measuring low value resistors

[Prairie Dawg]

Does anyone have a good idea as to how to measure some 1 ohm 5 per cent resistors? I'm building a tube matcher and I have some 1 ohm 2w 5% resistors, but I would like to pick the closest of the bunch and my multimeter doesn't do well on values this small.

I've been thinking that maybe I could take something that my multimeter does OK on, say, a higher value, and then add one of the 1 ohm resistors in series to get a more accurate reading.

[MWJB]

put your leads together, say your meter reads 0.3 ohms as it's default reading at 0 resistance, just subtract 0.3 from subsequent readings.

Or use 10ohm resistors instead of 1ohm.

[Kazooman]

I don't know how many you need, and I'm not certain whether or not you need exaclty 1 ohm or if you need well matched resistors. There may be a better way, but you at least match pairs by setting them up in series as a voltage divider and use that to compare them.

[J M Fahey]

Put them all in series , pass some current through them (say, between 100mA and 1A) and measure voltage drop across each one.
Your multimeter is *very* good measuring voltages; reasonably good measuring current, and poor measuring resistance, so turn the last one into the first one.
Remember to Sharpie label them for later reference.
That said, 5% is way over the real precision you need there.

[Prairie Dawg]

I ended up ordering some 1/2 per cent 2w 1 ohm resistors from Mouser. That oughtta be close enough.

[jrfrond]

You really don't need THAT kind of precision (and I'm not a fan of the 1ohm resistor thing anyway), but it does help to have a meter around with a low-ohms scale and zero adjustment to null the test lead resistance.

[mooreamps]

You really don't need THAT kind of precision (and I'm not a fan of the 1ohm resistor thing anyway), but it does help to have a meter around with a low-ohms scale and zero adjustment to null the test lead resistance.

No, and in the cold light of day, you probably don't need that kind of precision... But for the benefit of the poster, perhaps it's just as important to understand how do a two wire resistance measurement of a low value impedance...

-g

[km6xz]

I agree about the need for precision, tubes, now, are not precision devices. Any resolution exceeding 2.5 digits is is wasted in an analog tube specification.

The way precision resistance measurements are done is with a secondary voltage reference....primary standard cells are not to source current but are used to calibrate secondary reference voltage standards. Everything is derived from that reference voltage.
An easier way it to use a 4-measuring system, most lab type ohm meters have a separate current source pair of terminals, and a pair of sense terminals. A 4 wire system simple has the sense probes not carry the reference voltage. That is the job of the second pair of terminals. That way the voltage drop measured by the sense probes are not suffering from the error induced by having the lead resistance add ambiguity. The reference voltage is supplied by the second pair connecting only at the resistance to be measured. I have a few with that capability, HP, Keithley and Philips PM2525 which is on and in front of me as
I type from my bench in my tiny apartment. Probe resistance usually varies a lot from measurement to measurement. Probe tips are subjected to arcing, oxidation, chemicals and physical abuse so are the worst conductors for repeatable readings.
All this is trivia however, you do not need this close tolerance resistors. If matching is all you need, ignore the value and focus on balance between any two. A Wheatstone bridge can be extreme sensitive to match and is easy to wire up with any meter you have as the indicator of null. It measures ratios very well. You want a ration of 1:1.

[Prairie Dawg]

You really don't need THAT kind of precision (and I'm not a fan of the 1ohm resistor thing anyway), but it does help to have a meter around with a low-ohms scale and zero adjustment to null the test lead resistance.

They weren't that expensive (.94 each) and the .5 per cent tolerance was about as good as they had available. If it works at all I'll be a happy guy. If not, I can convert it into a Marshall clone.