Electrochemistry (batteries), Wheatstone bridge question.

[tabasko]

Hi, I am using a Wheatstone bridge to measure very low resistances (around 0,2ohms) and my measurements are changing from day to day. If I take the resistance measurement of the same resistor several times, I notice that each measurement will be lower than the next. But the next day, the resistance will be fairly high again, and decrease on subsequent measurements.

My hypothesis is that the battery is draining, or at least losing some kind of potential, and it "self recharges" overnight, (kind of like a gameboy that would run out of batteries, and run for a few seconds the next day? =S)

I think we are using a series of batteries (I don't want to disrupt my circuit to check, but it's just standard batteries you can buy anywhere.)

Is my hypothesis valid? Is there a name for this phenomenon? Is the battery really recharging (a bit) overnight?

 

[Bob S]

The last time I used a Wheatstone bridge to measure small (currents actually) resistances was about 1960. Usually the battery is a precision reference standard. Is that correct? Can you use a precision voltage ref instead? Do you have a galvanometer or a voltmeter across the bridge? How much current can you put in the resistor? What current are you putting in it? Could the resistor be heating up as you measure it?

 

[TurtleMeister]

I have used the Wheatstone bridge to quality check the resistance of windings used in electric motors. I don't remember observing the phenomenon you describe. I do remember that we had a mercury thermometer and a chart that we used to adjust for the copper temperature coefficient.

 

[tabasko]

The last time I used a Wheatstone bridge to measure small (currents actually) resistances was about 1960. Usually the battery is a precision reference standard. Is that correct? Can you use a precision voltage ref instead? Do you have a galvanometer or a voltmeter across the bridge? How much current can you put in the resistor? What current are you putting in it? Could the resistor be heating up as you measure it?

we definitely thought of the temperatures but I have the temperature coefficient for the resistor (it's actually a piece of alloy 20) I'm measuring, and the decreases would correspond to more than 100C of cooling down (since resistance is supposed to increase with temperature)

I put a voltmeter across a reference resistor (.15ohm) I'm using, I get ~15 millivolts, and the resistor is rated for 3W, which means I'm fine.

 

[tabasko]

I have used the Wheatstone bridge to quality check the resistance of windings used in electric motors. I don't remember observing the phenomenon you describe. I do remember that we had a mercury thermometer and a chart that we used to adjust for the copper temperature coefficient.

The resistance we are trying to measure is unknown, so we just want to find the resistance at room temperature. Once again I am positive that the temperature is not changing. Enough to account for that difference.

What were the resistances you were measuring? I measured several resistors (100-300ohm), and the problem did not occur. It only happens with my small resistors (.15ohm)

 

[TurtleMeister]

What were the resistances you were measuring?

Some windings were well below one ohm. In the same range as what you are trying to measure. I remember that the connections were critical at those resistance ranges. We used gold plated connectors and clips. I don't know what could be causing your drift problem. But just guessing, It sounds like a resistance change due to temperature. Lower resistances require more current, and more current generates more heat. I know you've ruled that out for your test sample. But what about the bridge resistors in the Wheatstone bridge?

 

[vk6kro]

It may be that you have a problem with contact resistance.

If you have a couple of digital multimeters, pass a measured 100 mA (10 volts variable through 100 ohms) through the test resistor and measure the voltage across it.
A 0.2 ohm resistor will give a reading of (0.1 times 0.2) or 20 mV which will be clearly visible on the low voltage range of most digital multimeters.