zog
Senior Member
- Location
- Charlotte, NC
LarryFine said:
A microhmmeter is a wheatstone bridge
Less than .1 ohm
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A microhmmeter is a wheatstone bridge
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
080905-2230 EST
zog:
I would not agree that a Wheatstone bridge is a solution to low resistance measurements. Can you show how a Wheatstone bridge can provide accurate low resistance measurements?
Also I would not define a bridge as a "microhmmeter". I would generally describe an ohmmeter as an analog (including digital) device using a measured current and voltage to display resistance.
A bridge for precision resistance measurement is a null balance means for comparing resistances, and does not require knowing current and voltage to determine resistance. This is also an answer to an incorrect statement somewhere on this forum that you can not measure resistance, but must calculate resistance from current and voltage.
In a Wheatstone bridge there is no capability to eliminate the error from lead resistance from the bridge to the resistor under test or the contact resistance at the connection to the resistor.
When you follow this problem from Wheatstone bridge via Wikipedia in the direction to a solution the result is incorrectly described and incoherent. A correct discussion and correct diagram exist on p 80 of "Basic Electrical Measurements", by Melville B. Stout.
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zog:
I would not agree that a Wheatstone bridge is a solution to low resistance measurements. Can you show how a Wheatstone bridge can provide accurate low resistance measurements?
Also I would not define a bridge as a "microhmmeter". I would generally describe an ohmmeter as an analog (including digital) device using a measured current and voltage to display resistance.
A bridge for precision resistance measurement is a null balance means for comparing resistances, and does not require knowing current and voltage to determine resistance. This is also an answer to an incorrect statement somewhere on this forum that you can not measure resistance, but must calculate resistance from current and voltage.
In a Wheatstone bridge there is no capability to eliminate the error from lead resistance from the bridge to the resistor under test or the contact resistance at the connection to the resistor.
When you follow this problem from Wheatstone bridge via Wikipedia in the direction to a solution the result is incorrectly described and incoherent. A correct discussion and correct diagram exist on p 80 of "Basic Electrical Measurements", by Melville B. Stout.
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zog
Senior Member
- Location
- Charlotte, NC
I always ignore your posts because they are so out there they dont deserve a response, all i have to say is you need some better references than "Wikipedia", that is not a reerence.
Try this for a reference, it is a DOE publication, not someones opinion like Wikipedia http://www.tpub.com/neets/book21/88.htm
Your definitions of ohmeters and microhmeters are just your opinon and go against equipment manufactures definitions.
Try this for a reference, it is a DOE publication, not someones opinion like Wikipedia http://www.tpub.com/neets/book21/88.htm
Your definitions of ohmeters and microhmeters are just your opinon and go against equipment manufactures definitions.
- Location
- Massachusetts
zog said:
I agree but I bet that thought will cause a bunch of posts.:smile:
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
080906-1033 EST
ZOG:
A Wheatstone bridge is not useful for low resistance measurements because it does not eliminate lead wire or contact resistance problems. A properly drawn Kelvin bridge does solve these problems.
Your reference does show a definition of a Wheatstone bridge by illustration.
The correct bridge for low resistance measurements is the Kelvin bridge.
As shown in your reference and under Kelvin in Wikipedia the drawings are incorrect because they do not show the 4 terminal connection of the reference and unknown resistors.
I do not depend upon Wikipedia for accurate descriptions. However, much of their material is accurate and a good reference that almost anyone has access to.
I did provide you with a reference to a reliable source that has a good drawing of a Kelvin bridge showing how it is used with 4 terminal resistors. Furthermore Stout's book has an extensive discussion of bridge circuits, and in his course a great deal of time was devoted to bridge circuits.
Ohmmeters and bridges are not the same thing and should not be equated as the same.
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ZOG:
A Wheatstone bridge is not useful for low resistance measurements because it does not eliminate lead wire or contact resistance problems. A properly drawn Kelvin bridge does solve these problems.
Your reference does show a definition of a Wheatstone bridge by illustration.
The correct bridge for low resistance measurements is the Kelvin bridge.
As shown in your reference and under Kelvin in Wikipedia the drawings are incorrect because they do not show the 4 terminal connection of the reference and unknown resistors.
I do not depend upon Wikipedia for accurate descriptions. However, much of their material is accurate and a good reference that almost anyone has access to.
I did provide you with a reference to a reliable source that has a good drawing of a Kelvin bridge showing how it is used with 4 terminal resistors. Furthermore Stout's book has an extensive discussion of bridge circuits, and in his course a great deal of time was devoted to bridge circuits.
Ohmmeters and bridges are not the same thing and should not be equated as the same.
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zog
Senior Member
- Location
- Charlotte, NC
iwire said:
Sure it will, everyone has an "Ohmeter" in thier van/truck and thinks it measures resistance, and probally think O.L. means "OverLoad" or worse yet, "Infinite" (My biggest pet peeve).
Understanding how your test equipment actually functions is very important IMHO.
zog
Senior Member
- Location
- Charlotte, NC
gar said:
Well at least we agree on that
gar said:
gar -
A bridge requires a calibrated scale on a resistor adjustment. If the mfg did not use current and voltage measurements to get the calibrated scale - how did they get it?
That calibrated scale on the bridge resistor is secondary standard calculated from current and voltage measurements.
The statement is correct.
carl
zog said:
zog said:
Help me out. I put these two statements together thinking you meant that current (no pun) equipment mfgs are producing lo-ohm measuring devices using Wheatstone bridge principles. Was this your intent?
If not, disregard.
However, if so:
I haven't seen a Wheatstone bridge is 20 years. Or if I did see one I didn't know it:-? . They pretty well went by the wayside with the advent of digital, accurate, inexpensive, current sources. The only continuing advantage of a W-bridge, that I know of, is one can limit the effects of heating by the current source.
Kelvin bridge: Except for a few specialized research lab tools, I haven't seen one of these in 15 years. Digital, accurate, inexpensive, current sources coupled with 4-wire ohmmeter measurement techniques pretty well ended these as well.
Then again, perhaps I am in error and today is a good day to learn. Perhaps you could post links to mfg sites showing using bridges for lo-ohm measurements.
carl
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
080906-1330 EST
coulter:
I do not need an accurate voltmeter and ammeter to make accurate resistance measurements. Nor do I need these to calibrate a variable resistance.
Take a piece of wire and call it X ohms and define this as my resistance standard. Make a few more similar ones. Take a long straight wire adjacent to a measuring stick with a sliding contact to make a potentiometer. Note a potentiometer is more than a variable resistance. Connect these in a Wheatstone bridge circuit. Thru an iterative process I can make a large number of resistors with the value of X to rather high accuracy. Then I can make resistors of X/N and N*X where N is an integer. From these I can make all sorts of combinations of high accuracy resistors.
In this process I never needed a voltmeter and ammeter to calibrate a variable resistor for use in the bridge.
In the early days accurate volt and ampere meters did not exist, yet it was probably possible to measure resistors relative to each other within 1 part in 100,000. Even today you can not make an analog meter with an accuracy much better than about 0.1 to 1%, or 1 part in 1000.
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coulter:
I do not need an accurate voltmeter and ammeter to make accurate resistance measurements. Nor do I need these to calibrate a variable resistance.
Take a piece of wire and call it X ohms and define this as my resistance standard. Make a few more similar ones. Take a long straight wire adjacent to a measuring stick with a sliding contact to make a potentiometer. Note a potentiometer is more than a variable resistance. Connect these in a Wheatstone bridge circuit. Thru an iterative process I can make a large number of resistors with the value of X to rather high accuracy. Then I can make resistors of X/N and N*X where N is an integer. From these I can make all sorts of combinations of high accuracy resistors.
In this process I never needed a voltmeter and ammeter to calibrate a variable resistor for use in the bridge.
In the early days accurate volt and ampere meters did not exist, yet it was probably possible to measure resistors relative to each other within 1 part in 100,000. Even today you can not make an analog meter with an accuracy much better than about 0.1 to 1%, or 1 part in 1000.
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Easy way:
Easy way:
One could use a low-voltage, current-limited DC lab supply to make the 4-point measurement.
Strip the ends of the wire back an inch or so.
Connect the bare ends to the binding posts on the supply.
Adjust the current limit to 10A or so.
Measure the voltage across the bare wires--not the binding posts.
If R = 0.1 Ohm, the voltage would measure 1V--easy enough for a decent meter.
If you are measuring romex, make a low R connection between black and white at the far and take this into account when computing the length.
L (in feet) = R/Rho (total length)
where Rho is the resistivity of the wire in Ohms/ft.
BTW, the 4-point method is used in the semiconductor industry to measure resistivity of semiconductor wafers.
Easy way:
One could use a low-voltage, current-limited DC lab supply to make the 4-point measurement.
Strip the ends of the wire back an inch or so.
Connect the bare ends to the binding posts on the supply.
Adjust the current limit to 10A or so.
Measure the voltage across the bare wires--not the binding posts.
If R = 0.1 Ohm, the voltage would measure 1V--easy enough for a decent meter.
If you are measuring romex, make a low R connection between black and white at the far and take this into account when computing the length.
L (in feet) = R/Rho (total length)
where Rho is the resistivity of the wire in Ohms/ft.
BTW, the 4-point method is used in the semiconductor industry to measure resistivity of semiconductor wafers.
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