Measuring ohms resistance

[Busker]

I have two old volt/ohm meters. One is an old RCA, which looks like it was good quality, the other is an el cheapo Radio Shack. I'm not sure I'm using them correctly, nor am I sure these things are calibrated correctly.

With a fresh battery and the switch on Rx1, I touch the two leads, zero the meter needle with the zero knob, and hook up the two leads to a pot. A pot that I'm pretty sure is 250K measures around 300K. A pot that is MARKED 500K (so I already know what that one is) measures around 1500K (hooking the red lead to the center lug, the black to an outside lug)

Are my meters bad? (they are old). Do pots vary a lot, or is a 250K pot supposed to measure exactly 250K?

Thanks

[Stan Hinesley]

Try hooking the leads to the outside terminals. And no, rarely will you find a pot that is exactly what it is rated. Most of them have a tolerance of +/- 10-20%.

[Rune Bivrin]

300K for a 250K pot is ok, +-20% isn't too bad. 1500K for a 500K pot isn't good at all, but it may well be the pot. The center lug is connected to the wiper, and if the contact inside is poor you might get a higher value. See what you get between the outside lugs.

While old analog meters have their place, measuring resistance isn't really one of them. You're doing it correctly, but since you're not getting reasonable results these meters should be relegated to the museum.

Get yourself a cheap digital multimeter. However shoddy quality, it's still going to work better for you than what you have.

[Jo6Pak]

I would also suggest using a higher scale than Rx1. Ideally, the needle should be near the center or somewhat to the high side of the face when making the measurement. Also, remember to re-zero when changing scales. Old analog meters are fine for making non-precision measurements such as audio taper pots. You should be fine in that regard...

[Diogenes]

What are the readings of each end of a full sweep of the pots?

Check the accuracy of your meters by measuring a good resistor... pots are essentially variable resistors and not good for basing the accuracy of your meters on.

[Rick Auricchio]

I presume after zeroing the meter you are selecting a reasonable ohms scale, e.g. Rx10k, Rx100k...

[Busker]

Hey all. I got a digital multi tester today. Seems like a pretty nice one for the price. Its a Gardner-Bender brand that I got at Ace Hardware for around $26.00

Anyway, the old RCA tester was measuring about 3 times actual value. That's why the 500K pots were reading 1500K. And those pots I assumed were 250K and reading about 300K? They were actually 100K pots. That surprises me, the 100K pots. These came out of an old junk Kay electric guitar. I didn't know they used 100K. I thought 99.9% of guitar pots were either 250K or 500K, and once in a while, 1meg. That's a new one on me.

[Jo6Pak]

The inaccuracy was likely due to having it on the Rx1 range (assuming it was zeroed prior to taking the measurement). I rarely use Rx1 except for continuity checks.