RMS multimeters

Discussion in 'Landscape Lighting' started by irrig8r, Nov 9, 2007.

  1. irrig8r

    irrig8r LawnSite Platinum Member
    Posts: 4,535

    Can't remember in which thread someone brought this up, so I did a seach on it and found this page. Looks like it was written by a distributor (see last paragraph).

    Here's the webpage where I found it:


    http://www.globaltestsupply.com/article-090806.cfm


    How Accurate is Your Multimeter and what is True RMS?

    Every electrical technician knows the difference between DC (Direct Current) and AC (Alternating Current). Every electrical technician also realizes the importance of taking accurate current measurements to protect conductors from exceeding their insulators' ability to withstand heat or assuring devices under power work properly. However, does every electrical technician realize that electrical current measurements aren't always what they appear to be?

    Direct Current (DC) is straightforward. When we use a multimeter to measure direct current, it is what it is. However, the plot thickens when we are dealing with Alternating Current (AC). AC current travels back and forth down a conductor and can best be described in graphical format. The most common graphical description of AC current is a sine wave. Because the amplitude of the sine wave continuously changes over the wave period (one complete cycle), at any given point in time, a current measurement would not be the same. Therefore, how do we accurately measure AC Current?

    One method to measure AC current would be take current measurements at increments across one complete cycle and average them together. This would give us an average value of the current. If the current is a perfect sine wave, mathematically, the average value is always 0.636 times the value of the peak amplitude.

    Another method to measure current is based on the current's ability to perform work when applied to a resistive load. The laws of physics tell us that when current passes through a resistive load, it dissipates energy in the form of heat, mechanical motion, radiation or other forms of energy. If the resistive load is a heating element and the resistive load stays constant, then the laws of physics tell us that the heat produced is directly proportionate to the current passing through the load. Therefore, if we measure the heat, we will know the current.

    Mathematically, the relationship between heat and current is such that the heat produced is proportional to the square of the current applied to a resistance.

    (Power or Heat) = (Current) ^2 * (Resistance)

    If the current is continuously changing, as in AC current, the heat produced is proportional to the average (or mean) of the square of the current applied to a resistance:

    (Power or Heat) = Average [ (Current) ^2 * (Resistance) ]

    Using algebra, the above formula can be rewritten to read:

    Current = Square Root [ (Power or Heat) / (Resistance) ]

    AND this is called the Root Mean Square Current or RMS Current.

    For AC currents that are graphically represented by a sine wave, the RMS current will always be 0.707 times the peak current. With that said, we can calculate current by multiplying peak measurements by 0.707 if the current is a perfect sine wave. However, perfect sine waves are rare in most commercial and industrial applications. This is because resistive loads in commercial applications are not linear which results in unpredictable or variable current requirements.

    In order to get a True RMS measurement, we can measure the heat dissipated by a constant resistive load and perform the above calculations. The result is a True RMS measurement.

    Now that we got all the technical discussion out of the way, which is the best method to calculate current? Should we 1.) measure a current average 2.) multiply current peaks by 0.707 to get an RMS current, or 3.) measure the heat from a resistor and calculate a True RMS current value?

    Although Global Test Supply sells multimeters that can calculate current using any of the above methods, the most accurate way to calculate current in my opinion is a True RMS method. Average current values often are 40% less than True RMS values and that could mean the difference between blown circuit breakers, malfunctioning motors, or worst case, potential fire hazards. True RMS multimeters only cost about 20-30% more than the alternative. How much is an accurate current reading worth to you?

    About the author:
    Robert Preville is the President and CEO of Global Test Supply, LLC, a distributor of test and measurement equipment, and welcomes question from those looking for a True RMS Multimeter.
     
  2. JoeyD

    JoeyD LawnSite Silver Member
    Posts: 2,933

    Funny you post this Gregg. I was laying with a mini 75 w TF that converts 120v to 12v. I could not figure out why I had a 35 w lamp that looked to be burning at optimal range but when i tested it with my VM it read that it ws only gettting 4v. It wasnt until my product engineer saw me tripping out on this that he told me that I needed a RMS volt meter to test it. I still dont understand why but it is coincidental that you made this post only 2 days after I went through this.

    Joey D.
     
  3. Chris J

    Chris J LawnSite Silver Member
    Posts: 2,837

    As you lay there, were you smoking a cigarette and telling the transformer how much you loved her? :laugh:
    You and your mis-spelled words kill me sometimes!
    Funny stuff Joey!
     
  4. ChampionLS

    ChampionLS LawnSite Bronze Member
    Posts: 1,066

    I went over this a few months ago.. could probably find it in past discussion. I use a Fluke #87 True RMS Multimeter, and whether it be true or not, I still use a analog multimeter (the one with the pointer) because at times, its easier, accurate and less subject to the criteria as outlined above.

    -Anthony
     
  5. Pro-Scapes

    Pro-Scapes LawnSite Platinum Member
    Posts: 4,180

    This is where you seperate the men from the boys. Do yourself a favor... use a good quality true RMS meter. Make sure you have a clamp on to read amps as well.

    Anthony this is NOT the type of info we need to pass on to future guys installing lighting. Using an analog meter is about 1 step above using a 12v automotive test light or no meter at all. About 12v is not close enough if your serious about lighting
     
  6. ChampionLS

    ChampionLS LawnSite Bronze Member
    Posts: 1,066

    Well Billy... I got some bad news for you. Unless your planning on generating your own power, and regulating your voltage to within that fine spectrum your talking about, there's not much you can do.

    According to the FPL (Florida Power and Light), The Utility Companies can crank out anything in-between and at any time.
    Heres what they say:

    What is a voltage fluctuation?

    A voltage fluctuation occurs when there is a dip or spike in the electrical flow to your home. The state of Florida requires that utilities normally operate within a range of +/- 8 volts (112 - 128) from standard voltage (120), but FPL prefers to keep this range within +/-6 volts (114 - 126). Fluctuations of more than 6 volts may cause your lights to dim or brighten.

    I am sure this occurs Nationwide, and is partially why most transformers have voltage taps that are in 0.5 to 1.0 volt increments.

    Check the voltage at your home each day for one week and see how much it changes.

    -A
     
  7. Pro-Scapes

    Pro-Scapes LawnSite Platinum Member
    Posts: 4,180

    im sure your sweeper needle meter will pick up these fluctuations better than my RMS meter.

    I always check the line voltage at the house and I also go a step further and make sure the AC is on when I check it. Around here its pretty stable in any given nieghborhood. I have seen anywhere from 117 to 123 without much fluxuations. And yes I check the line voltage at each and every maint visit and especially thoes with multiple burned out lamps.

    I like you anthony but you can keep your pierce points and 0-6inch methods and buy all the analog meters you want. I usually rather enjoy your interpretations of how to do things per code and accuratly.
     
  8. irrig8r

    irrig8r LawnSite Platinum Member
    Posts: 4,535

  9. irrig8r

    irrig8r LawnSite Platinum Member
    Posts: 4,535

    BTW, I used to have a Greenlee which I think was true RMS, but it disappeared when I was careless at a job site.

    :hammerhead:
     
  10. ChampionLS

    ChampionLS LawnSite Bronze Member
    Posts: 1,066

    lol...no problem there Billy. I know you do good work. I'd hire ya. :drinkup:
     

Share This Page