A multimeter 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.
Electrical Terms
AC volts (VAC) - Alternating
Current, this is household voltage. Most irrigation solenoids
operate on AC voltage. Rain Bird voltage specifications are
117 VAC ± 10% (105 - 129) volts.
DC volts (VDC) - Direct Current,
the normal source is a battery. DC voltage is polarized, meaning
that there is a positive (+) and a negative (-), sometimes
referred to as ground. The meter must be connected properly
to prevent meter damage, the RED lead is (+), and the BLACK
lead is (-).
Resistance (ohms )
- A measurement of how difficult it is for the current to
flow through the electrical system. This is very similar to
friction loss through a piece of PVC pipe.
These may be easier to understand if they are compared to
irrigation terms that are more familiar.
Irrigation term
Electrical term
Pressure (PSI)
Voltage (volts - V)
Friction Loss (PSI)
Resistance (ohms - )
Flow (GPM)
Current (amps - A)
Additional Terms Used in Measuring Resistance
Resistance is the most useful function for troubleshooting
irrigation systems. For irrigation applications, a solenoid
is considered to be good if its resistance is between 20 -
60 ohms. It is best to compare resistance measurements to
that of a new solenoid. Example, if a solenoid measures 44
ohms, it would be considered good because it is between 20
- 60 ohms. However, if a solenoid of the same brand measures
24 ohms when new, it would be an indication the solenoid is
deteriorating and should be replaced.
Short - when the resistance
measured is below 20 ohms for a single solenoid. This allows
excess current to flow through the circuit breaker or fuse.
If the amount of current exceeds the rating of the devise,
it will open, therefore stopping the 24 volts to the valves.
Open - when the resistance
is above 60 ohms, the flow of current to the solenoid is reduced.
Think of this as having a rock lodged in the mainline of a
sprinkler system. The resistance may increase to the point
that the solenoid does not get enough voltage to operate.
HINT... To help remember the difference between an open and
a short. An open drawbridge stops the flow of traffic. A short
is a short cut for the flow of current through the solenoid.
NOTE...When two valves are operated together on a single
station, the total resistance will be less than the resistance
of the lowest solenoid. Example: if a 52
solenoid and a 24
solenoid are combined on the same station, the total resistance
is about 17,
less than the resistance of the lowest solenoid (24.)
Using The Multimeter
The following instructions have been written specifically
for the Radio Shack catalog No. 22-179 Digital Multimeter.
AC volts - Turn the dial to
the AC V position, AC will show in the lower left corner of
the display. Touch the meter probes to the circuit that you
wish to measure. NOTE...When the mV is active to the right
of the display, divide the reading by 1000. Example, the display
shows 061 mV, the actual measurement is 0.061 volts.
DC volts - Turn the dial to
the DC V position. DC voltage has a positive (+) and a negative
(-) terminal. The red lead is (+), and the black is (-). If
the leads are reversed, the display will show a minus (-)
sign to the left of the reading. NOTE...When the mV is active
to the right of the display, the reading must be divided by
1000. Example, the display shows 061 mV, the actual measurement
is 0.061 volts.
Resistance - To prevent meter damage,
it is very important that the circuit power be turned off
when measuring resistance. Turn the dial to the K
symbol (resistance), .OL M
will show in the display. This indicates infinite resistance,
an open circuit. Touch the probes to the circuit you wish
to measure. The display indicates the resistance.
NOTE...When the K
is active to the right of the display, the reading must be
multiplied by 1000. When the M
is active to the right of the display, the reading must be
multiplied by 1,000,000. Example, the display shows 41.2 K,
the actual measurement is 41,200 ohms.
NOTE...When measuring resistance from the controller, you
are measuring the resistance of the wires leading to the solenoid
through the wire connector, the solenoid, the second wire
connector, and the wire leading back to the controller. Resistance
of the control wires do not add much to the total resistance.
UFUL 14 is approximately 2.5 ohms per 1,000 feet, 16 gauge
UFUL wire is approximately 4.0 ohms per 1,000 feet, and 18
gauge UFUL multi-conductor wire is approximately 6.5 ohms
per 1,000 feet.
How To Interpret Display Characters
Voltage
0.0126 volts, or 12.6 millivolts
Multiply the reading by 0.001 when the MV is active.
Resistance
1,004 ohms
Multiply the reading by 1,000 when the K
is active.
Knowing how to operate a meter will save considerable time
when testing the wiring on a job site. It is possible to inspect
the solenoid and field wiring condition from the controller.
The procedure is as follows:
Resistance Measurements
1. Disconnect the common wire(s) from the terminal strip
on the controller.
2. Turn the dial to K.
3. Connect one of the meter leads to the common wire, not
the controller common terminal.
4. Touch the second meter lead to each of the station terminals,
and record the resistance readings. Compare your readings
to the acceptable range of 20 - 60 ohms.
5. If the measurements are within the acceptable range (20
- 60
) then the electrical circuit for that station is good. NOTE...This
test only inspects the condition of the wiring, the station
may not operate properly because of controller and/or valve
problems.
6. If the resistance range is below 20 ohms (a short), proceed
to the valve and disconnect the solenoid from the field wires.
Test the resistance of the solenoid only. If the measurement
is still low, then the solenoid must be replaced. If the solenoid
resistance is acceptable, then the short is in the field wiring
itself (two solenoids connected to the station can also produce
a low reading). Wire tracing equipment should be used to locate
the problem.
7. If the resistance is above 60 ohms (an open), as in step
7, test the solenoid without the field wires connected. Replace
the solenoid if its resistance is above 60 ohms. More than
likely the solenoid will test within proper limits of between
20 - 60 ohms. If the solenoid is good, cut out the wire connectors
and connect the station and common wires together at the valve
location. From the controller, re-test the resistance without
the solenoid in the circuit. The resistance should now read
very low, possibly 5 ohms or below since only the resistance
of the field wires is being measured. If the resistance is
this low, then the problem was a faulty wire connector. Install
new waterproof wire connectors on the existing solenoid and
test the resistance again at the controller.
If the resistance is still high when the common and station
wires are connected together, then there is an open somewhere
between the valve and the controller, possibly caused by a
faulty wire or wire connector. This fault can only be found
with the use of wire tracing equipment.
8. This final field wiring test will determine shorts directly
into the earth. In addition to the common still being disconnected,
remove each of the station wires from the controller. Connect
one of the meter leads to a piece of bare wire wrapped around
the metal shaft of a screwdriver. Insert the screwdriver into
the ground (it may be necessary to wet the ground to assure
a good connection). Touch the second lead to the station wires
and the common one at a time. Each of these measurements should
be above 700K (700,000) ohms. A resistance reading of below
700,000 would indicate a section of the wire has a nick in
the insulation and is making contact with the earth. Wire
tracing equipment should be used to locate the problem.
9. Transformers can also be tested using a multimeter. Connect
the meter leads to the primary winding, that is, the transformer
input wires or plug in connectors. You will either get a resistance
reading or an open. A resistance reading shows the internal
windings are intact, an open indicates the transformer's internal
fuse is bad and the transformer must be replaced. The secondary
winding, the transformer output, is tested the same way. Connect
the meter leads to the output wires. An open indicates that
the transformer must be replaced. The 20 - 60 ohm rule does
not apply to transformers. It is common to find resistance
as low as 3 ohms.
10. A multimeter may also be used to determine what type
of battery should be used in a solid-state controller. The
controller should be powered up and the battery removed from
its connector. Rotate the dial to the DC V setting. There
are two battery connectors, a large one (female) and a smaller
one (male). Touch the red probe to the large connector, the
black probe to the small connector. If the controller is designed
to accept an alkaline battery, the reading will be close to
zero volts. If it is designed to accept a rechargeable NiCad
battery, the reading will be between 7 and 13 volts DC. Never
use an alkaline battery in a controller designed to accept
a rechargeable battery.
Note...Do not use rechargeable alkaline batteries in solid-state
controllers!
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