Circuit Troubleshooting: Use the Right Meter
Technicians have been using average responding meters, an industry standard, for years, and for years they’ve been appropriate. However, the digital waveform on most of today’s electrical loads requires measurement with a true root mean square (RMS) meter. Using the average responding meter can put hvacr troubleshooters on the wrong track.
This, in turn, can lead to more warranty replacements of components that have nothing wrong with them, and more callbacks for the contractor.
Dave Boyd, national training director of ATP (formerly Amprobe), a manufacturer of measuring devices, took service technicians attending the recent Garden State-RSES (Refrigeration Service Engineers Society) Service Seminar here for a trip down memory lane. His subject: “Stop the Insanity! True RMS and Circuit Troubleshooting.”
Remember when the state of the art in calculations was the slide rule? “That was a step above an abacus,” Boyd said.
In 1973 the first calculator hit the market, with a $500 price tag and the ability to calculate sine and cosine.
Pulsing SignalsBy 1980, 10% of electric wiring in the United States included some sort of electronic (pulsing) signal, Boyd said. This changes the smooth, alternating current (ac) 60-Hz sinusoidal waveform signal to the boxy “on-off” shape of the electronic signal, which only draws what it needs, when it needs it.
By 1998, 60% of electric wiring was running through electronics, Boyd said.
And by 2005, 90% of electric wiring is predicted to run through electronics.
Those older average responding meters, Boyd said, are very good at measuring the sinusoidal waveform. However, when used to measure wiring with electronic pulse signals, their readings can be 10% to 50% off. In short, while true RMS meters cost more (about $50 more), they’re more accurate for measuring today’s currents.
The increased accuracy can affect bottom-line profits at all levels of the market, from the consumer and contractor on up to the system or component manufacturer. Part of the solution is for contractors and techs to make better-informed choices when it comes to buying new meters.
Say a technician is examining a rooftop unit, and he accidentally drops his meter off the roof. He goes to the wholesaler and says, “I need a new meter. Mine fell off the roof.” The guy at the counter will show the tech two models, the averaging model and the true RMS meter. The tech asks, “What’s the difference?” The counter guy says, “Fifty bucks.”
Human nature being what it is, Boyd said, most would buy the less expensive averaging meter with 1% accuracy and feel that they had made a good decision.
However, more and more furnaces and a/c units use variable-speed blower motors and electronically controlled, solid-state compressors, which put a pulse signal in the current. Then, “What the heck good is having 1% accuracy if you’ve got the wrong tool for the job?” asked Boyd.
Do You Feel Lucky?He called up audience members to take line measurements of a high-efficiency lighting demo unit using both averaging and true RMS meters. For the high-efficiency lightbulb demo, the true RMS reading was 2.5 A; the averaging meter was 1.8 A — a 30% spread. Such loads can affect the reading a technician takes for a compressor, he added, if they’re on the same load. And as electronics continue to grow in popularity, the chances increase that the hvacr load a technician measures has a pulsing electronic signal. “The washer, dryer, and my wife’s hair dryer are the only smooth loads in my house,” he commented. This has wide-ranging implications for the hvacr industry and could solve a few mysteries. For instance, compressor manufacturers say that 40% to 60% of the compressors that come back under warranty have no defects. Contractors have gotten a pretty bad rep over this, but while some poor installation practices play a role, is it as large as was assumed? “Thermostats and transformers also have electronic variable loads,” Boyd pointed out. “The majority of contractors use average responding meters.” (At this point, more than a few lightbulbs went off for the audience members.) “Say you’ve got an a/c system with a compressor and a motor with a variable-frequency drive,” Boyd continued. “You read the compressor’s vital signs — volts, amps, and resistance. The system is working, but the compressor readings are 30% low.” Diagnosis: Change the compressor. However, the new one has the same “problem.” “You may be working on a circuit with no electronics, but how can you be sure?” Boyd asked. In Dirty Harry, Clint Eastwood’s character asked, “Do you feel lucky today?” Technicians who encounter blown fuses also can’t assume that someone else hasn’t put an extra load on the a/c line. “In the real world, people tap off stuff all the time. One lady said that after the air conditioning guy came, she could only dry her laundry at night. That’s because the a/c guy tapped the condensing unit into the dryer line. “Say you get to a jobsite where they’ve complained of a lack of cooling,” he said. “A 40-Amp breaker has tripped. You pull out the averaging meter. The breaker reads 34 Amps.” Diagnosis: Change the breaker. However, a true RMS meter would have read 44 or 46 A. In this case, somebody put another load on it, but if the technician only replaced the breaker, he’d keep getting called back to the jobsite for the same problem, until it is eventually figured out or the customer calls another contractor.
Get The Word On The StreetSo why haven’t contractors and technicians heard more about this?
“Testing companies don’t know about compressor manufacturers’ returns,” said Boyd, who has worked at both a testing company and for a compressor manufacturer. He said he has made it his duty to share his insights with field technicians and service engineers.
Like Jake and Elwood Blues in The Blue Brothers, he’s on a mission.
Publication date: 04/09/2001