How to use a Milliamp Clamp Leakage Meter to Diagnose Two Wire (2-Wire) Irrigation Sprinkler Systems

Today I want to talk to you about
troubleshooting 2-wire systems with a milliamp clamp meter. If you’re working
with 2-wire systems, a milliamp clamp meter is a must. All the manufacturers
recommend you have one of these; the Irrigation Association teaches that you
need to have one of these. These are a ‘gotta-have’ tool for a 2-wire system.
The reason they are a ‘must have’ is because 2-wire systems work on
milliamps and this meter, called the Pro93 milliamp clamp meter, can measure milliamps. There aren’t very many multimeters out there that can do that,
so when you go to select on a multimeter to do this job
make sure absolutely it has the milliamp capability. On this unit, the milliamp
capability is shown here by the ‘mA’. When I move the dial to the mA position, it’s
now ready to measure milliamps. The way you measure milliamps is through the
clamp portion of it. You take one, not two, but one wire of the system and you’re
going to put it through that clamp and it’s going to give you a reading here of
what your milliamps are. Now, what we’re looking for or what we’re trying to
troubleshoot is what is the drop in milliamps from one decoder to the next?
Let me take you through a chart or diagram that will show you how that
works. This is what a typical 2-wire system looks like and this is a diagram
of how you would use a Pro93 in that system to diagnose. Out of your
2-wire controller comes a certain amount of milliamp current that is
measurable by the Pro93. You see in the first instance here on the elbow, the 93
is going to measure and let’s say there’s 200 milliamps coming out of that
that controller. Then you see a series of decoders here. In this case, we have three
of them but the system is going to continue on down. Typically, a decoder
will draw .6 milliamp per decoder but specs will change based on the manufacturer, it might be higher. It couldn’t be a whole lot lower. As you
test, as we see here in the third Pro93 here, you’re going to test
on one leg of the 2-wire system, wiring, that comes into the decoder. Again, you
don’t want to test both wires because it tends to cancel itself out, you get zero
for a reading. You get a test on one side and you’re going to take a milliamp
clamp or a milliamp reading on the inbound of that decoder. Let’s say in
this diagram we start off with 200 milliamps and each one takes .6. I
want to see a .6 milliamp drop across the decoder every time I test. So
on the inbound, let’s say, if I’ve got 100 milliamp, on the outbound of that
decoder I should see 99.4. On the next one I should see
99.4 milliamps coming into the decoder and the outbound is just
a 98.8 and so on down the line. You know you have a fault
when you get to a decoder where the inbound and the outbound are not a .6 milliamp difference. If you drop by 30 milliamps on the inbound and the
outbound of a certain decoder, you know that that decoder is bad. That’s the beauty of
these things, but the only way you can do that is if you have a meter, a clamp, that
can measure milliamps. That’s what the Pro93 can do. In addition, you
also want to have a true RMS capability on your multimeter. True RMS
means that the meter is accurate 100% of the time, no matter what your AC sine
wave looks like: square wave, sine wave, distorted wave, it doesn’t make a
difference. It’s going to be accurate all the time. If you just get an averaging
milliamp clamp meter, it may or may not be accurate. Now, is it going to be off by
tremendous amounts? No, but you want the most accurate meter you can get and they’re really not that much more expensive to get than a true RMS version.

One thought on “How to use a Milliamp Clamp Leakage Meter to Diagnose Two Wire (2-Wire) Irrigation Sprinkler Systems

  1. if u have a 200 mil. line and a decoder draws .6 mil do u subtract .6 from and add all the decoder in line to c what thay are

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