Re: Question regarding thermocouples and a voltage terminal block

Late at night, by candle light, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> penned this immortal
opus:

On 15 Dec 2006 06:38:49 -0800, "cloudnine" <benzy300@xxxxxxxxx> wrote:

Spehro Pefhany wrote:
On 13 Dec 2006 08:13:57 -0800, the renowned "kd6532" <kd6532@xxxxxxx>
wrote:

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
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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 *** 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.

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