Measuring the voltage across a heated wire?

F

Funky

Guest
How would you go about measuring the voltage across a piece of wire due to
the two ends being at different temperatures?

Perhaps one way is to connect it across a capacitor, air-cored maybe,
disconnect and then measure the voltage across the capacitor. I'll try this
tomorrow at work.

Funky
 
Funky wrote:
How would you go about measuring the voltage across a piece of wire due to
the two ends being at different temperatures?

Perhaps one way is to connect it across a capacitor, air-cored maybe,
disconnect and then measure the voltage across the capacitor. I'll try this
tomorrow at work.

Funky
Click to expand...
Your biggest problem is gonna be probing the wire.
Methinks you'll get more voltage at the junction than along the wire.
Assuming you mean homogeneous conductive wire, the free electrons will
try very hard
to make the (average value of the) potential difference along the wire
pretty near zero.
mike

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mike <spamme0@juno.com> wrote:
Funky wrote:
How would you go about measuring the voltage across a piece of wire due to
the two ends being at different temperatures?

Perhaps one way is to connect it across a capacitor, air-cored maybe,
disconnect and then measure the voltage across the capacitor. I'll try this
tomorrow at work.

Funky




Your biggest problem is gonna be probing the wire.
Methinks you'll get more voltage at the junction than along the wire.
Assuming you mean homogeneous conductive wire, the free electrons will
try very hard
to make the (average value of the) potential difference along the wire
pretty near zero.
Click to expand...
Seebeck says not.
There is a non-zero voltage across a bit of wire with a temperature
differential across it.
It's small.

Two wires made of different materials going back to a measuring device with
cold junction comparison at the end will measure it.
 
Ian Stirling wrote:
mike <spamme0@juno.com> wrote:

Funky wrote:

How would you go about measuring the voltage across a piece of wire due to
the two ends being at different temperatures?

Perhaps one way is to connect it across a capacitor, air-cored maybe,
disconnect and then measure the voltage across the capacitor. I'll try this
tomorrow at work.

Funky




Your biggest problem is gonna be probing the wire.
Methinks you'll get more voltage at the junction than along the wire.
Assuming you mean homogeneous conductive wire, the free electrons will
try very hard
to make the (average value of the) potential difference along the wire
pretty near zero.


Seebeck says not.
There is a non-zero voltage across a bit of wire with a temperature
differential across it.
It's small.
Click to expand...
Tell me more. How small? As in volts/degree C??

Two wires made of different materials going back to a measuring device with
cold junction comparison at the end will measure it.
Click to expand...
Well, yes, probing is the biggest problem yes? no? ;-)
mike




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mike wrote:
Funky wrote:
How would you go about measuring the voltage across a piece of wire due to
the two ends being at different temperatures?

Perhaps one way is to connect it across a capacitor, air-cored maybe,
disconnect and then measure the voltage across the capacitor. I'll try this
tomorrow at work.

Funky




Your biggest problem is gonna be probing the wire.
Methinks you'll get more voltage at the junction than along the wire.
Assuming you mean homogeneous conductive wire, the free electrons will
try very hard
to make the (average value of the) potential difference along the wire
pretty near zero.
mike

--
Return address is VALID.
Bunch of stuff For Sale and Wanted at the link below.
Toshiba & Compaq LiIon Batteries, Test Equipment
Honda CB-125S $800 in PDX
Yaesu FTV901R Transverter, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/
Click to expand...
"Connecting a capacitor" just adds errors and complexity.
Making a connection is the problem; one should use the finest wire
possible consistent with (a) adequate strength and (b) minimise thermal
conductivity away fromwire to be measured.
I suggest you use tungsten, for the high melting point (and reasonable
strength), so that a reasonably fine wire could be used (#44 or finer),
and laser weld it onthe wire to be measured.
Use a slide-back voltmeter (or equivalent) for zero current; add in
correction for thermal generation as needed.
 
mike wrote:
Ian Stirling wrote:
mike <spamme0@juno.com> wrote:

Funky wrote:

How would you go about measuring the voltage across a piece of wire due to
the two ends being at different temperatures?

Perhaps one way is to connect it across a capacitor, air-cored maybe,
disconnect and then measure the voltage across the capacitor. I'll try this
tomorrow at work.

Funky




Your biggest problem is gonna be probing the wire.
Methinks you'll get more voltage at the junction than along the wire.
Assuming you mean homogeneous conductive wire, the free electrons will
try very hard
to make the (average value of the) potential difference along the wire
pretty near zero.


Seebeck says not.
There is a non-zero voltage across a bit of wire with a temperature
differential across it.
It's small.

Tell me more. How small? As in volts/degree C??


Two wires made of different materials going back to a measuring device with
cold junction comparison at the end will measure it.

Well, yes, probing is the biggest problem yes? no? ;-)
mike

--
Return address is VALID.
Bunch of stuff For Sale and Wanted at the link below.
Toshiba & Compaq LiIon Batteries, Test Equipment
Honda CB-125S $800 in PDX
Yaesu FTV901R Transverter, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/
Click to expand...
Millivolts/C is the magnitude; depends on the two metals.
If the "probe" wire is of the same alloy/type as the test wire, then
thermoelectric effects will be near zero.
Makes it harder to have very thin diameter probe wires (see my other
response).
 
Robert Baer wrote:
mike wrote:

Ian Stirling wrote:

mike <spamme0@juno.com> wrote:


Funky wrote:


How would you go about measuring the voltage across a piece of wire due to
the two ends being at different temperatures?

Perhaps one way is to connect it across a capacitor, air-cored maybe,
disconnect and then measure the voltage across the capacitor. I'll try this
tomorrow at work.

Funky




Your biggest problem is gonna be probing the wire.
Methinks you'll get more voltage at the junction than along the wire.
Assuming you mean homogeneous conductive wire, the free electrons will
try very hard
to make the (average value of the) potential difference along the wire
pretty near zero.


Seebeck says not.
There is a non-zero voltage across a bit of wire with a temperature
differential across it.
It's small.

Tell me more. How small? As in volts/degree C??


Two wires made of different materials going back to a measuring device with
cold junction comparison at the end will measure it.

Well, yes, probing is the biggest problem yes? no? ;-)
mike

--
Return address is VALID.
Bunch of stuff For Sale and Wanted at the link below.
Toshiba & Compaq LiIon Batteries, Test Equipment
Honda CB-125S $800 in PDX
Yaesu FTV901R Transverter, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/


Millivolts/C is the magnitude; depends on the two metals.
Click to expand...
This can't be as there is only one metal in the wire.
The effect you're talking about has to to with the junction
of dissimilar metals (when probing, which was my point). Quite a
different thing.
I was asking Ian about the effect causing voltage to
be generated along a uniform wire with a temperature difference from end
to end (no dissimilar metals present).
mike

If the "probe" wire is of the same alloy/type as the test wire, then
thermoelectric effects will be near zero.
Click to expand...
This works if you don't have anything different on the other ends of the
probe wires. no connectors, no meters, nothing of different metal.

Yes, you can make it "near zero" and that's why I asked about the
magnitude of the effect being measured.
mike

Makes it harder to have very thin diameter probe wires (see my other
response).
Click to expand...


--
Return address is VALID.
Bunch of stuff For Sale and Wanted at the link below.
Toshiba & Compaq LiIon Batteries, Test Equipment
Honda CB-125S $800 in PDX
Yaesu FTV901R Transverter, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/
 
mike <spamme0@juno.com> wrote:
Robert Baer wrote:
mike wrote:

Ian Stirling wrote:

mike <spamme0@juno.com> wrote:


Funky wrote:


How would you go about measuring the voltage across a piece of wire due to
the two ends being at different temperatures?

Perhaps one way is to connect it across a capacitor, air-cored maybe,
disconnect and then measure the voltage across the capacitor. I'll try this
tomorrow at work.

Funky




Your biggest problem is gonna be probing the wire.
Methinks you'll get more voltage at the junction than along the wire.
Assuming you mean homogeneous conductive wire, the free electrons will
try very hard
to make the (average value of the) potential difference along the wire
pretty near zero.


Seebeck says not.
There is a non-zero voltage across a bit of wire with a temperature
differential across it.
It's small.

Tell me more. How small? As in volts/degree C??


Two wires made of different materials going back to a measuring device with
cold junction comparison at the end will measure it.

Well, yes, probing is the biggest problem yes? no? ;-)
mike

--
Return address is VALID.
Bunch of stuff For Sale and Wanted at the link below.
Toshiba & Compaq LiIon Batteries, Test Equipment
Honda CB-125S $800 in PDX
Yaesu FTV901R Transverter, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/


Millivolts/C is the magnitude; depends on the two metals.

This can't be as there is only one metal in the wire.
The effect you're talking about has to to with the junction
of dissimilar metals (when probing, which was my point). Quite a
different thing.
I was asking Ian about the effect causing voltage to
be generated along a uniform wire with a temperature difference from end
to end (no dissimilar metals present).
Click to expand...
I was under the impression that the differential resistance due to
temperature would cause a slight voltage change along the wire.
I could of course be completely wrong.
 
"Robert Baer" <robertbaer@earthlink.net> wrote in message
news:405978BA.124AE8D4@earthlink.net...
mike wrote:

Funky wrote:
How would you go about measuring the voltage across a piece of wire
due to
the two ends being at different temperatures?

Perhaps one way is to connect it across a capacitor, air-cored maybe,
disconnect and then measure the voltage across the capacitor. I'll try
this
tomorrow at work.

Funky




Your biggest problem is gonna be probing the wire.
Methinks you'll get more voltage at the junction than along the wire.
Assuming you mean homogeneous conductive wire, the free electrons will
try very hard
to make the (average value of the) potential difference along the wire
pretty near zero.
mike

--
Return address is VALID.
Bunch of stuff For Sale and Wanted at the link below.
Toshiba & Compaq LiIon Batteries, Test Equipment
Honda CB-125S $800 in PDX
Yaesu FTV901R Transverter, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

"Connecting a capacitor" just adds errors and complexity.
Making a connection is the problem; one should use the finest wire
possible consistent with (a) adequate strength and (b) minimise thermal
conductivity away fromwire to be measured.
I suggest you use tungsten, for the high melting point (and reasonable
strength), so that a reasonably fine wire could be used (#44 or finer),
and laser weld it onthe wire to be measured.
Use a slide-back voltmeter (or equivalent) for zero current; add in
correction for thermal generation as needed.
Click to expand...
What about the voltage drop across the tungsten wires because of the thermal
gradient?
I tried using the capacitor method I suggested and it worked perfectly.
Around 100mv/200C for a length of 8x0.2mm copper wire. I thought it might be
a lot higher since thermocouples give around 40uv/C even though the two
voltages across the wires subtract.

Funky
 
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