From the MACS technical think tank
While the Fluke 88V (Figure 139) is probably the “gold standard” of automotive test meters, many A/C shops buy lower-priced meters and get usable results – even if those meters don’t have all the features of premium multimeter kits, or their long-term durability, or even quite their accuracy.
Some examples of comparative range and accuracy: on DC voltage, the Fluke 88V has a 1000 volt maximum and an accuracy of +/- 0.1% plus/minus one digit. A premium multimeter (but much lower priced) sold by Radio Shack has a 600 volt maximum and an accuracy of +/- 1.0% plus/minus 2 digits up to 400 volts (the accuracy drops to +/- 1.5% plus/minus 2 digits at 600 volts, but that’s beyond even a high-voltage hybrid battery pack, so we’ll ignore it). Within the automotive range, roughly that means that the Fluke 88V has a 90% closer tolerance on accuracy.
Let’s review the meaning of the plus/minus digits. The digits are an addition to (or subtraction from) the meter reading after adjustment for the percentage tolerance. If a 12.72 volt reading is subject to a 1% tolerance and two digits, it means the reading really can be as low as 12.57 or as high as 12.87 volts. Explanation: 1% of 12.72 is 0.1272, which rounds up to 0.13 volt (to get the same number of decimal places). Add two digits and the tolerance increases from 0.13 to 0.15 volt. Add and subtract that 0.15 from 12.72 volts to get a high of 12.87 and a low of 12.57—the actual voltage is somewhere in that range.
So how can you tell if your voltmeter has remained reasonably accurate? A well-charged battery should read 12.75 to 12.8 volts, a fresh AA battery should read 1.6 volts, and a fresh 9-volt battery should read about 10 volts. With those references, you shouldn’t have to guess, because a defective voltmeter will show up somewhere in one or all of those tests.
With ohms, you might have other concerns, primarily any resistance in the meter leads and probes. Touching the probes together should produce a zero reading, but it rarely does. Most new multimeters have an REL (relative) button that subtracts the initial resistance of the probes and leads from a subsequent reading. If your meter is one without the REL feature, be sure to subtract the initial reading from the leads and probes from the one you get when you make the circuit check.
With amps, if you’re taking readings for blower fans and radiator electric fans, you have to use an inductive clamp. With a multimeter and its 20-30 second maximum and 10-amp limit (or 20 amps with a Fluke), the meter itself can only be used to take readings of milliamps (like parasitic current draw with accessories off), or just a few amps, such as A/C clutch coil current draw. With these tests, the accuracy tolerances are small enough, even with less expensive meters, that any meter’s rated accuracy wouldn’t make a difference, as long as it’s reading within its rated accuracy.
Making a purchase decision should be based on the features you need, as premium multimeter kits perform automotive test functions well beyond the lower cost multimeters. You may not need as much accuracy as a premium meter, but you may want a magnetic rpm sensor, long-reach alligator clips, piercing probes, peak min-max hold for readings, etc. The glitch capture capability found on premium meters is also often a diagnostic winner.
The bottom line is whichever meter you choose, take care of it, and periodically test it for basic accuracy.
When having your mobile A/C system professionally serviced, insist on proper repair procedures and quality replacement parts. Insist on recovery and recycling so that refrigerant can be reused and not released into the atmosphere.
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The 32nd annual Mobile Air Conditioning Society (MACS) Worldwide Convention and Trade Show will take place January 18-20, 2012 at the Rio All Suite Hotel and Casino in Las Vegas, NV.