Know your specs when evaluating multimeters

One multimeter is the same as the next, right? Not so. While a multimeter remains as one of your best diagnostic investments, it’s one you shouldn’t take lightly. With just a little insight into what to compare, you’ll be better prepared when you plunk down your hard-earned cash.

Here’s a look at some of the most common meter specs, and what they really mean when comparing one meter against another. There’s a lot of jargon flying around when it comes to meter specs, but with a little patience you should be able to frame it into understandable terms.

INPUT IMPEDANCE

Expressed in megohms (millions of ohms), this is the amount of resistance that the multimeter's circuitry yields during testing. It’s critical for a couple of reasons. A high-impedance multimeter (at least 10 megohms) only causes minimal loading of the circuit under test. This is extremely critical on today's electronically controlled systems. If a low-impedance meter were used, circuit readings would be affected because of the meter's load. In some cases, it could even damage the system’s computer.

RESOLUTION

This reflects the meter's ability to make small measurements. For instance, if you need to make measurements in the millivolt (1/1000th of a volt) range, you'll want to make sure that the meter is capable of displaying voltage three places to the right of the decimal point.

DIGITS

This refers to the number of places in a numbered display. The most common specifications for multimeters are displays that number 3-1/2, 3-3/4, or 4-1/2 digits. A full (whole number) digit expresses numbers from 0 to 9. A 1/2 digit only expresses 0 or 1 and a 3/4 digit only expresses from 0 to 3.

When counting display digits, count from right to left. Let's see what the digit numbers mean in the following examples.


3-1/2 digit display:
1/2 digit 3rd 2nd 1st
1 9 9 9
3-3/4 digit display:
3/4 digit 3rd 2nd 1st
3 9 9 9

Since the 3/4 digit can be 0, 1, 2 or 3, it gives the meter a much better display resolution. In fact, the 3-3/4 digit display has two times the resolution of a 3-1/2 digit display.

COUNTS

This expresses the total number of different values, or digital steps, that a meter has. Since the counts increase in direct proportion to the meter's display, both work together in determining a meter's final resolution. The bottom line: the higher the number of counts, the greater the resolution.

To determine the total counts, add one to the meter's maximum display value. For example, a 3-1/2 digit display has 2,000 counts, a 3-3/4 digit display has 4,000 counts and a 4-1/2 digit display has 20,000 counts.

ACCURACY

This spec refers to the margin of error between the meter's displayed value and the actual value. Accuracy specifications are usually given as a ± percentage of the reading, and there will be different specifications for a meter's various measurement capabilities.

Although resolution should never be confused with accuracy, it's important to keep both in mind when comparing meters. For instance, a 4-1/2-digit meter with an accuracy of ±10 percent fares poorly against a 3-1/2 digit meter with an accuracy spec of ±0.5 percent.

UPDATE RATE

This is simply the rate at which the meter refreshes the display with test information. Typically, most meters update their information at a rate of 3-4 times per second. Meters with bar graph displays update faster. Although a fast update rate is desirable, keep in mind that readings will appear more unstable as update rates increase.

RMS

An acronym for Root Mean Square, it's a mathematical equation for measuring the voltage of alternating current that has a true sine wave. An example of this would be the voltage available at a wall outlet. Meters with AC scales using averaging circuitry will only be accurate when used on AC voltages with a true sine wave.

TRUE RMS

The AC voltages produced by most automotive magnetic sensors do not produce a true sine wave. In other words, the voltage and frequency of the signal varies. Examples of these types of sensors include ABS wheel-speed sensors and some crankshaft sensors. To get an accurate voltage reading from these types of sensors, look for a meter with true RMS sampling circuitry. A meter with averaging circuitry will display a significantly different — even erroneous — reading.

Of course, there are even more specs beyond these, but the ones outlined here represent the main “guts” of most meters. So, with due diligence and awareness of your diagnostic needs, you’ll be able to compare your way to exactly the multimeter for your specific needs.

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