Whether you call it a multimeter, multitester or VOM (volt/ohm/milliammeter) or just a meter, it's an essential piece of electrical test equipment that deserves a place on any workbench or toolbox. So what are these devices used for?
A VOM is actually four measuring devices in one. They can be used to measure AC or DC voltage, amperage and resistance or continuity. Some even have additional functions such as a thermometer. Common uses include measuring voltage checking whether a circuit is live, testing batteries, and determine exact levels of voltage, amperage and resistance.
Analog or Digital?
One of the first things you are likely to notice when looking at meters is they have one of two types of displays—analog or digital. Analog meters, which have a needle indicator pointing to numbers on a scale (or often multiple scales) have been around for about a century while digital meters with numerical displays first appeared some 40 or so years ago and have steadily grown in popularity over time. Which type of meter to buy is largely just a matter of personal preference. The perform exactly the same function.
Analog Advantages
Traditional analog meters have a small learning curve as one learns which scale to read for a particular function. These are labeled and often color coded to correspond with the scales on the selector switch. Sometimes a scale requires multiplying by tens or sometimes hundreds to take an accurate reading. The main advantage of an analog meter is it's easier to see how much a reading varies.
Unlike a digital meter, which uses battery power for all functions, an analog meter uses it only on the resistance scale, where it sends a small current through a circuit to measure resistance. On the voltage and amperage settings, the device is measuring power flowing through the circuit without the need for any other power source.
Some of my favorite vintage analog meters are from Radio Shack (Remember them?), sold under their Micronta brand. Models worth looking for include the 22-202, 22-203, 22-204, 22-210 and 22-214. Although these models have been out of production for years, they are plentiful on the second-hand market so hold out for a good example. Also worth considering, if have the money to spend is a Simpson 260. The epitome of professional grade, this venerable line has been in constant production since the 1930s and is still made in the USA. Most models use standard AA, D and 9v cells but some take obsolete 15v batteries that are absolutely unobtainable. New Simpson meters are priced in the hundreds of dollars but good second-hand units are available for far less.
Digital Precision
Digital meters show exact numerical values but they can be hard to read if that value is shifting too rapidly. Nonetheless, they are often preferred because they have fewer moving parts and thus are often more durable than their analog counterparts. Because they have largely replaced analog devices, economies of scale often make digital meters a little less expensive than a comparable analog device.
New digital meters range in price from a few dollars to hundreds. Harbor Freight has even been known to give away their cheapest digital meter for free with a coupon.
What to Look for in a Vintage or Second-hand Meter
As mentioned above, I prefer older, analog meters because they are usually built better than newer analog models but there is also a bit of risk involved. Three signs of trouble to watch out for are damage or excessive wear to the exterior, signs of battery leakage, and burn marks on the interior circuitry.
If the meter looks beat up on the outside with lots of dirt, scratches, cracks or missing pieces of plastic, chances are it has seen some heavy use or even abuse. Check the knobs and switches to make sure they move properly. If they don't, this is also a sign the meter should be avoided. As a final part of the exterior inspection, look for missing case screws as they are a sign someone has been inside it before.
Because these meters can often sit idle for long periods, battery leakage is fairly common. Residue from alkaline batteries can easily be cleaned off with white, distilled vinegar. Buff the contacts with sandpaper and everything should be fine as long as the residue was confined to the battery compartment. Even if the batteries appear good, check the expiry date, replace any that are excessively old and make a habit of checking them periodically for leakage.
Burn marks are a sure sign that a meter has been subjected to an overload and the meter should be avoided unless you are comfortable replacing the damaged components, which will usually require some soldering skills. Of course, an overload could also damage the circuit board itself.
Finally, check the fuse(s). A cylindrical glass fuse can also be examined visually to see if the filament is intact If a fuse is opaque or discolored, it is probably bad. The definitive test, of course, will be with a multimeter set on the ohms setting. If the needle moves all the way across on an analog meter or you see the number one on a digital unit, you have continuity and the fuse is good. No movement or the number zero indicates a blown fuse. Make sure you replace it with one of the same rating. Even though fuses may look alike, they have different amperage ratings. Read what's inscribed on the metal contacts at the ends of the fuse or even better, check to see if the fuse requirements are on the device itself or the instructions, if you have them.
