The volt-ohm-milliammeter (YOM) is also called a multimeter. The multimeter is used for all phases of service work including installation, preventive maintenance, and troubleshooting.

The multimeter is used to measure high -level AC voltages in power and load circuits and low-level AC voltages in control circuits (Figure 1-24 ). In units containing direct current (DC) operated control circuit devices, it is used to measure low-level DC voltages and currents. In troubleshooting, the multimeter is frequently used to measure voltage and make continuity checks on system and component wiring. It is typically used to check motor windings, relay coils, and motor starter/contactor coils for resistance values and for sh o rt, open, or grounded circuits. Another common use of the multimeter is to check the start and run capacitors of motors for a shorted or open condition.

Both pocket-size and hand-held analog multimeters (VOMs) or digital multimeters (DMMs) are used for field service work. Analog multimeters (Figure 1-25) are best for observing changes, instantaneous response, and peak or dip indications when making adjustments. However, the technician must identify the proper scale to use when making measurements and must interpret the pointer location to obtain the reading. Scale divisions may limit resolution. The accuracy of an analog meter is based on percent of full scale reading and is typically in the range of ± 1 .0 to ± 5 .0 percent. While electrical protection is normally provided, analog meters can be damaged by measuring in the wrong mode or range.

Digital multimeters (Figure 1-26) have direct-reading, high resolution displays that give accurate readings without the need for scale interpretation. Accuracy is generally in the range of ±0.1 to ±0.5 percent. DMMs normally have fuses for current protection and input limiting on volts and ohms. Because they have no moving parts, digital multimeters are less likely to fail or lose their calibration than analog multimeters.

The multimeter feature most needed for field servicing HVAC equipment is the capability to accurately measure A C voltage over several scales, ranging from 0 volts to 1,000 volts. Generally the multimeter should also measure D C voltages over several ranges, but the 0 to 50-volt DC range is the most important for servicing low-voltage DC control circuits. The full scale accuracy for the readings should be about ±0.5 to ± 2 percent.

The multimeter should be capable of measuring resistance over several scales, ranging from 0 ohms to 30 million (meg) ohms. The ability o f the multimeter to accurately measure resistance is very important because of the need to make critical motor and relay/solenoid winding resistance measurements as well as checks for shorted, open, or grounded circuits.

Since the clamp-on ammeter is the most frequently used instrument for making A C current measurements, the current measuring function of the multimeter is not as important as its ability to measure voltage and resistance. However, most of the multimeters that meet the voltage and resistance measurement requirements for HVAC servicing can also measure A C and DC currents over several scales ranging from 0 to 10 amperes. Digital multimeters can normally measure low-level A C and DC currents in the microampere range. This feature is useful for servicing the flame sensing system on heating equipment.

Many special-purpose accessories are available for use with digital multimeters. Thermocouple and thermistor temperature probe accessories and non-contact infrared surface temperature probes enable the digital multimeter to measure temperature. Non-contact probes measure the surface temperature or temperature in a refrigerant line by simply pointing the probe at the surface or line to be measured. Clamp-on ammeter accessories are also available that enable the digital multimeter to be used to measure high-level A C currents up to about 400 amperes.

In addition, there are accessories that enable the digital multimeter to measure pressures up to about 350 psig.

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