Wheatstone bridge problem

I guess I should mention one or two other things here. The Kelvin measurement of resistance shown above didn't show the voltmeter:

` ___ / \\ ` .-|___|-( A )----. ` | 10 ohm \\_/ | ` | o

Hi, Matty. The quality of the instrument doesn't matter if it's being used to measure something it's not made to do. I'd be fairly confident that if there are only two terminals, it's probably not made to accurately measure less than an ohm. So don't worry about blaming the instrument. From what you've said, that's the first place I'd look, too.

Unless you have a resistance standard, you'll kind of have to wing it on bouncing low ohm measurements. The first quick check is actually the force current/measure voltage method. If you have a good handheld DC voltmeter like the Fluke 77 (around 0.1% DC Volts accuracy), your resistance measurement accuracy will be almost entirely dependent on the accuracy of your ammeter. If you can crank 1 amp through your Rx, another Fluke 77 measuring DC Amps will give you a resistance measurement accuracy of 1.5%, which should be fine for what you're doing. Inferring resistance by forcing a measured current and then measuring voltage is theoretically sound as well as practical. Your boss is making sense on this one. But in order to get more accuracy or give you confidence in your measurements, Ohmite, Dale and others make those aluminum-housed 50 watt 0.1% 100 milliohm Evanohm wirewound resistors, which can be convenient, especially if you just want a quick sanity check. If you thermal cycle the resistors in an oven several times to stabilize them, measure resistance with a quality calibrated instrument, and then make sure never to apply more than 1 amp of test current to them (

I've used a Wheatstone Bridge to measure the resistance of 4 large solenoids. The readings were: 0.3, 0.28, 0.27, 0.26 ohms. I measured all of them again and then they were all about 0.20 ohms. As you can see, the readings have slowly dropped from 0.3 to 0.2 ohms. Can anyone offer suggestions why the readings keep changing? I assume that the battery condition is independent of the measurements.

Each time I did a measurement I reconnected the alligator clips to the terminals and wriggled them around to get a good contact, and checked the reading three times for each solenoid. Each of those three readings was the same. i.e. I've done a total of 24 readings.

When I connect the two leads from the Bridge together I get a reading of

0.05 ohms. There appears to be several hundred feet of 14 gauge wire on each solenoid. Each time I press the check button on the bridge the meter needle swings slightly to the left of its final position. Each solenoid weighs about 20kg. There is no way they can be changing temperature!

Hi, Matty. A 100 amp analog meter certainly isn't going to be good enough here. Their accuracy is specified as a percentage of full scale. Even a very good 100 amp meter might be specified as +/-2%, which would be +/- 2 amps. You're not going to get there from where you are. That meter will make you want to push higher current to get a more accurate reading, which will cause Rx heating and ruin your measurement.

The same is true for DC current applied. Don't think large -- you'll ruin your measurement. Think small current, like 1 amp.

Get yourself a couple of cheapie $10 USD DVMs from the hardware store if you have to. They'll be far more accurate for what you're doing than a 100 amp analog meter.

And don't fall into the trap of thinking that unless you're reading ohms directly off a brand name meter, the measurement can't be as good. If you can accurately measure an appropriate DC current being forced through Rx, and you can accurately measure the volts being impressed across Rx, you _have_ measured Rx. It's ohm's law.

R = V / I

It works. The rest is just using common sense to avoid changing the resistance too much with your measurement.

Good luck Chris

I'll have a look at the instrument tomorrow. It's a quality instrument, and I used an identical one 40 years ago at University. There are only two terminals. I do suspect self-heating in an internal resistor. I am being criticized for blaming the meter. Do you have any references for inaccuracy in measuring low resistance, i.e. around 0.2 ohm?

The boss has already suggested doing something like that, and is going to bring his Avometer and other meters tomorrow. We have already put 30 amps through another solenoid, and it seems to work fine. We have a large battery with a 100 amp meter and can change the current with a compressible pile of carbon slabs. All we want to do is to check whether all the solenoids are the same, and whether they have shorted turns. Measuring the precise resistance is not important. I was just wondering why it keeps changing.

In use, the solenoids get a variable high current (maybe 30 or more amps) for a maximum of a minute or so. According to tables I've just read, 100 feet of 14 AWG is about 0.25 ohms, which is in line with what I get.

For the record I have found the following references saying that the simple Wheatstone Bridge is unsuitable for measuring resistances under 1 ohm:

Two of the more common types of bridges are the Wheatstone Bridge, which is used to measure resistances of 1 ohm to 100,000 ohms, and the Kelvin Double-Bridge, which is used to measure resistances in the range of ..0001 ohms to 1 ohm.

A Kelvin bridge is recommended for measuring resistances lower than 1 ohm.

There are lots of "Wheatstone Bridge" circuits and the simplest of these are NOT suitable for measuring low resistances below about 1 Ohm. So for a typical resistance wire investigation the simple Wheatstone Bridge can not be used.

Test: Winding DC Resistance Measurements Detects: Broken strands, loose connections, bad tap changer contacts. Tool: Wheatstone Bridge (1 ohm and greater), Kelvin Bridge micro-ohmmeter (Less than 1 ohm)

I have analogue meters up to 100 amps that seem to work fine, and as much DC power as anyone could want. There might even be a Kelvin Bridge around here somewhere. Clearly we need something like that. I am only trying to compare all the solenoids. I've already measured the pull force for one solenoid and that's OK. There's more than a dozen to check.

Hi, Matty. Electronics is a specialized branch of physics. The trick with the resistance is just a trick. You're on the right track here.

Two of the best electrical engineers I ever worked with started out with degrees in physics.

You might want to check around at hamfests and such, and see if you can find the manual for your "Punt-avi" bridge. It will tell you what you need your coworkers to know.

Age doesn't necessarily bring wisdom, but it does bring experience. Experience has taught me to RTFM (read the manual). If they don't have the manual, get it.

It's funny sometimes (or so the War Department tells me) how much the wisdom of age actually looks like just being tired. ;-)

Good luck Chris

I have fed current to the solenoid and measured the current and voltage with Avometers. The current was 1.45 amps (on the 1.5 amp scale) and the voltage 0.38 volts, which works out to 0.26 ohms, which is the same as what the Wheatstone bridge measured initially yesterday, and again today. I've avoided pressing the test button too much this time.

The Wheatstone brand is "Pontavi" or some name like that. The name has a strange font so I can't be certain it's right. I cannot see that name on the Net. I'm told it has a 9 volt battery in it. If the lowest range used two 5 ohm resistors in the bridge, when measuring a 0.25 ohm resistance it would be trying to push nearly 2 amps through the bridge resistors, which could heat them up to have the effect I got before when the bridge readings kept going down for each measurement. There is no "sensitivity" switch (which is badly needed). There is no switch to put power on the resistors before the meter is used, so the large impedance of the solenoid makes the meter move when the test button is pressed.

Everybody but me in the workshop says that the Wheatstone bridge is fine to measure resistances of way under 1 ohm. So clearly you and I and all the references that I gave before are wrong. All my University work in physics measurements and statistics was clearly wasted. :) Anyway, thanks for your help.