Reduced resistance of resistor

[engrguy42]

Okay, science project time... 

I have a 0.5 ohm, 100 watt resistor. Applied current and voltage via my bench power supply, and got consistent values of 0.595 ohms.

Checked my multimeter...leads shorted gives an ohm reading of "0.0". Measuring the resistor gives a reading that stays pretty much on "0.6" but occasionally bounces down to "0.5".

So...hell, I dunno...I guess the only bit of useful info from that is if you want some accurate ohm readings at that low range try using your bench supply and a calculator since it will probably give V and I values with at least 3 digit accuracy? 

[GlennSprigg]

Everything is fine with that resistor. The letter 'R' shows the decimal point of the value:
R56 = 0.56 Ohm
5R6 = 5.6 Ohm
56R = 56 Ohm

So the value show on your meter is OK as with such low resistance values you have to take the resistance of the cables and such things into account. For example connect the cables together and you'll most likely see a value > 0.

Twoflower said it all. The 'R' replaces the decimal point, for readability !
This also caused some confusion over the decades, when photo-copied "Blue-Prints" of German circuitry showed 'Commas' instead of full-stops between numbers, as the 'dot' would often be lost in such old prints. Like... 25,6 Volts...

[Twoflower]

The commas in Germany are still used as decimal point. To be honest I don't know what the exact background of this difference is. The Wikipedia shows Germany is not the only one :-)

There's some additional potential pitfall: For German people
1.000 + 2,000 = 1.002,0
Solution to that: In Germany the point is optionally used as the thousands separator. In english that would translate to:
1,000 + 2.000 = 1,002.0
Confusing, isn't it? 

And with this the thread is officially derailed (but hopefully answered).

[engrguy42]

The commas in Germany are still used as decimal point. To be honest I don't know what the exact background of this difference is. The Wikipedia shows Germany is not the only one :-)

There's some additional potential pitfall: For German people
1.000 + 2,000 = 1.002,0
Solution to that: In Germany the point is optionally used as the thousands separator. In english that would translate to:
1,000 + 2.000 = 1,002.0
Confusing, isn't it? 

And with this the thread is officially derailed (but hopefully answered).

Yeah, lucky guess 

But how about my questions about multimeter accuracy with resistance?

[Siwastaja]

Just never ever use either , or . as a thousands separator and there is no problem. Even if you ignore the international aspect, both symbols look too much alike and can be mistaken ("100,000.00" and "100.000,00" visually suck even if both are technically well defined in some countries).

Use spaces instead - both "100 000.00" and "100 000,00" are unambigious to anyone, regardless of language background.

[Twoflower]

Oh, sorry, I missed the follow up question.

To measure low resistance resistors you can use better equipment of course. For better accuracy they use 4-Wire measurement methods (one set of wires apply the current while the second pair is used to measure the voltage across).

Or you do this using a bench supply as you suggested. Which comes with its own problems. As for example the power supply must be stable driving such high currents at low voltages (e.g. 1V with 1.78A). And you might do the voltage/current measurements with external meters (at least for cheaper supplies I don't trust the current readings untested). Ideal would be two meters. One for the voltage over the resistor (connected direct at the resistor leads) and one for current. This way you have the same setup like the 4-wire setup mentioned above. But again: You need to take cate about the accuracies of your meter. For example many meters lack a 2A range, so you have to use a 20A (or 10A) range. Which gets problematic with the +/-n digit error. Especially if you have only a low digit meter.

In general: Low resistance measurements ain't not easy if you want to get good and reliable results. Unless you have the right tools.

[engrguy42]

Thanks, but my big question was regarding multimeter resistance accuracy down in the range of the OP's resistor (ie, 0.56 ohms), and specifically what can be expected with his reading of 0.7 ohms. If the spec'd accuracy of the meter is 5 digits, does that mean the 0.7 reading can be off by +/- 0.5 ohms?

[Twoflower]

Argh, that was on page one. Sorry.

But your assumptions are right. Mainly the +/-n digits kill you in this case (+/-0.5Ohms in this case). And your calculations are right. The +/-m% ain't adding much (about +/-0.05Ohm). So the total value is 0.7Ohm +/-0.55Ohm. Which gives a total range of 0.15...1.25Ohm. Which is a huge error +/-467% if my math didn't fail me.

One digit more in resolution would gain much in the accuracy here.

[engrguy42]

Argh, that was on page one. Sorry.

But your assumptions are right. Mainly the +/-n digits kill you in this case (+/-0.5Ohms in this case). And your calculations are right. The +/-m% ain't adding much (about +/-0.05Ohm). So the total value is 0.7Ohm +/-0.55Ohm. Which gives a total range of 0.15...1.25Ohm. Which is a huge error +/-467% if my math didn't fail me.

One digit more in resolution would gain much in the accuracy here.

Cool, thanks. I was kinda addressing what you mentioned about the possible disparity between the OP's measurement of 0.7 for a 0.56 ohm resistor, and your response that it might be due to multimeter lead resistance. Seemed to me that it was more likely due to the poor meter accuracy at those levels. 

[Twoflower]

I haven't had looked up the accuracy of the meter itself. I should have done that, especially as I saw only one valid digit. So you had the right answer about the discrepancy of the reading. 

Somehow I drop such one digit measurements mentally and never think about it except how do I get a valid measurement. And then one can easily fall into the next problem. Like the mentioned contact and lead resistance.

[engrguy42]

Yeah, among all the speculations surrounding thermals and resistor properties and lead resistance and 4 wire measurements and European commas and so on, we seem to miss the elephant in the room...

According to mfr specs, a reading of 0.7 ohms can be off by +/- 0.5 ohms with many/most multimeters (my Klein has about the same specs as the Hioki, 1% +/- 5 digits).

 

[wraper]

It's not only about multimeter specs but also about lead and contact resistance.

[engrguy42]

It's not only about multimeter specs but also about lead and contact resistance.

Yes. That statement is sufficiently general and non-specific to be true and relevant in some cases.