K-type thermocouple

[Phil Allison]

"Bob Parker"

From http://www.sensoray.com/support/tcapp.htm .....

"There is a misconception of how thermocouples operate. The misconception
is that the hot junction is the source of the output voltage. This is
wrong. The voltage is generated across the length of the wire.

** This is a pedantic and misleading comment - the hot tip IS the source of
voltage in a thermocouple used in the normal way.

Or, more precisely, voltage is only generated where there is a temperature
gradient along the pair of wires coming from the joint ( or tip) to the
meter.

The magnitude of this voltage depends ( in a non-linear way ) only on the
actual temperature difference - so the length of these wires becomes
irrelevant.

So - if the tip it at a certain temp, you will get the same voltage output
at the meter as long as NOT all the wire leading to the meter is at a
different temp to the meter.



...... Phil

 

[-=Spudley=-]

IAWD.

"Davo" <Dave@gmail.com> wrote in message
news:48a26163$1_7@news.peopletelecom.com.au...

Bob Parker wrote:
On 13/08/2008 02:03 Taylor wrote:
-=Spudley=- wrote:

The tip of the thermocouple is generally the only place that is
effected by
heat, this is because it is the junction of the two dissimilar metals
that
forms the thermocouple that produces the voltage that is being read by
the
meter.

Ding! All makes sense now; that corresponds to the link that Bob
Parker showed on thermocouple theory.

Thanks!




From http://www.sensoray.com/support/tcapp.htm .....

"There is a misconception of how thermocouples operate. The misconception
is that the hot junction is the source of the output voltage. This is
wrong. The voltage is generated across the length of the wire. Hence, if
the entire wire length is at the same temperature no voltage would be
generated. If this were not true we connect a resistive load to a
uniformly heated thermocouple inside an oven and use additional heat from
the resistor to make a perpetual motion machine of the first kind.

I don't understand this at all, it seems like a lot of gobbledegook.
The voltage generated is related to the difference between the hot and
cold junctions. In most modern measuring devices the cold junction is
compensated for within the device. A long thermocouple may have some
resistance but with modern high impedance measuring devices the effect is
minimal. The voltage is generated at the junctions.

The erroneous model also claims that junction voltages are generated at
the cold end between the special thermocouple wire and the copper
circuit, hence, a cold junction temperature measurement is required. This
concept is wrong. The cold-end temperature is the reference point for
measuring the temperature difference across the length of the
thermocouple circuit."

 

[Bob Parker]

Phil Allison wrote:

"Bob Parker"

From http://www.sensoray.com/support/tcapp.htm .....

"There is a misconception of how thermocouples operate. The misconception
is that the hot junction is the source of the output voltage. This is
wrong. The voltage is generated across the length of the wire.


** This is a pedantic and misleading comment - the hot tip IS the source of
voltage in a thermocouple used in the normal way.

Or, more precisely, voltage is only generated where there is a temperature
gradient along the pair of wires coming from the joint ( or tip) to the
meter.

The magnitude of this voltage depends ( in a non-linear way ) only on the
actual temperature difference - so the length of these wires becomes
irrelevant.

So - if the tip it at a certain temp, you will get the same voltage output
at the meter as long as NOT all the wire leading to the meter is at a
different temp to the meter.



..... Phil

It seems that the theory's more complex than it first appears.
According to some 'experts', the dominant voltage is generated by the
temperature gradient along the dissimilar wires, plus there's a smaller
voltage generated at the junction at the tip, plus other minor factors
affect the reading.
Anyway I just thought I'd throw that in to get people discussing the
subject. I'm no expert on thermocouples.


Bob

 

[Davo]

Bob Parker wrote:

Phil Allison wrote:
"Bob Parker"

From http://www.sensoray.com/support/tcapp.htm .....

"There is a misconception of how thermocouples operate. The
misconception is that the hot junction is the source of the output
voltage. This is wrong. The voltage is generated across the length of
the wire.


** This is a pedantic and misleading comment - the hot tip IS the
source of voltage in a thermocouple used in the normal way.

Or, more precisely, voltage is only generated where there is a
temperature gradient along the pair of wires coming from the joint (
or tip) to the meter.

The magnitude of this voltage depends ( in a non-linear way ) only on
the actual temperature difference - so the length of these wires
becomes irrelevant.

So - if the tip it at a certain temp, you will get the same voltage
output at the meter as long as NOT all the wire leading to the meter
is at a different temp to the meter.



..... Phil

It seems that the theory's more complex than it first appears.
According to some 'experts', the dominant voltage is generated by the
temperature gradient along the dissimilar wires, plus there's a smaller
voltage generated at the junction at the tip, plus other minor factors
affect the reading.
Anyway I just thought I'd throw that in to get people discussing the
subject. I'm no expert on thermocouples.


Bob




Why would you get a voltage caused by a temperature gradient along a

wire? Copper-Constantan thermocouples are fairly commonly used, do you
get a voltage generated by a temperature gradient along a copper wire?
None of the copper wires I've ever seen develop a voltage from a
temperature gradient. What are these people talking about?

 

[Phil Allison]

"Davo"

Why would you get a voltage caused by a temperature gradient along a wire?

** It's a fact of nature - very likely a quantum effect.

Copper-Constantan thermocouples are fairly commonly used, do you get a
voltage generated by a temperature gradient along a copper wire?

** Yes.

http://en.wikipedia.org/wiki/Thermocouple#Attachment

None of the copper wires I've ever seen develop a voltage from a
temperature gradient.

** They do.

But only when you have dissimilar metals, in a series loop, will the effect
show up.

Contacts made of dissimilar metals show the same effect, but it is normally
only a few uV.

Plays havoc with some types of strain gauges and their pre-amplifiers.



....... Phil

 

On Aug 12, 8:08 pm, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:

Den wrote:
The boiling water test suggested by Bob is a good idea, also maybe iceblocks
in water continuously stirred should give close to 0C.

Would indeed have to be continuously stirred. Ice itself may well be much below
0C.

Grahams right (for once) but if the bath is continuously stirred so
that the water is moving all the way through the ice (or a least
through the ice in the immediate vicinity of the sensor) and the water
is pure (or at least de-ionised) the temperature is 0C plus or minus a
few thousandths of a degree Celcius/Kelvin.

If you dig through the American National Bureau of Standards web pages
you could find a more detailed exposition (I did the last time I
looked but that was about five years ago).

--
Bill Sloman, Nijmegen

 

[Bob Parker]

Taylor wrote:

I was attempting to verify my oven thermometer using a k-type
thermocouple attached to my multimeter. Putting the probe into the
oven, the displayed temperature quickly rose to match my oven
thermometer, then slowly kept climbing until it was well over 100
degrees above the thermometer. I know the oven wasn't really that
hot. I'm not certain I'm using the thermocouple correctly. Does
having a significant length of it exposed to the temperature affect
its reading?

Thanks

According to the thermocouple theory at
http://www.capgo.com/Resources/Temperature/Thermocouple/Thermocouple.html
it shouldn't make any difference if a fair bit of the thermocouple
wire's exposed to the high temperature.
Does it correctly read 100C when you measure the temperature of
boiling water?