etc – into a measurable voltage that can be interpreted by the multi meter and displayed as a digital readout. The circuits that do this conversion are prone to variation and tolerance as all circuits are. The accuracy of all meters is outlined for each one. If all you’re measuring is batteries or crude “dead or alive” voltages, then virtually any meter will serve you well. If you’re measuring discreet components and need a high degree of accuracy, then this tolerance could undermine the work you are doing. Basic DC voltage accuracy for instance, can vary between almost 1% on a hobbyist meter, but is more like 0.06% on our professional grade multi meters. This won’t matter much if you’re measuring car batteries (11.9VDC or 12.0VDC still provides you with the information you need).
Auto ranging was once a luxury, but now fairly standard when it comes to features. Basically it means that the meter automatically selects the optimum range based on what you’re measuring. This saves time when measuring relatively unknown sources, but also means you get the most information from the meter. In a real-world example, take a humble AA battery. there’s not much point seeing 0.001kV (1V) when you’re measuring a battery is there. But 1.305VDC is probably quite useful to help determine the state of that battery. On the flip side, if you try and measure a car battery on a 2V range setting, the meter will simple tell you it’s over range, without any sensible values. Not very useful! So auto ranging when used will automatically switch it to an appropriate range and give you the most useful information. Of course you can generally override this which is particularly handy when you’re measuring something erratic, to stop your ranges jumping all over the place.
NON-CONTACT VOLTAGE DETECTION
This is primarily a safety feature. It gives you a very fast way to determine if a wire is live. It’s not just for electricians though – even if you’re hanging pictures in your home, if you’re drilling into a wall and unsure if there’s live wires behind it, it will alert you to any power running nearby. If you’re in a house, it might save you a tripped circuit breaker. If you’re in an apartment it could save you from cutting shared services, thousands in repair bills, not to mention some angry neighbours.
RMS stands for Root Mean Squared. It’s a mathematical term used for measuring waveforms such as AC voltage. Some meters use “average” methods, others use True RMS. Both will give a relatively accurate result for a nice and clean, pure sine wave (or at least, regular shape waveform) signal. The problem comes in however when it’s erratic. If the frequency isn’t consistent, or the waveform is a strange one, average measurement might not be accurate. A True RMS meter uses this superior calculation method to provide you with a more accurate value, regardless of the shape of the waveform. If you’re only measuring DC voltages, and will never measure AC, this feature will be less useful to you.
Like all devices, this is only useful if you require it, but critical if you do. If your multi meter is going to live on your electronics bench, the most you have to worry about is a spilled coffee. But if you’re working outside, in a garage, or anywhere there’s potential for liquids, a meter with a high IP rating is a great idea. Of course water and electronics don’t really go together, so it’s more for protection of the meter, than measuring something underwater (but you know that already!).