Question regarding thermocouples and a voltage terminal block

[kd6532]

Right now some peers and myself are designing a pv module testing
system, in which, one of the things we will be testing is temperature.
To do so we will be using four thermocouples on each solar panel, and
we wanted to join the four into a voltage terminal block to reduce the
amount of wires we will be running to our multi tracer. The problem is
we are having trouble finding a terminal block that will establish a
CLEAN connection and allow us to connect the four wires in and have
just one coming out for both the copper and constantan leads. Is their
a better way to approach this? We found one promising terminal block at
Phoenix Conacts that is specifically constructed for thermocouples
composed of copper/constantan, but four leads cannot go into one of the
inputs, and the tech. support basically told us to try another route.
Any suggestions are welcomed, and thanks for your time.

Kris

 

[John Larkin]

Right now some peers and myself are designing a pv module testing
system, in which, one of the things we will be testing is temperature.
To do so we will be using four thermocouples on each solar panel, and
we wanted to join the four into a voltage terminal block to reduce the
amount of wires we will be running to our multi tracer. The problem is
we are having trouble finding a terminal block that will establish a
CLEAN connection and allow us to connect the four wires in and have
just one coming out for both the copper and constantan leads. Is their
a better way to approach this? We found one promising terminal block at
Phoenix Conacts that is specifically constructed for thermocouples
composed of copper/constantan, but four leads cannot go into one of the
inputs, and the tech. support basically told us to try another route.
Any suggestions are welcomed, and thanks for your time.

Kris

Are you going to parallel the tc's into one signal pair? If so, *any*
terminal block will work fine. Just find one big enough to squish all
five wires together. Or crimp them into a butt splice, any wires in
either or both ends (3 in one end, two in the other maybe), doesn't
matter.

John

 

[kd6532]

Interesting. I am wondering, since tc's basically convey the
temperature as the potential difference between the two conductors,
using a different metal at the junction would not alter this potential
that we will be measuring across the copper and the constantan. And
just out of curiousity, using a metal doesn't typically change the pot.
difference does it?

Thanks again,
Kris

 

[Geo]

Interesting. I am wondering, since tc's basically convey the
temperature as the potential difference between the two conductors,
using a different metal at the junction would not alter this potential
that we will be measuring across the copper and the constantan. And
just out of curiousity, using a metal doesn't typically change the pot.
difference does it?

google for :-/ thermocouple "cold junction"

Geo

 

[John Larkin]

Interesting. I am wondering, since tc's basically convey the
temperature as the potential difference between the two conductors,
using a different metal at the junction would not alter this potential
that we will be measuring across the copper and the constantan. And
just out of curiousity, using a metal doesn't typically change the pot.
difference does it?

Thanks again,
Kris

As long as the junctions are isothermal, they can't generate any
potential. As a practical matter, scrunching some number of wires
under a screw, or crimping them into a butt splice, will be
isothermal.

And oh, we like to bottom-post here.

John

 

[Spehro Pefhany]

Right now some peers and myself are designing a pv module testing
system, in which, one of the things we will be testing is temperature.
To do so we will be using four thermocouples on each solar panel, and
we wanted to join the four into a voltage terminal block to reduce the
amount of wires we will be running to our multi tracer. The problem is
we are having trouble finding a terminal block that will establish a
CLEAN connection and allow us to connect the four wires in and have
just one coming out for both the copper and constantan leads. Is their
a better way to approach this? We found one promising terminal block at
Phoenix Conacts that is specifically constructed for thermocouples
composed of copper/constantan, but four leads cannot go into one of the
inputs, and the tech. support basically told us to try another route.
Any suggestions are welcomed, and thanks for your time.

Kris

I don't understand- are you paralleling the T/Cs? If so, for type T,
just solder the leads together and be done with it.



Best regards,
Spehro Pefhany

 

[John Perry]

As long as the junctions are isothermal, they can't generate any
potential. As a practical matter, scrunching some number of wires
under a screw, or crimping them into a butt splice, will be
isothermal.

....Well, that's a bit too simplified, I believe.

Different conductors in contact generate a potential difference.

Differences in temperature influence this potential difference in
particular ways depending upon the specific conductors connected.

The key is to know the temperature of all other connections (called
junctions in thermocouple language) to different conductors so that you
can infer the temperature of the test junction based on the known
temperatures and the known junction relations.

So, you can use just two different metals, join them at one junction,
and connect them to a measuring device. BUT, this device has to be at a
specific temperature if there's any other metal involved -- for
instance, your voltmeter certainly has only copper in its circuitry, so
where it connects to the constantan wire, you have a second junction.
You have to know the temperature of that junction before you can infer
the test junction's temperature from the thermocouple relation.

This means you have to either control the voltmeter's terminal
temperature, or measure it in some independent way. For decades, the
usual method was to chill the second junction in ice water (thus the
standard term cold junction). Later, it was controlled at a higher
temperature. Now, we simply measure the temperature of the second
junction and calculate the test junction's temperature from the
temperature difference between the two junctions.

But the key is to know the temperature and the materials of every
junction in the circuit.

And oh, we like to bottom-post here.

And we ought to edit irrelevant text from our messages.

John Perry

 

[John Larkin]

...Well, that's a bit too simplified, I believe.

Different conductors in contact generate a potential difference.

Differences in temperature influence this potential difference in
particular ways depending upon the specific conductors connected.

The key is to know the temperature of all other connections (called
junctions in thermocouple language) to different conductors so that you
can infer the temperature of the test junction based on the known
temperatures and the known junction relations.

So, you can use just two different metals, join them at one junction,
and connect them to a measuring device. BUT, this device has to be at a
specific temperature if there's any other metal involved -- for
instance, your voltmeter certainly has only copper in its circuitry, so
where it connects to the constantan wire, you have a second junction.
You have to know the temperature of that junction before you can infer
the test junction's temperature from the thermocouple relation.

This means you have to either control the voltmeter's terminal
temperature, or measure it in some independent way. For decades, the
usual method was to chill the second junction in ice water (thus the
standard term cold junction). Later, it was controlled at a higher
temperature. Now, we simply measure the temperature of the second
junction and calculate the test junction's temperature from the
temperature difference between the two junctions.

But the key is to know the temperature and the materials of every
junction in the circuit.


And we ought to edit irrelevant text from our messages.

John Perry

The OP wasn't entirely clear, but it sounded to me that all 5 wires at
each junction were the same tc alloy. That being true, the temp of
those junctions doesn't matter, since each junction is isothermal and
mostly all the same stuff.

If that's not true, well, the temps do matter. As usual, the exact
situation is unclear. A diagram would help.

John

 

[Glen Walpert]

...Well, that's a bit too simplified, I believe.

Different conductors in contact generate a potential difference.

....Well, that's just plain wrong. Different conductors in contact do
not generate any potential difference unless a poor connection is
made, and that effect has nothing to do with proper thermocouple
operation. The voltage in a properly assembled thermocouple is
generated entirely by the temperature gradient along the conductors.
The junctions serve only to complete the circuit so that the
difference in voltage produced by the same temperature gradient along
two different wire materials can be measured. Of course the junction
temperatures are the temperatures of the ends of the two different
wires and so determine the total temperature gradient (and therefore
voltage) along the length of both wires, so the junction temperatures
are used in all thermocouple calculations even though the junctions do
not generate any voltage.

So in order to make a proper temperature measurement with a
thermocouple the connections to the copper voltmeter terminals must be
at the same temperature, in order that the temperature gradient along
both thermocouple wires is the same, and one end (either one) must be
at a known temperature so that the other temperature can be
calculated.

http://www.electronics-cooling.com/Resources/EC_Articles/JAN97/jan97_01.htm

 

[John Perry]

...
...Well, that's just plain wrong. Different conductors in contact do
not generate any potential difference unless a poor connection is
made, and that effect has nothing to do with proper thermocouple
operation. The voltage in a properly assembled thermocouple is
generated entirely by the temperature gradient along the conductors.
...

Yes, I'm familiar with all that theoretical verbage. And, as you point
out later, it's effectively indistinguishable from the practical reality
I spelled out. Which is much easier to introduce to one who doesn't
already know it all.

jp

 

[cloudnine]

I don't understand- are you paralleling the T/Cs? If so, for type T,
just solder the leads together and be done with it.



Best regards,
Spehro Pefhany

The TC's are being paralleled together.

Do you think by soldering the leads together of the type T
thermocouples would give as accurate a measurement as they could with a
terminal block of the correct metals?

Attempting to get as accurate as possible paralleling those 4 TC's into
the input and having 1 output is our main objective.

Thanks

 

[cloudnine]

I don't understand- are you paralleling the T/Cs? If so, for type T,
just solder the leads together and be done with it.



Best regards,
Spehro Pefhany

The TC's are being paralleled together.

Do you think by soldering the leads together of the type T
thermocouples would give as accurate a measurement as they could with a
terminal block of the correct metals?

Attempting to get as accurate as possible paralleling those 4 TC's into
the input and having 1 output is our main objective.

Thanks

 

[John Popelish]

The TC's are being paralleled together.

Do you think by soldering the leads together of the type T
thermocouples would give as accurate a measurement as they could with a
terminal block of the correct metals?

Attempting to get as accurate as possible paralleling those 4 TC's into
the input and having 1 output is our main objective.

The solder adds no error, as long as all of it is at the
same temperature (no thermal gradient across the soldered
section, since temperature gradient is responsible for the
thermocouple voltage).

I assume you are paralleling thermocouples to average many
temperature measurements. If so, keep in mind that the
resistance of each of those thermocouples plays an important
part in how each one contributes to the average. Lower
resistance couples will contribute a proportionately larger
part of the average result.

 

[cloudnine]

The solder adds no error, as long as all of it is at the
same temperature (no thermal gradient across the soldered
section, since temperature gradient is responsible for the
thermocouple voltage).

I assume you are paralleling thermocouples to average many
temperature measurements. If so, keep in mind that the
resistance of each of those thermocouples plays an important
part in how each one contributes to the average. Lower
resistance couples will contribute a proportionately larger
part of the average result.

If all metals were placed in a box at the same temperature, is there
any reason to believe the soldier section would have a thermal
gradient?

Thanks,

Pete

 

[John Popelish]

If all metals were placed in a box at the same temperature, is there
any reason to believe the soldier section would have a thermal
gradient?

Not unless one side of the box were exposed to heat and one
side exposed to cold. Wrap the box in thermal insulation
and the internal gradients will be very low.

 

[John Larkin]

The TC's are being paralleled together.

Do you think by soldering the leads together of the type T
thermocouples would give as accurate a measurement as they could with a
terminal block of the correct metals?

Attempting to get as accurate as possible paralleling those 4 TC's into
the input and having 1 output is our main objective.

Thanks

If you are paralleling identical-alloy leads, scrunching them together
in a clamp-type terminal block or under one screw, the terminal block
material *doesn't matter*. And a crimped butt splice would be just as
good. Or twist, solder, and tape. All good to micro-kelvins. The
biggest source of error will be alloy differences between the t/c
leads and the extension wire, so keep both of the 5-wire junctions
close together; but that's way second-order, still a minute error.

Besides, thermocouples aren't super accurate to start with, and I
can't imagine the solar cell measurement has to be ultra-precise. So
just do it.

John

 

[John Larkin]

If all metals were placed in a box at the same temperature, is there
any reason to believe the soldier section would have a thermal
gradient?

Thanks,

Pete

Really, none of that sort of thing is necessary. Connect the 5 wires
together at one point, any way you can, and it will be a negligable
contribution to system error. This is *not* an issue.

What kind of signal conditioning will you be doing on the other end?
That error will probably swamp the junction thing by, literally,
1000:1. Most commercial t/c acquisition stuff has really mediocre
reference junction compensation.

John

 

[Glen Walpert]

Yes, I'm familiar with all that theoretical verbage. And, as you point
out later, it's effectively indistinguishable from the practical reality
I spelled out. Which is much easier to introduce to one who doesn't
already know it all.

jp

I can't agree with that view at all. The verbiage "thermocouple
voltage produced by the junction" is complete nonsense; how can it
possibly help anyone's understanding? It does not take much
investigation to actually understand the physical basis for the
function of thermocouples, which in my opinion is far better than
relying on a completely fictitious explanation even if that fiction
can be used to make correct calculations.

 

[Spehro Pefhany]

The TC's are being paralleled together.

Do you think by soldering the leads together of the type T
thermocouples would give as accurate a measurement as they could with a
terminal block of the correct metals?

Sure. As someone else said, you're actually going to be getting a
weighted average of the temperatures, based on the thermocouple
resistances, but if they're all similar materials and similar lengths
it will be good enough for your purposes.

Just don't do anything silly like putting the solder blobs near a heat
source. You can tack them down by putting them into a terminal strip
or whatever. You certainly do not need T/C material in the strips.

Attempting to get as accurate as possible paralleling those 4 TC's into
the input and having 1 output is our main objective.

Thanks

Best regards,
Spehro Pefhany

 

[YD]

Late at night, by candle light, John Larkin

If you are paralleling identical-alloy leads, scrunching them together
in a clamp-type terminal block or under one screw, the terminal block
material *doesn't matter*. And a crimped butt splice would be just as
good. Or twist, solder, and tape. All good to micro-kelvins. The
biggest source of error will be alloy differences between the t/c
leads and the extension wire, so keep both of the 5-wire junctions
close together; but that's way second-order, still a minute error.

Besides, thermocouples aren't super accurate to start with, and I
can't imagine the solar cell measurement has to be ultra-precise. So
just do it.

John

At best the actual reading will be the average of all four TCs. No way
to tell which one is out of whack if the readings look suspicious.

TCs are common in industrial applications and eg. Devicenet devices
can have several inputs and transmit the readings to a central
receiver over a single wire. Try taking it up in a controls and
instrumentation forum. Go the whole hog and get a PLC and some SCADA
for a few kilobucks.

- YD.