# Ohms(resistance) Testing



## John (Jan 22, 2007)

wallace7777 said:


> I am not sure where there question would be best suited so I am goign to start here.
> 
> I am working on a Engine Test Probe for a Pratt and Whitney Engine however, my question is more in general Ohms Testing of an Electrical Circuit.
> 
> ...


You may need a Milliohm Meter and the resistance bridge is part of the measurement process for a Milliohm Meter. With a Milliohm Meter you use 4 test leads to get an accurate measurement. Some meters will measure to a resolution to 100µÙ and you might want it to be NIST calibrated due to what you are working on.


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## micromind (Aug 11, 2007)

For a simple test such as this, with the type of test equipment you're using, when the probes are shorted together the reading becomes the base, and must be subtracted form all subsequent readings. 

Using 0.3 Ohms with shorted probes as an example, if the actual test read 0.5 Ohms, then the resistance of the device under test is 0.2 Ohms. 

Some DMM's have a zero Ohms feature, most don't. 

A miliohm meter is the best way to test low resistances, as stated above. 

Rob


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## wallace7777 (Feb 17, 2007)

*Thanks for the reply guys!*

I do have everything calibrated to NIST as I use MIL-STD-45662A for my program and the FAA requires this for all of our equipment.

I actually think I have a Milliohm Meter, I will check it with that here in a few to see what I get.

What is the purpose of a resistance bridge and how is it adjusted?

Thanks for all your replies guys!

Eric


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## John (Jan 22, 2007)

wallace7777 said:


> I do have everything calibrated to NIST as I use MIL-STD-45662A for my program and the FAA requires this for all of our equipment.
> 
> I actually think I have a Milliohm Meter, I will check it with that here in a few to see what I get.
> 
> ...


The purpose of the bridge is to accurately measure the low resistance. I do not believe that it needs to be adjusted because it is part of the meter electronics.

Since we gave you some information, how about showing us some pictures of what you are working on? :thumbsup:

We are here to learn also.:whistling2:


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## wallace7777 (Feb 17, 2007)

*Thanks!*

I will post picks of what I am working on when I get the resistance bridge in. I have worked in the Aviation field for almost 20 years and am constantly learning new things, keeps me interested. What ever happened to the days when someone works 20 years and tehy retire... I would be there already! 

Thanks again guys!

Eric


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## randomkiller (Sep 28, 2007)

wallace7777 said:


> I will post picks of what I am working on when I get the resistance bridge in. I have worked in the Aviation field for almost 20 years and am constantly learning new things, keeps me interested. What ever happened to the days when someone works 20 years and tehy retire... I would be there already!
> 
> Thanks again guys!
> 
> Eric


 
I think most of those guys past away shortly after coming to a full stop. I intend to retire into another job.


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## John (Jan 22, 2007)

The day you stop learning is the day that you die. I intend to find something more enjoyable and less wear and tear on the body.


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## randomkiller (Sep 28, 2007)

John said:


> The day you stop learning is the day that you die. I intend to find something more enjoyable and less wear and tear on the body.


 
Ditto, that is what I have in mind.


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## nick (Feb 14, 2008)

*ohms testing*

i agree with John , if your using a standard digital ohm meter ,you can not measure less than one ohm .you may think you can ,but it is not going to happen in this life time . you need a milli ohm meter or a micro ohm meter john you can correct me if you like ? but its called a[ Klevin connection ]you have 2 wires on each test lead and must be a good clean and solid connection when you test .this will read less than one ohm. even the temperature of the wire or connection is important and time you test meaning length, of the test . we use this to test transformer windings and breaker contacts or transfer switches it will detect a problem if they are not making good contact like old breakers or bad contacts. because it will measure 1000ths of a ohm or millionth of a ohm and its the only way to go.your standard meter can read .400 .300 .200 but that is as low as that meter gos ,your meter . it is not what you are actually measuring it only gos that low and stops . but yes if you put your leads together and see .300 thats just giving you the lowest reading with that meter it would be a short for that meter but not a actual short , that other tester is a wheatstone bridge. but this is not for less than one ohm its for medium or high resistance testing . but back to your meter it is not a short on something less than one ohm . example in understanding a four wire test if you put[ one amp] thur a metal object or contact or a wire what ever ,and apply[ one volt] to this metal object you are measuring the voltage drop across it, ohms law . thats your real resistance the voltage drop of that metal undertest ,and thats why its a four wire tester volts / amps= resistance one volt one amp =resistance thats how a milliohm meter works but they use milli volts milli amps to accomplish the same results . hope this helps iam not a expert just pull feeders and run pipe all day , but best to ya good luck.


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## jbfan (Jan 22, 2007)

John said:


> The day you stop learning is the day that you die. I intend to find something more enjoyable and less wear and tear on the body.


Fishing comes to mind for me!:thumbup:


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## John (Jan 22, 2007)

jbfan said:


> Fishing comes to mind for me!:thumbup:


I don't like fish(to many fish sticks when I was younger), so fishing is not going to be a good choice for me.:no:


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## nick (Feb 14, 2008)

*resistance bridge test*

just a added input , and question for you is this resistance bridge test calling for a dc test or a ac test? just a little input after thinking on this the bridge test is a high ohms test more than one ohm meaning your part is bad . i used to work on 400 cycle power generators in the military ,we had a tester for terminals or connectors , was for power support stuff it had test resistors of precise one ohm .001 ohm ect ect . you could adjust this tester in the field to match your test area , meaning adjust voltage adjust current of test item. like 400 cycle stuff which needed testing every month it was basic stuff but highly precise , better than most testers i have seen today , could be used on anything needed, did your sensor check out or what happen?


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## ohmontherange (May 7, 2008)

A good way to measure low resistances is to apply a known DC current across the load and measure the voltage drop across it. I say DC assuming the load is resistive if there are reactive components involved and AC current used we are really talking impedance. I know, I know one in the same... kind of.

Using simple Ohm's law you can calculate the resistance.


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## nick (Feb 14, 2008)

*ohms law back to basics*

ya i agree , playing around in my shop we got a home bru current regulator and a voltage regulator from old junk parts ,1 amp output 1 volt output . can adjust to milli amps or milli volts also . like building stuff its a habit of mine .we can take a roll of 10 wire or 12 wire or 500 mcm and i read total ohms reading by voltage drop . we than measure the one foot of any size and divide that into the total measured ohms of the roll,guess we got a cable length meter home made for 30 dollars in mostly scrap parts , and they sell these for high dollars ,its pretty close can get within two or so feet of actual length .and will measure copper or alum, wire any size any length ,and you just need a sample one foot of wire than get your total and then calculate out. meaning divide one foot ohms into the total ohms ,i used a greenlee cable length meter to test my home bru and it was the same the only thing mines not good to take out to the job, take care best to ya


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## brian john (Mar 11, 2007)

Could use a DLRO/DUCTOR/micro-ohm meter.

http://www.metercenter.com/cgi-bin/...wdcaklun121454213426&command=link--bi3049.htm


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## nick (Feb 14, 2008)

*dlro*

yes you most certainly can, we have one at work they use ,but i use my homemade one for small projects like pin sockets for microchip stuff 5volts meaning the tiny pins and connections just fun stuff , but just for kicks we checked it out on wire spools to see if it would match the greenlee length meter , and i can not afford a real micro ohm meter yet, so i build my own basic testers just to get by .take care best to ya brian john


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## ohmontherange (May 7, 2008)

Brian John - You sir have all the toys.


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## 76nemo (Aug 13, 2008)

Some DMM's such as some of the Fluke's have a relative function. If you place your test leads together firmly and press "Relative", the DMM will automatically take into account for it's leads resistance and give you a much accurate reading after being placed across the load.


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## frank (Feb 6, 2007)

For the best measure at the low levels you are working at Wallace I would use a valve (tube/ solid state mains powered ohmmeter). These things are just great. They measure resistance by injecting a dc and self adjusting prior to display.For bench use there is nothing to beat them.To tell the truth I like the old analogue type of meter best even though I have bag full of DMM's. They are steady in response and and are not subject to the fluctuations you get with DMM. But then - to everyone his own as they say.

Frank


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