Digital multimeters, and the three words on the box that decide it

A multimeter is the tool that turns “is this dead” into a number. It tells you whether a battery is flat, whether a fuse has gone, whether the car’s alternator is charging, and whether the socket you are about to work on is really off. Every meter on this page will do all of those. The specifications in the titles are about how well, and one specification that is mostly missing is about how safely.

True RMS, and why cheap meters read wrong

Mains electricity is a smooth sine wave, and any meter can read that. What comes out of a dimmer, an LED driver, a motor controller or a solar inverter is not smooth. A basic meter guesses the value from the average and gets it wrong on those, sometimes by a lot.

True RMS means the meter works out the real value however ugly the waveform. If you will ever put the probes on anything electronic rather than a plain battery or a plain socket, it is the specification to insist on. Every title here states it.

Auto ranging, and the dial

An old-style meter has a dial with a dozen positions: 200mV, 2V, 20V, 200V. You choose volts, then guess the range. Guess too low and the meter shows an overload; on a cheap one, guess too low on current and the fuse blows.

Auto ranging means you choose volts, or ohms, or amps, and the meter picks the range itself. It is slower by a fraction of a second and it is the right choice for anyone who is not doing this every day. Every title here states it too, so on this shelf it is not a deciding factor.

Counts, which is resolution

A meter’s display has a maximum number it can show before it moves the decimal point. 6000 counts means it shows up to 5999, so a reading of 5.999 volts, and then goes to 06.00. 9999 counts and 22000 counts show more digits on the same reading. Five titles here state a counts figure; the rest leave it out, which usually means the small one.

For checking a battery, a fuse, a socket or a car, 6000 counts is more resolution than the job needs. The 22000 count meter is for somebody measuring small differences on a bench, and they will already know that.

CAT, the word that is mostly missing

A meter’s CAT rating says how big a surge it can survive without the front blowing off in your hand. CAT II is for things plugged into a socket. CAT III is for the socket itself, the consumer unit and the wiring in the walls. CAT IV is the supply coming into the building.

One title on this page states a CAT rating, and it is the oscilloscope. That does not mean the other eleven have none; it means the seller did not think it worth the words. It also means you do not know. The honest rule is simple: a meter whose CAT rating you cannot find should stay on batteries, cars, and electronics, and should not go into a mains socket or a consumer unit. If you need a meter for mains, find the CAT III 600V in the listing or the manual before you buy, not after.

If the words CAT III mean nothing to you, that is itself the answer: keep the probes out of the socket.

NCV, the beeping wand

Four titles here state NCV, non-contact voltage detection. Hold the top of the meter near a cable and it beeps if there is mains in it. It is useful for finding which cable is live behind a wall.

It is not a safety test. It misses cables with metal shielding, it can beep at static, and it cannot tell you a circuit is dead. Silence from the wand is not proof. The only proof is a reading on the probes, on a meter rated for the job.

The clamp meter, and the oscilloscope

Two listings here are different tools that share the word. A clamp meter measures current by closing a jaw round one wire, without cutting into the circuit. Electricians use them constantly. For checking a battery or a fuse they are the wrong shape.

The oscilloscope multimeter draws the waveform on a screen as well as reading it. For someone repairing electronics it is a bargain. For everyone else it is a meter with an extra menu.

Rechargeable, which is not the feature it sounds like

One title here is a USB rechargeable meter. That is a lithium cell inside a device that measures mains, and a meter that is flat when the boiler stops working at 11pm. A 9V battery lasts years in a meter and is in every supermarket. It is one of the few places where the old answer is the better one.

What I would actually do

For the drawer, the ANENG 616. Its title states the three words, auto range, 6000 counts and True RMS, and the screen is large and lit.

For a name that will still be in business when you need a manual, the UNI-T UT139C. UNI-T is the brand people on repair forums assume you have. Its title says nothing about CAT, and its own photographs show CAT III 600V printed on the case beside the probe sockets, which is the household mains rating. A picture is a weaker promise than a title, so confirm it in the manual, but it is the one meter on this page with the rating in sight.

For a bench, where small differences matter, the 22000 count meter. Its title names no brand, and its own photographs show a UNI-T badge on the case. A picture is a weaker promise than a title, so treat it as something to confirm with the seller rather than as settled, but it is worth knowing.

And whichever you buy, look for CAT III 600V in the listing. If it is not there, the meter is for batteries and cars, and that is still most of what a meter is for.