Basics of using a voltmeter

A simple voltmeter will read AC voltage, DC voltage and AC /DC current. This type of meter will help you accomplish the basic tests needed to diagnose most problems.

AC voltage is usually the 120 volts that supplies power to the panel and controls. To read this voltage, set the meter dial to the closest upper range (example set to 200 volts to read 120) and touch one meter lead to the black (hot) terminal and the other meter lead to the white (neutral) terminal. A typical reading will be around 120 volts. Anything higher than 125 or lower than 110 could be a sign of a problem, contact an electrician to investigate.

DC voltage is what is produced after 120 AC line voltage is converted in the power supply. Most of the equipment within a Micro-Comm RTU panel is powered by DC voltage. Note the output of the power supply is set at 13.8 volts DC to properly charge the backup battery. To check the DC power supply output, set the voltmeter to read DC volts at the next highest scale (usually 20). Put the red meter lead on the positive output terminal (red wire connected) and the black meter lead on the negative terminal (blue wire). You should read something close to 13.8 volts DC. Any reading lower than about 13.0 volts may be an indication of a faulty power supply. Likewise, a reading above 13.8 could indicate a problem with the power supply as well.

DC milliamps (current) is the measurement used to test transducers and flow meters with 4-20 milliamp output. On some voltmeters, the red meter lead must be put into a different socket on the meter to read current, check your model. Set the meter to read on the next highest DC current milliamp scale (usually 20m). To measure the current, the meter must be in series with the connection, meaning that you must remove one wire from the device first, usually the power (+) wire. After doing this, touch the red meter lead to where the wire was previously connected; touch the black meter lead to the red wire; this will complete the circuit and you can read the meter for the milliamp value.

Frequency (hertz) is only available on some more expensive voltmeters. This feature would be used to check the output from a pulse type flow meter. To test, set the meter to read frequency at the next highest range (usually 20 volts). Touch one meter lead to each wire from the flow meter (polarity not important. The frequency you read on the meter must be calculated to tell you if the reading you see on the meter is correct for the flow rate.

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