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Registered Member #621
Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119
Why does my Fluke read 0 ohms when i set to impedance and touch both terminals together, but from my understanding and readin 0 ohms doesn't exist except in superconductance. If you put 0 ohms into ohms law formula you get 0 current flow, would't current flow perfectly free and fast with 0 ohms resistance? So what number would I put into a formula if I had NO resistor...instead of 0?
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
You are asking 2 different questions here, 1. There are 2 possible reasons for your fluke to read 0 ohms (it is probably a combination of both of them), the first is that it has a limited precision (probably only to the .1ohm) and that there is is an offset adjustment on it to account for the resistance inside the meter, probe resistance, etc. When you just touch 2 wires together, if they are decently clean, you should get about .02ohm (very rough, it depends a lot on pressure, etc) which if the meter only reads to the .1ohm will show up as 0ohms. It is probably also calibrated so that the resistance of the probes are subtracted from the reading.
The second question is a little harder. Depending on what you are calculating there are different values you can use. For some things, 0ohms is a decent approximation, like when you are calculating the resonant frequency of a LC circuit. For other things, a 0r estimate will wreak havoc, so you need to take your best guess at it, and use that. Most of the time when you are dealing with very low resistances you are dealing with a piece of wire, so you can just look up the resistance/foot in one of the numerous charts out on the net.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
You are right, the 0 ohms isnt really true... but compared to say 100 or 1000 ohms (which is what most meters are accurate at), its not much error from what the resistance really is (likely around .01 ohms for a multimeter and leads).
If R is zero, then I tends to infinity (not zero).
If you need to know the resistance of a conductor, you may wish to search for either a resistance chart for various wire gauges, or if you are working with a different form factor (buss bar, PCBs, or something else) you can work out the resistance by knowing the materials resistivity, its cross sectional area and the length. I think its just (resistivity*length)/area (make sure your length and area are the same units).
Registered Member #621
Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119
Wow, thanks for clearing that up for me. See I was thinking that a piece of straight wire would be 0 ohms, obviously the ohm scale covers a very vast region of values then. I appreciate you guy's pateince with my lack of alot of beginning knowledge. I've always played around with this stuff (i had a little bug zapper step up transformer i ripped from my dad's old bug zapper when I was a kid. I hid it from my parents I used to burn stuff with the arcs LOL!!!) But I skipped past alot of the fundamentals I now need in my project, I'm gonna look for a more beginner forum to post more of these types of questions so I'm not wasting space.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
For low resistances, take a measurement with just the probes together. This is the resistance of the probe wires, and then take the measurement of the device. Subtract the two to get a better measurement. Of course assuming your meter has the resolution.
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Zero Ohms
