Thermocouple

it will be average when both cables have same length (resistance)
you can use a simple relais to switch over then you will have both temperatures.
 
shooter said:
it will be average when both cables have same length (resistance)
you can use a simple relais to switch over then you will have both temperatures.
Click to expand...

A simple relay is going to have copper or nickle plated terminals and this will offset the reading when the thermocouple wires are connected.
 
Jeff23spl said:
yes + contact resistances that change each time it open and close will make the reading erratic
Click to expand...
This topic piqued my interest.
How far off will the readings be through the relay contacts? In other words, if I have two thermocouples wired through different contacts on a relay and they should be reading approx. 350 degs. each and I'm sure that the wires are the same length...will the readings be 5, 10, 20, or more degrees off? Also, will they hold the same difference throughout the range of their capabilities? In other words can I simply tell the PLC "Hey when the thermocouple is telling me it's 345 degs., interpret this to be 350 degs. and keep the 5 deg. difference throughout"
 
1 the CJC is on a terminal strip so rest not important.
the resistance is also not important as this depends on the resistance of the meter or converter.

and yes fish you are correct as long as you calibrate once in a while.
 
gbeaker said:
A simple relay is going to have copper or nickle plated terminals and this will offset the reading when the thermocouple wires are connected.
Click to expand...

Jeff23spl said:
yes + contact resistances that change each time it open and close will make the reading erratic
Click to expand...

Wire resistance should never be a problem with Thermocouples, unless you are running thousands of feet. TC inputs ( in the last 20/30 years) are not a current sink in any way.

You can use anything at all to hook up/switch thermocouples, hook them up with steel and aluminum wire, and switch them through a bronze knife switch, as long as the entire conductive path is completely isothermal (the exact same temperature at every connection point) everything will be fine.

Of course, making the entire conductive path isothermal is a problem, especially with electromechanical relays, as the coil of the relay (when energized) generates heat.
 
yes from TC way of work but from the plc card reading side, if you open and close the path all the time, the module averaging will goes crazy.
 

Similar Topics

A
Allen-Bradley 5069-IY4 Thermocouple Help
I am having an issue getting a IY4 to read my thermocouple correctly. We have a R type and K type hooked up and sitting at a known room...
Replies
5
Views
521
danw
danw
B
Delta ISPSoft - Establishing communication with thermocouple module
Posted by u/mr_biodtox 44 minutes ago Delta ISPSoft - Establishing communication with thermocouple module Hi, How do I establish communication...
Replies
1
Views
334
biodtox
B
K
Thermocouple not steady values
Hi, We have an ongoing project with a lot of thermocouple involved that monitor the temperature of heaters. There are more than 20 PID...
2 3
Replies
32
Views
3,467
MaxK
M
G
Problems with the SLC 500 Thermocouple Module
Hello. PLC beginner here, so apologies in advance for any incorrect terminology or poor communication. I have an SLC 500 Thermocouple/mV Analog...
Replies
1
Views
649
KuulKuum
KuulKuum
C
Compact logix 5069-IY4 input module - thermocouple
Hello Guys, Trying to scale a thermocouple directly into my IY4 card. Connected into the terminal +\-. I’ve scaled the range of my J type to...
Replies
4
Views
1,660
danw
danw
Back
Top Bottom