Electrochemistry (batteries), Wheatstone bridge question.

Click For Summary

Discussion Overview

The discussion revolves around the use of a Wheatstone bridge for measuring very low resistances (around 0.2 ohms) and the observed variability in measurements over time. Participants explore potential causes for this variability, including battery performance and temperature effects, while discussing the technical setup and methodologies involved in such measurements.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant hypothesizes that the variability in resistance measurements may be due to the battery losing potential and "self-recharging" overnight, comparing it to the behavior of batteries in devices like a Gameboy.
  • Another participant questions the use of a precision reference standard for the battery and suggests using a precision voltage reference instead, while also inquiring about the current applied to the resistor and the presence of a galvanometer or voltmeter across the bridge.
  • Concerns are raised about the heating of the resistor during measurements, with one participant noting that they have accounted for temperature effects using a temperature coefficient for the resistor material.
  • Some participants share their experiences with measuring resistances in similar ranges and note that they did not observe the same phenomenon, suggesting that the issue may be specific to the participant's setup or connections.
  • There is a suggestion that contact resistance might be affecting measurements, with a recommendation to measure voltage across the test resistor while passing a known current through it.

Areas of Agreement / Disagreement

Participants express differing experiences and hypotheses regarding the variability in resistance measurements. While some acknowledge the potential for temperature effects and contact resistance, there is no consensus on the exact cause of the observed phenomenon.

Contextual Notes

Participants mention the importance of connection quality at low resistance measurements and the potential impact of heating on resistance values. There are also references to specific measurement techniques and equipment used, which may influence the results.

Who May Find This Useful

This discussion may be useful for individuals interested in precision measurements in electrical engineering, particularly those working with low resistances and Wheatstone bridges.

tabasko
Messages
8
Reaction score
0
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?
 
Engineering news on Phys.org
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?
 
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.
 
Bob S said:
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.
 
TurtleMeister said:
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)
 
tabasko said:
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?
 
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.
 

Similar threads

Wheatstone bridge and conditions for current to flow between 2 points
  • · Replies 8 ·
Replies
8
Views
4K
What could be draining the car battery and how can it be fixed?
  • · Replies 10 ·
Replies
10
Views
6K
How do I use full-bridge strain gauge circuit to measure F?
  • · Replies 2 ·
Replies
2
Views
3K
Deciding batteries for my circuit
  • · Replies 3 ·
Replies
3
Views
2K
System design for charging battery from turbine alternator
  • · Replies 8 ·
Replies
8
Views
3K
Wheatstone bridge offset-nulling circuit
  • · Replies 2 ·
Replies
2
Views
6K
Increasing the Battery Life of an LED Circuit
  • · Replies 26 ·
Replies
26
Views
4K
Battery, voltage, current and loads
  • · Replies 10 ·
Replies
10
Views
2K
Modifying a Micro-Heater for Rechargeable Cell Phone Battery Use
  • · Replies 5 ·
Replies
5
Views
2K
How to Measure a High Impedance Wheatstone Bridge at 50kHz?
  • · Replies 28 ·
Replies
28
Views
7K