VOLT OHM-METER ( VOM )
A volt ohm-meter is an inexpensive piece of test equipment that is capable of measuring AC volts, DC volts, and resistance. By learning how to use this equipment, it is possible to troubleshoot controllers, solenoids, field wiring, and to verify AC and DC voltage levels.
There are two styles of VOM’s, analog and digital. An analog VOM is the style that has a needle that moves across a face (similar to a speedometer). The DMM (digital multi-meter) has a digital display. DMM’s normally have additional features than are available with an analog VOM, and therefore are more expensive. For this reason the following instructions are designed to work with an analog style VOM.
AC volts (VAC) - Alternating Current, this is household voltage. Most irrigation solenoids operate on AC voltage.
DC volts (VDC) - Direct Current, this would normally be from a battery. DC voltage is polarized, meaning that there is a positive (+) and a negative (-), sometimes referred to as ground. The VOM must be connected properly to prevent VOM damage, the RED lead is (+), and the BLACK lead is (-).
Resistance - a measurement of how difficult it is for the current to flow through the electrical system. This would be similar to being able to actively measure the friction loss when water flows through a piece of pipe.
Comparisons between water and electricity
Pressure (psi) Friction Loss (psi) Flow (GPM)
Voltage (volts-V) Resistance (ohms-) Current (amps-A)
For irrigation applications, a solenoid is considered to be good if its resistance is between 20 - 60 ohms
Short - when the measured resistance is below 20 ohms for a single solenoid.
Open - if the resistance is above 60 ohms when measuring the field wiring.
Note…when two valves are operated together on a single station, the resistance will be approximately ½ of a single solenoid.