Wheatstone bridge

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  • #1
RCB
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Main Question or Discussion Point

Why would one use a bridge measurement circuit instead of a wheatsone bridge (I know for increase accuracy)

This website I found explains it (I think) but its far to complex:
http://www.sensorland.com/HowPage002.html

Can someone please explain it simply
 
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Answers and Replies

  • #2
AlephZero
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One reason for using a bridge is because the resistances are very small. The resistances of the connecting wires etc can not be ignored, and any electrical interference like voltages induced from AC mains wiring or stray magnetic fields would cause errors.

If you build a bridge using "identical" components all mounted the same way, these errors are cancelled out.

A Wheatstone bridge circuit that had to be balanced by a human operator would be too slow to use for large numbers of gauges, and could not be used at all for measuring strains that vary with time, for example if the component is vibrating. Strain gauges can easily measure the response of a structure at frequences up to tens of kHz.
 
  • #3
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Why use a 'bridge measurement circuit' (=voltmeter) instead of a wheatstone bridge?

What you are measuring is the change of resistance, not its absolute value.

Well the 'output' of a Wheatstone bridge consistes of numbers on a dial. these have to be manually read and converted to resistance change by calculation.

The output of the voltmeter is, well, volts. For small chnges the voltage output is directly proportional to the resistance change.
Furthermore the output is continuous and can be read by an automated device such as a data logger.
The output can also be used to control self balancing bridges in more sophisticated circuits.

As an aside, interference errors increase with circuit impedance (resistance) so to minimise these you need as low an impedance as practicable.
 
  • #4
RCB
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One reason for using a bridge is because the resistances are very small. The resistances of the connecting wires etc can not be ignored, and any electrical interference like voltages induced from AC mains wiring or stray magnetic fields would cause errors.

If you build a bridge using "identical" components all mounted the same way, these errors are cancelled out.

A Wheatstone bridge circuit that had to be balanced by a human operator would be too slow to use for large numbers of gauges, and could not be used at all for measuring strains that vary with time, for example if the component is vibrating. Strain gauges can easily measure the response of a structure at frequences up to tens of kHz.


How would the errors be cancelled out?
Based on your last paragrah, are Wheatstone bridges used at all in industry?
 
  • #5
AlephZero
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How would the errors be cancelled out?
IF there are any induced EMFs, approximately the same EMF will be induced in all the wires if they follow the same geometrical path, therefore the resultant is only the (small) difference between the EMFs, not the (bigger) total induced EMF.

Based on your last paragrah, are Wheatstone bridges used at all in industry?
The original Wheatstone bridge was invented in the 19th century, before the era of electronics. Apart from as a teaching tool (i.e. they are a circuit that you can analyse and experiment with) electronic measuring instruments have made them completely obsolete.
 
  • #6
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Simple methods are not obsolete, and are still used effectively in harsh environments that might upset delicate electronics, especially for slowly changing measurements such as the opening of a crack in a rock face or a building.

Here are some pics of a portable device.
 

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  • #7
RCB
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does this mean strain gauges are no longer used (generally speaking)
 
  • #8
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No strain gauges are being used in ever greater numbers and types.

It is just the associated instrumentation that has changed.
 
  • #9
RCB
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so what's changed
 

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