The Real Deal on Multimeters

SO YOU’VE PROBABLY GOT A DIGITAL MULTIMETER ONBOARD YOUR BOAT, BUT HOW MUCH DO YOU KNOW ABOUT THIS RATHER COMPLICATED LITTLE DEVICE?

OKAY,

BY CAPT. BILL PIKE PHOTO BY EDDIE BERMAN

RANGE: Most digital multimeters have an autoranging function and a manual function for measuring voltage, amperage, and resistance. Switching back and forth between the two is usually easy. While automatic ranging (allowing the unit to pick a parameter setting that maximizes a reading’s resolution and accuracy) is a helpful feature, it’s sometimes better to go manual in iffy situations, working your way down from the highest set of parameters the meter offers to the lowest rather than vice versa. The point is to avoid blasting an overly low or restrictive setting with a high-input jolt, which can either blow a meter’s fuse or flat-out fry it.

OHMS: This function and the two voltage functions are the most useful (and most used) on most multimeters. Turn the power off before taking a resistance reading. Your meter is not measuring the rate of movement of electrons— it’s measuring resistance to said movement (in ohms) using a minute electrical charge of its own.

MAX CAPABILITY: Whether they’re cheap or expensive, most multimeters are sold with maximum AC and DC voltages they can safely test. The Mastech unit shown here is rated to 750 volts (AC) and 1,000 volts (DC) and costs approximately $23. A similarly rated meter from a top-shelf manufacturer can cost $300 or more. In general, the voltage limits prescribed for the latter type of product are realistic. However, most electricians will tell you that using an inexpensive multimeter to trifle with the very same voltage limits, even if they’re officially inscribed on the unit, will likely prove hazardous, perhaps even catastrophic. Got big juice onboard your boat? Spend a couple of hundred bucks on that digital multimeter. AMPS: Avoid using these settings. The average multimeter is not especially useful for determining amperage (the volume of electron flow in a given direction) in situations that don’t involve circuit boards and other marineelectronics minutiae. A clamp-type ammeter (with two jaws that open to temporarily surround an electrical conductor) works much better.