A Myth About Valve Circuits
MYTH: IF A CIRCUIT HAS CONTINUITY, A VALVE SHOULD WORK ELECTRONICALLY.
You'd think that'd be sound logic — but think again. This reasoning isn't sound at all. I am sure you might be wondering why. As an irrigator troubleshooting a valve circuit, you will likely use a circuit tester that flashes a green light or emits a tone when it detects continuity in a circuit. No continuity results in a red light or no tone. Even multimeters that are set to measure ohms emit an audible tone to indicate that a circuit has continuity. What often happens is a technician will interpret the test instrument's tone, beep or green light to mean that the circuit being testing is free of faults. Stop right there.
This is a myth. A valve wire circuit can have continuity and still not work electrically. Believing in this myth could lead to a tech proceeding as if there's no problem with the circuit. Talk about wasting valuable time for the irrigation service business! And the customer might even be charged more than if an accurate diagnosis had been given in the first place.
To understand why merely verifying continuity is not a complete fault test, you must first understand how an electrical circuit works. While the water in an irrigation system only needs to travel from point A to point B, electricity has a more complicated journey. It must not only travel from its source to the solenoid, but it must also return back to the source to create a complete circuit. While reviewing the typical electrical circuit path of an irrigation system, you need to think about all the places that path could fail. Just as water flows through a pipe, electrons flow through wire. The flow of electrons through a circuit is called current and is measured in amperes. The current is what lifts the plunger inside a solenoid.
For a valve circuit to work properly, there must be enough current going to the solenoid to open it. Many standard valve solenoids require about 0.40 to 0.50 amps to open their valves. Let's put that into perspective. It takes 6,241,000,000,000,000,000 (6 quintillion, 241 quadrillion) electrons per second to create 1 amp. That means if a solenoid needs 0.40 amps to open, about 2,496,400,000,000,000,000 electrons per second must reach it to create the electromagnetic force that lifts the plunger and opens the valve. That's a lot of electrons!
So what does this mean? When using continuity alone to test a valve wire circuit, the beep of the continuity tone or the green light only means that some electrons are passing through the entire circuit. They won't tell you if there are enough amps — or too many — to do the work of lifting the plunger inside the solenoid. You can now see why testing for continuity does not give you enough information to resolve a problem efficiently. It can also provide false information that not only wastes your time but the customer's money. No one wants that!
The accurate test
If a continuity test can make a circuit look good when it's not, then what's a better, more accurate way to test? The answer is to test the circuit's resistance. It's easy to measure and can identify a good circuit or indicate the type of fault the circuit has. Resistance measurements can also be compared over time to see if a circuit is trending toward failure. Don't rely solely on a multimeter's tone, beep or green light to tell you if a circuit is free of faults. Resistance is the opposition to the flow of electrons in a wire and is measured in ohms. It's like friction in a pipe. As resistance goes up, current and voltage go down. Problems in a circuit occur when there's too much current or not enough. Measuring the resistance in ohms of a valve wire circuit will determine if the circuit is sound or if one of the three types of faults exists: an open spot, a short or a partial connection.
The ohm readings establish the facts
Ohmmeter readings will tell you what's right or wrong with a circuit. A sound circuit with one solenoid on the wire will have a reading of between 20 and 60 ohms, when measuring a valve circuit from a controller. In irrigation, a sound circuit means that the power and the common wires, the splices and the solenoid are all without faults or “good.”
An open circuit will produce a reading on a digital multimeter of 1 or OL. It means that the resistance to the flow of electrons is so high, none of them can move along the circuit. Common causes include:
- a disconnected splice;
- a broken wire;
- a wire disconnected from a terminal or other connection.
- a damaged solenoid with a short inside.
- two valve wires touching directly.
- two valve wires touching through soil or water.
A partial connection would give you a reading over 60 ohms (probably hundreds of ohms or more) when measuring a valve circuit with one solenoid from the controller. A partial connection means that the flow of electrons is being restricted, which results in higher resistance because electrons aren't moving freely. The cause could be:
- a corroded splice.
- a splice where the wires are not making complete contact.
- a partially broken wire, most likely inside its insulating jacket.
There are some exceptions to these interpretations of ohm readings. The most common is when two or more solenoids are on a single valve wire. The expected ohm reading of a sound valve circuit with two solenoids on it will be almost half the ohm reading of a single-valve circuit. Doesn't make sense, unless you think about the additional paths electrons can take when there are multiple solenoids. If two or more solenoids are on one wire, electrons can take more paths to complete the circuit. This reduces the current on any one path, reducing total resistance. That reduction in ohms compared to a single-valve circuit continues like that for circuits with three or four solenoids on one valve wire.
Relying on continuity to tell us the condition of a circuit is a mistake; a continuity test can only indicate an open circuit. Fine for testing fuses, but for every other electrical component we need to know not only that there's a fault but exactly what type it is. Testing a valve circuit using ohm readings will check the wires, the splices and the solenoid. We start by measuring the ohms at the controller. If there's a fault, the readings will tell us where to look for it. The repair can be made, and you can get paid.
By Kurt Thompson
Kurt K. Thompson, CIC, CID, CIT, CLIA, CLWM, is the owner of K. Thompson and Associates, a water-use evaluation and training company, a principal of IrriTech Training, an online and face-to-face training company. He can be reached at firstname.lastname@example.org.