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  1. #1
    Join Date
    Mar 2007
    Location
    San Diego CA.
    Posts
    16

    measuring 4-20 ma

    A simple question what is the proper way to measure a 4-20 ma signal. Do you have to skin the wire that the signal is coming in on or do you read accross the signal wire and the com wire?

  2. #2
    Join Date
    Aug 2007
    Location
    Western Colorado
    Posts
    497

    resistor

    Attach a 499ohm (500ohm) .5 watt resistor to the end of your test leads, set the scale to dc volts, take your reading at the sensors common and signal terminals, then convert back to milliamps.
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  3. #3
    Quote Originally Posted by rad1 View Post
    Attach a 499ohm (500ohm) .5 watt resistor to the end of your test leads, set the scale to dc volts, take your reading at the sensors common and signal terminals, then convert back to milliamps.
    If you do this (and the sensor wires are still landed on the device which is reading the sensor) you will have a parallel resistance circuit, you will not get the right reading.

    If the sensor wires are still landed, you should be able to just land your meter acros the input terminal (and set to voltage)

    If you want to get a real mA reading, you need to wire your meter in series with the circuit (and set to mA)

    if the sensor wires are not landed on the reading device (IE. controller), then you can do the 499 ohm resistor circuit as described

  4. #4
    Join Date
    May 2002
    Location
    Baton Rouge, Louisiana
    Posts
    3,924
    If your ready to drop about 400 dollars(which I did) you can purchase this


    http://us.fluke.com/usen/products/Fluke+771.htm

  5. #5
    Join Date
    Mar 2005
    Location
    Virginia
    Posts
    1,144
    The Fluke 771 is without a doubt very nice, but you can find it for 200-250$ US.
    “It is impossible for one to learn what one thinks they already know"

  6. #6
    Join Date
    May 2002
    Location
    Baton Rouge, Louisiana
    Posts
    3,924
    Quote Originally Posted by codewriter View Post
    The Fluke 771 is without a doubt very nice, but you can find it for 200-250$ US.

    dam code, dont hold out on me, where at?

    I may eventually get the Pressure meter and its 400 bucks too

  7. #7
    I have seen them for about $375, but never that low. Ill get one in a second for 200

    Do you still tink it was worth the investment Joey?

  8. #8
    Join Date
    May 2006
    Location
    Western NY
    Posts
    4,356
    Quote Originally Posted by rad1 View Post
    Attach a 499ohm (500ohm) .5 watt resistor to the end of your test leads, set the scale to dc volts, take your reading at the sensors common and signal terminals, then convert back to milliamps.
    Hmmm....interesting thread. How do you covert back to ma?
    "Nearly all men can stand adversity, but if you want to test a man's character, give him power."
    -Abraham Lincoln

  9. #9
    Join Date
    Jan 2008
    Location
    Near Philly
    Posts
    514

    m-20 ma reading

    Kele had an excellent 4-20 refresher in the last newsletter, check it out, page 5/8 on the Kele web site.

  10. #10
    Join Date
    May 2002
    Location
    Baton Rouge, Louisiana
    Posts
    3,924
    Quote Originally Posted by freddy-b View Post
    I have seen them for about $375, but never that low. Ill get one in a second for 200

    Do you still tink it was worth the investment Joey?
    Its a nice meter, even though I havent used it.

    My issue with 4 to 20 has always been how you had to properly check what your reading, thats why I use 2 to 10 98% of the time. With the 771 you dont have to worry about taking a wire loose and possibly missing the problem if its a bad connection.

    Mine takes a little while to turn on, about 20 seconds, it has a LED built in that seems to light up pretty good that points towards the clamp. It comes with a soft canvas case(I would have preferred the grey leather case most people put there meters in for that price point). The clamp is removable and there is a velcro strap holding part of the lead(I dont know how long the lead is, I dont undo stuff, I try to keep it like original, of course thats one of the benefits of being OC )

    Freddy, I have been an a/c tech first, controls second, until last year now that I'm doing mostly controls. Since then even though my company furnishes my tools since doing controls I have been buying all my tools related to it, my cart, conduit benders, etc. , just because I wanted to own my tools myself(I take pride in knowing those nice tools are mine ) and the 771 just seemed like a must buy to me, just like I want to get this
    http://www.testtools.fluke.dgtlpub.c...0498/home.html (the Fluke 922)

    I also picked up one of these at Home Depot, they were cheaper than the regular Tic Tracer, higher quality pouch, and I have used it and really like it.
    http://www.flukenetworks.com/fnet/en...ategorycode=TP

    sorry for the long winded post

  11. #11
    Join Date
    May 2006
    Location
    Western NY
    Posts
    4,356

    Thumbs up

    Quote Originally Posted by mechdorn View Post
    Kele had an excellent 4-20 refresher in the last newsletter, check it out, page 5/8 on the Kele web site.
    Thanks, mechdorn. I'll check it out.
    "Nearly all men can stand adversity, but if you want to test a man's character, give him power."
    -Abraham Lincoln

  12. #12
    Join Date
    May 2006
    Location
    Western NY
    Posts
    4,356
    Good read. Here it is for others who are interested. Scroll down to page five.

    http://www.kele.com/webservices/arch...s/Fall2007.pdf
    "Nearly all men can stand adversity, but if you want to test a man's character, give him power."
    -Abraham Lincoln

  13. #13
    Join Date
    Aug 2007
    Location
    Western Colorado
    Posts
    497

    O.m.i.f.

    My apologies to coremech. I spoke before I thunk.
    You can use the method I mentioned to meter a controllers 4-20ma AO. To measure the AI, I agree with choosewisely; you need to put your meter in series with the sensor.
    RealEyes
    Realize
    RealLies!




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