To do this job right you need very fine resolution on your meter
Following a few quick set-up procedures, the Fluke 187 or 87 is capable of measuring low ohms while compensating for lead resistance.
Half an hour ago, a key piece of production equipment stopped dead in its tracks - and the pressure is on. You've managed to get the system to give you an error code, but you can't identify what in particular is wrong. You call the equipment vendor, and explain what you've found. The vendor says, "Well, that can mean a bad connection at one of 16 different points, or possibly a circuit board crack. Or, it could even be a bad RTD input. If we could send a tech out there with a low ohms instrument, we could identify the problem."
You smile. Clipped to your belt is a Fluke 87. Your smile widens, because you also have a Fluke 187. And you know either instrument will allow you to measure low ohms. After getting some instructions from the vendor, you can start making measurements. But, do you know how to take such measurements accurately?
To do the job right, you can't use just any DMM. Resolution is one reason why. Many models of DMM have a resolution of only 0.1 ohms - the Fluke 87 and 187 do 10 times better than that. But at this resolution, another factor comes into play: lead resistance - which typically adds 0.1 to 0.3 ohms to your reading. So, the meter needs fine resolution and it needs to somehow compensate for lead resistance - or the measurement will be grossly inaccurate. The Fluke 87 and 187 provide both the resolution and the means of compensating for lead resistance. Before we explain how to take accurate low ohms measurements with your 87 or 187, let's look more closely at why you might make such measurements in the first place.
Making the measurements
This is actually pretty easy. Essentially, you measure the resistance of the test leads, then subtract that value from the actual measurement. You could do this with paper and pencil, if your meter doesn't have relative mode. But, then you would be working with the display resolution rather than the internal resolution - you'd get the kind of "rounding errors" people encounter when manually using results from calculators or spreadsheets. Plus, who really has the patience to do those calculations for every measurement? The easiest and most reliable way is to let the meter do the job.
First, set the meter to the Ohms mode. Next using the Fluke 87, push the yellow button (far left side) for one or two seconds (depending on the meter vintage) to place the meter in the high-resolution mode. The 187 will go there automatically. You now have 0.01 ohms of resolution on the 400-ohm range.
Next, short the test leads directly together, making sure to get solid contact. Don't try to short them, by, for example, clipping one to an enclosure bolt and the other to the enclosure frame - remember, we are measuring low ohms now. When the leads are shorted, the meter will display the resistance of the leads. At this point, press the Relative (REL) button. You'll see the display go to zero, and you'll also see a small triangle appear off to the left of the scale.
You can now make an accurate low ohms measurement, with those test leads "zeroed out." When you're done, short them together again and press the Relative button. You will see the lead resistance again. If you are making many critical measurements, short the leads between measurements and check that the display reads zero. There's one other thing you should do, to ensure accurate critical measurements. The Fluke 87 has an internal 1K precision resistor. Test your meter against this by connecting a test lead from the V-ohms jack to the microA/A jack while the meter is in Ohms mode. Don't forget to zero out that test lead, first - simply put the lead between the V-ohms and COM jacks and press the Relative button.
Keep in mind that dirt, oil, solder, flux, and corrosion that you can't even see can alter your resistance reading. Therefore, make sure you have solid contact between clean surfaces. If, for example, you are checking a grounding connection, you aren't interested in measuring the surface corrosion of the wires going into the connection. You want to be on bare, clean wire with your probes.
With your Fluke 87 or 187, you can easily make low ohms measurements while making sure that your leads don't get in the way.