Home · Articles · Irrigation · Miscellaneous · The Mysterious Ohm Meter

The Mysterious Ohm Meter

JOE GOLSON | Miscellaneous

My colleagues always joke, "If you ask Joe what time it is, he'll tell you how to build a watch!" This quip by my buddies in the office is partly true. I’ve been working in my present job for many years and seem to have developed quite a following among our contractors. They call with all sorts of problems, with the hopes of stumping me in the process.

This last call I received concerned the use of a volt/ohm meter. The contractor relayed the fact that he was getting an error message on zone two of an electronic controller. He assured me that he had performed a diagnostic of the system with his analog volt/ohm meter but still could not find out what was causing the error message. I asked him what the readings were and he confided in a halting manner that he didn’t know. In fact, he told me that he did not know how to read the meter! I felt sure I had instructed this fellow before on the ohm meter’s operation, and knew I was in for a tough time.

Many times, in trying to educate a customer, a feeling of frustration creeps in and I begin searching for an alter native method to convey the information.

While talking with my customer, a vague feeling overtook me and I found myself in a laboratory with the contractor strapped to a table. I had just finished attaching electrodes to his head. Complicated machines whined and Jacob’s Ladders sparked while my hunchbacked assistant helped me download the information needed to properly operate an ohm meter directly to his cerebral cortex. This experience, while intense, lasted only mili seconds and, feeling victorious, I was able to continue instructing my victim, er— customer.

The points listed below are a synopsis of our conversation.

First, what is being measured?

In any electrical equipment, electricity is being forced through a circuit by a power source. The electrons on their journey will have to pass through constrictions within the circuit. This causes power to be dissipated and is called resistance, which is measured in ohms.

Since my customer was using an analog meter (the type with a needle that sweeps over a graduated scale), I went through the simple calibration procedure. He inserted the red and black probe leads into the ports marked for volts and ohms. Next he located the ohms adjustment wheel on the face of the meter. While holding the probes together, he was instructed to move the wheel right or left until the needle was centered over the zero on the ohms scale.

He was instructed to remove the common wire from the controller. This is done to protect the meter from any voltage present when a resistance reading is taken. The controller can also be disconnected from the power supply, as long as it is equipped with a way to hold the program in memory.

With one probe held against the common wire and the other probe held against a zone terminal, he will be able to “see” if the wire path to the valve is complete or the valve solenoid is operational or not. The wire path is unbroken if a reading of 20-60 ohms is present. This also indicates the valve solenoid is functional.

If the needle does not move but points to infinity, there is a break in the circuit. If the needle moves to the far right on the scale and pegs out at zero, this could indicate a shorted or burned out solenoid or field wiring that has been damaged and is shorting out.

With this information in hand, and feeling a bit more confident, my customer checked the resistance on all the zones, including zone two, which had shown an error. He made a startling discovery! All the field wiring and the valve solenoids checked out. Apparently, a small surge had come in through the power supply and momentarily locked up the processor. He unplugged the unit to let it reset itself, thanked me and went to lunch. He called back later to report that the controller was working perfectly.

Still, I cannot help thinking, what is so complicated about the operation of this useful device? Even my father who, at 96, may be the oldest member of the Southern California Bar Association, understands how to use a volt/ohm meter. He told me step by step how he was going to proceed troubleshooting a balky toaster.

You see, it’s so simple even an attorney can use one!

It seems I have a call on line two with a question about a controller that keeps blowing fuses. I wonder, should I answer with the Rolex or the Timex version?

Opposite page: Calibrating an ohm meter. This page above: Inserting a probe into an ohm port. This page , left: Ohming a zone.

Photos courtesy: Joe Golson

EDITOR’S NOTE: Joe Golson is with W.P. Law, Inc., an irrigation distributor in Lexington, South, Carolina

 
  • Currently 3.5/5 Stars.
  • 1
  • 2
  • 3
  • 4
  • 5
 
 
 

 

 
01.26.2012 at 03:03 Reply

joe you never cease to amaze me have you written any articles showing how an italian can get through this  life on earth ??????

 

good article

 

 
 
Close
Close
Close