Dual thermocouple wiring 1

[cooleyjo]

I was wondering if there is a way to wire two thermocouples so that they send the highest of the two signals to the signal processor. I have a single temp reader but there are two things that I need to measure the temperature that should remain about the same temperature all of the time. I need to know if I can simply wire them in series or paralell in order to get the signal from one of them without halfing or doubling the signal or have any other undesired effects.

 

[nbucska]

Each thermocouple needs a cold junction and a subtracting
amplifier. A thistd amplifier may be connected to output
the highest of the two.

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[cooleyjo]

Im a Mechanical engineer so I dont really know what a thistd amplifier is I googled it but it came up with nothing if you could elaborate on what that is that wousl help maybe its just a spelling error but I dont know.

 

[itsmoked]

cooleyjo I've never heard of that either... methinks maybe a typo?

You can always hook two T/Cs in parallel which will give you the average. Never in series!

Generally instead of the hassle you are heading for, you would just acquire a T/C scanner and see both. Some scanners would probably let you select for "highest".

 

[cooleyjo]

so if you hook them up in paralell it will give you the average of the two TCs?

 

[nbucska]

it depends on the internal resistance of the two TC-s.
If they are the same: yes else : no.


<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[itsmoked]

cooleyjo nbucska is correct you would want to use the same style T/Cs when paralleing.

We often hooked up to 12 in parallel for large sterilization chambers. In other cases we would hook up three sets of three.

 

[cooleyjo]

Well I wired the two thermocouples in paralell and it seems that the tepmerature is wayyyy too hot for what its supposed to be I would say around twice as hot as it should be any suggestions about why this is acting up like this??

 

[IRstuff]

Are you sure you have the correct polarity of the wires?

TTFN

 

[itsmoked]

Double?!?

It sounds like you hooked them in SERIES!

 

[cooleyjo]

no I definately hooked them up in paralell I thought that would be the problem too. but just to make sure I hooked 2 T/C + to + on plug and 2 T/C - to - on plug is there only one way to hook up a thermocouple isnt it just two disimilar metals? I will try to hook reverse polarities on the two TCs tonight Hope it works anyone have any other suggestions

 

[itsmoked]

This is wierd. What are you using to monitor the T/Cs?

 

[GTstartup]

I suggest the following:

Measure each T/C individually with a multimeter on mV DC setting, convert to Temperature using the tables for your type T/C (ie. J, K or whatever). Don't forget to add on ambient temperature of the location where your multimeter is. Calculate the average of the 2 measurements.

Then parallel the T/Cs and do it again. You should get the same result. Otherwise you are doing something wrong.