checking a wire with an ohm meter

Is there a way to establish what the resistance is on a given size stranded wire of a known length? If so where to look for the answer.
Thanks Angle Iron
 
There are plenty of wire tables and calculators you can use to calculate the resistance of a conductor of known gauge, length and material. But your subject line implies that you want to measure wire resistance with your ohmmeter. That's a bit trickier, because the resistance of a typical wire is far too low to measure accurately with a common multimeter. There are lab meters that can measure down in the milliohm range, but no handheld meter has that capability.

What's the problem you're trying to solve?
 
A friend was given a resistance reading to verify if a wire on a exit safety loop for a commercial gate opener/closer was still good. I believe he told me somewhere in the mid 30s was the # he was told and he said it was 25 or so on the meter when he tested it. He is chasing a problem with very little to go on and no reference data on the unit. I found a O&M manual to help him. It has a short trouble shooting section that may help a little. The question that came to me was how could they give him a resistance on a wire that they do not know the size or length of? This was a tech rep for the gate opener MFR that has not seen the install so does not have this info. If I understand how it is run you cut a groove in the driveway, run a single run of stranded wire from the controller out to the loop witch can be anywhere from 4ft by 8ft to perhaps 4ft by 18 ft. you run three passes in the groove then back to the controller. If I did the math right the difference between 8 ft wide and 18 ft wide in this set up would be over 200 ft of wire so the question about a set resistance came up. So that's the reason for the question. I do not know how these safety loops work and was trying to get a little help understanding a small part of it.
Thanks Angle Iron
 
Do you have a meter with inductance measuring ability, hook
up to the loop and pass a big metal shovel across it and see if
you see it, if you see it the loop controler should see it.
 
As an electrical engineer, if somebody told me a conductor's resistance should be around 35 ohms and I measured 25, I'd say "close enuff". If it was off my an order of magnitude (ie. less than 4 ohms or more than 350) I'd be concerned.

Understand that it's not the actual resistance you're concerned with. What you need to know is whether the loop is open or shorted, and you don't need an accurate measurement to determine that.

Inductance loops sense the presence of a vehicle by detecting the change of the loop's inductance when a vehicle enters the loop. But you can't measure inductance with a typical multimeter; they measure resistance using a DC current source, and you need to apply AC to the loop to measure inductance. But a resistance test will tell you if the loop is open or shorted, which is all you really know. (You could use an inductance meter or bridge to measure the actual inductance, but not too many folks own such instruments.)

Make sure you also check for a short to ground, as described in the information posted by Cliff.
 
I want to thank everyone for the help and info. I will pass all this along to my friend and I am sure it will help. I have a much better understanding of how it is supposed to work now. That really isn't saying much because I had no clue before. The thing that puzzled me was a single set answer to the resistance the tech gave him got me interested. Again thanks I'm sure this will help.
Angle Iron
 
exactly.

Resistance is generally a pretty sloppy measurement.

Even resistors used in electronics are commonly rated at 10% tolerance.

Meaning if two resistors are within 10% of each other, they're close enough to be considered the same value.

Unless there are test specs that specify an upper and lower limit - I'd guess you're close enough.
 
I think he said they were going to work on it again next week. I did give him the O&M manual along with the info on the three links Cliff posted. He will read it this weekend and go from there. He did say he could check impedance with his meter so we will see how it goes. I am not sure just what is going on but think maybe a gate/door opener company has already spent some time on it and the customer gave up on them. My friend and his brother take care of other maint on their equipment, trucks, trailers and other items. They have already fixed a couple problems and I am not sure what is going on with the safety loop but they are working on it. He does ok but is not willing to learn to use a computer. And as long as he can get me to spend my time, ink and paper I guess he doesn't need to.
The flip side is I haven't paid any labor bills on repairs for a while
Thanks again Angle Iron
 

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