Clamp Meter

Discussion in 'Maintenance and Troubleshooting' started by geome, Jan 6, 2011.

  1. geome

    geome Member Forum Leader

    Is a clamp meter capable of reading 600 volts AC and 600 amps AC sufficient for testing our units (Envision 2-ton split, 3-ton packaged unit, 10kW & 15kW heat strips)?

    Also, the meter I am looking at can measure current inrush, so I want to make sure I don't make a mistake and buy a 600 amp meter when I should have bought the 1000 amp meter in order to use this feature on the compressors.

  2. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    I employ a very inexpensive meter ~$30. It does everything I need it to and even has a thermistor for temp measures. 600V and amps is fine, but don't spend more than about $50.
  3. geome

    geome Member Forum Leader

    Is this a clamp meter, or a multi-meter with a clamp accessory? Could you be specific about a reasonably priced model? I'm having trouble finding a reasonably priced model that can measure current. I prefer to use a clamp as opposed to probes (I don't want to touch the wrong thing.)

    I thought a model with an inrush feature would be helpful in determining a compressor problem before the compressor gives out. Am I on the wrong track, or are there other ways of determining compressor health with the less expensive meters?
  4. dgbair

    dgbair Just a hobby Forum Leader

    I picked up a cheap one from Harbor Freight for $20.. how knows how long it will last. :D Seems ok for what I want to measure.

    Digital Clamp-On Multimeter
  5. geome

    geome Member Forum Leader

    Thanks dgbair. Funny how I never think of HF for this type of thing.

    Is having a True RMS meter better than not for my purpose (testing geothermal?) Where would not having a True RMS meter be a problem when working on other things in the house? Can you tell that I really want to just make this purchase once :)
  6. teetech

    teetech Member Forum Leader

    I don't think an amp meter would be very useful in determining compressor health.
    A "decent" ohm meter would be of more value for checking compressor windings.

    You get what you pay for when it comes to accuracy.
  7. Looby

    Looby Member Forum Leader

    A true RMS meter is useful mainly for dealing with highly non-sinusoidal
    waveforms -- e.g., square waves, half-wave rectifiers (dimmer switches),
    cheap A/C generators, or some DC-to-AC inverters.

    I wouldn't expect true RMS meters to have any advantage for making
    measurements of stuff from the local electric utility. With well-formed
    sine waves, the various tricks/calculations used to make "true RMS"
    corrections are more likely to degrade accuracy than improve it.
    Last edited: Jan 8, 2011
  8. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    What he said. With accurate amp draw and a verified capacitor, compressor performance is well indicated.
    Last edited: Jan 8, 2011
  9. Verdae

    Verdae Member

    There is apparent power, this is what a standard volt/amp meter measures, and there is actual power, this is what the electric meter on the building measures. Then there is a thing called power factor that compares apparent power to actual power. A compressor has a power factor of about .82 until a run capacitor is added. A properly sized and functioning run capacitor will allow the compressor to run with a power factor of about .92. I was told by one engineer at a large compressor manufacturer that failure of the run capacitor is a major cause of compressor failure.
    To measure actual power the angle between the apparent and actual power has to be measured, this is where the true RMS meter comes in. However, the meter needs to be able to also measure power factor in order to measure actual power. A watt meter is supposed to be able to do the calculations and give actual power. The cost of these is going down dramatically but are currently still in the $200-$300 range.
    For most applications measuring apparent power is enough. Although I would suggest testing the run capacitor as well. Be sure to replace a run capacitor with one supplied by the equipment manufacturer as many after market capacitors are jun and the capacitor must be properly sized to work.

    Clamp on current meters need to be properly aligned and closed completely or they will give false readings and misalignment can give significant errors. They are generally good enough for diagnostics when used by an experienced service person.
  10. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Yep, Lloyd has it as I mentioned above with the "verified capacitor". A cheap amp clamp and a good capacitor tester would be my pick.
    I would also test any replacement capacitor, but don't share the belief that they must come from the heat pump manufacturer as long as they are a match.
  11. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    I need a good capacitor checker.

    Capacitors drop like flies around here.
  12. geome

    geome Member Forum Leader

    Regarding true RMS, does the following make any sense? I treat sales literature, even from Fluke with some skepticism.

    "Now bear in mind, many of these electronic controls involve nonlinear loads. Essentially, any control system containing semiconductors in the power supply or in the controllers would be considered a nonlinear load. Normally, when troubleshooting an HVAC equipment failure or nuisance trips due to an electrical problem, your first instinct would be to check the panel for tripped circuit breakers or overloading. However, if a nonlinear load, such as a new high efficiency heat pump, is on that circuit, you’ll need a truerms test tool to accurately measure the true load current to determine where the problem is—is the circuit faulty, is it overloaded, or is the problem with the load itself? The photographs demonstrate a standard averaging meter and a true-rms meter. Notice that the difference between the two meter readings is approximately 32 percent."
    Last edited: Jan 8, 2011
  13. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    I doubt that's an issue on any standard single or dual stage heat pump. Working on an inverter-based true variable capacity system may trigger that issue. I will keep it in mind when I go to upgrade my clamp.
  14. Looby

    Looby Member Forum Leader

    On second thought, true RMS might make sense regarding ECM blowers;
    (I don't know for sure, I've never looked into how the EC is implemented).

    I believe geo compressor and circ pump motors use garden-variety
    relays (a.k.a. "contactors") -- with no transistors/triacs in series --
    so those loads should be essentially linear.

    BTW, true RMS voltage and current measurements must still be
    corrected for reactive loads (power factor). RMS and power factor
    are separate subjects (except that the definition of "phase angle"
    gets kinda fuzzy for non-sinusoidal waveforms).
  15. geome

    geome Member Forum Leader

    Thanks for bearing with me on this.

    It sounds like the following features may be good to have (either in one or multiple units):

    possibly true-rms
    What is the minimum volt and amp capacity that should be considered?
    How about a thermocouple?
    Anything else?

    What brands do you guys like (Fieldpiece, Fluke, etc.)?

    Regarding power factor, is this something that techs consider when comparing readings to the unit's specs listed on the label on the machine? If yes, how?

    Do you look at safety ratings (CAT III, CAT IV, etc.) when making a choice between models?
  16. Looby

    Looby Member Forum Leader

    If you're looking for a clamp type meter, be careful to check the
    measurement ranges for simple things like ohms, DC mA, etc.
    Some "entry level" clamp-ons have ridiculously limited ranges;
    for example, less than 10 kOhm maximum.

    You might be better with a minimal clamp-on plus a regular DMM.

    Fluke is good, also check Agilent, (formerly Hewlett-Packard,
    until HP spun-off their Electronic Test & Measurement Group,
    about ten years ago).

    Full disclosure: retired 30+ year HP & Agilent employee.
    Last edited: Jan 9, 2011
  17. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    I think most techs have a DMM and a separate clamp. The two resistance measurements of interest 97% of the time are zero and infinity
  18. geome

    geome Member Forum Leader

    Thanks guys. I'll shorten my list of questions to just one:

    What is the minimum volt and amp capacity that should be considered for a clamp (for testing geothermal systems)?

    But anyone can still feel free to answer the other questions too. :)
  19. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Your system likely has nothing bigger than a 10k coil so highest amp draw would be LRA of ~100amps.
    Volts 250.
    Cheap clamps will cover ya.
  20. geome

    geome Member Forum Leader

    Ok, I'll look for a cheaper one. :)

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