R
THERMOCOUPLE--IF THE THERMOCOUPLE JUNCTION JUNCTION BOX IS VERY HOT, IT WILL GIVE ACTUAL READING OR NOT?
THE COLD COMPENSATION IS PROVIDED AT CONTROL ROOM.
THE COLD COMPENSATION IS PROVIDED AT CONTROL ROOM.
D
d
if the connections are made properly and proper thermocouple extension wiring and terminal blocks are used then yes.
> if the connections are made properly and proper thermocouple extension wiring
> and terminal blocks are used then yes.
To just expand a little on what d wrote, any time a thermocouple is joined to thermocouple extension wire in a junction box care must be taken to ensure <b>EIHER:</b>
--BOTH thermocouple leads <b>DO have</b> cold junctions,
<b>OR</b>
--both thermocouple leads <b>DO NOT have</b> cold junctions.
If <b>BOTH</b> thermocouple leads have cold junctions in a junction box, then any ambient temperature the junctions are exposed to in the junction box will be canceled out by each other (the two cold junctions), and the reading provided at the other end of the thermocouple extension wire will be the temperature measured at the thermocouple. (Cold junction compensation is typically provided for the termination of the wires to the control system.)
If both thermocouple leads are solidly connected to their respective thermocouple extension wire leads in a junction box, then there is NO cold junction in the junction box and the ambient temperature in the junction box will not affect the reading provided at the other end of the thermocouple extension wire. That is, the reading at the other end of the thermocouple extension wire will be the reading from the thermocouple, without any affect of temperature (high or low) in the junction box.
The key is: there must be a cold junction on both of the thermocouple junctions (the positive <b>AND</b> the negative), <b>OR</b> there must be <b>NO</b> cold junctions on <b>either</b> of the thermocouple junctions (the positive <b>OR</b> the negative). A single cold junction on either thermocouple lead in the junction box will result in some adverse affect on the reading at the control system.
What is a cold junction? When the thermocouple leads (of either the thermocouple <b>OR</b> the thermocouple extension wire) is <b>NOT</b> touching it's respective thermocouple lead. HOWEVER, if thermocouple terminals with the proper materials are used for the terminations of both leads of the thermocouple/thermocouple extension wire this, effectively, eliminates the cold junction and is, effectively, the same as touching the respective thermocouple leads to the thermocouple extension wire leads. But, this can be expensive, and many such thermocouple terminal boards are not made to withstand high temperatures, so non-thermocouple-type terminal boards are typically used in high temperature junction boxes.
It's difficult to describe, but, again, the key is: both leads MUST have cold junctions, or both leads MUST NOT have cold junctions--and the reading at the control system end will be the thermocouple reading, regardless of the temperature (high or low) in the junction box. You can twist the two positive leads together and the two negative leads together and put them under separate terminals, or you can use set-screw wire nuts to put the two positive leads and the two negative leads together--either method eliminates the cold junction for that circuit.
But, if there is a cold junction on <b>EITHER</b> the positive <b>OR</b> the negative lead of the thermocouple circuit in a junction box, any ambient temperature experienced in that junction box (high or low) will have an impact on the reading at the control system end of the thermocouple circuit.
> and terminal blocks are used then yes.
To just expand a little on what d wrote, any time a thermocouple is joined to thermocouple extension wire in a junction box care must be taken to ensure <b>EIHER:</b>
--BOTH thermocouple leads <b>DO have</b> cold junctions,
<b>OR</b>
--both thermocouple leads <b>DO NOT have</b> cold junctions.
If <b>BOTH</b> thermocouple leads have cold junctions in a junction box, then any ambient temperature the junctions are exposed to in the junction box will be canceled out by each other (the two cold junctions), and the reading provided at the other end of the thermocouple extension wire will be the temperature measured at the thermocouple. (Cold junction compensation is typically provided for the termination of the wires to the control system.)
If both thermocouple leads are solidly connected to their respective thermocouple extension wire leads in a junction box, then there is NO cold junction in the junction box and the ambient temperature in the junction box will not affect the reading provided at the other end of the thermocouple extension wire. That is, the reading at the other end of the thermocouple extension wire will be the reading from the thermocouple, without any affect of temperature (high or low) in the junction box.
The key is: there must be a cold junction on both of the thermocouple junctions (the positive <b>AND</b> the negative), <b>OR</b> there must be <b>NO</b> cold junctions on <b>either</b> of the thermocouple junctions (the positive <b>OR</b> the negative). A single cold junction on either thermocouple lead in the junction box will result in some adverse affect on the reading at the control system.
What is a cold junction? When the thermocouple leads (of either the thermocouple <b>OR</b> the thermocouple extension wire) is <b>NOT</b> touching it's respective thermocouple lead. HOWEVER, if thermocouple terminals with the proper materials are used for the terminations of both leads of the thermocouple/thermocouple extension wire this, effectively, eliminates the cold junction and is, effectively, the same as touching the respective thermocouple leads to the thermocouple extension wire leads. But, this can be expensive, and many such thermocouple terminal boards are not made to withstand high temperatures, so non-thermocouple-type terminal boards are typically used in high temperature junction boxes.
It's difficult to describe, but, again, the key is: both leads MUST have cold junctions, or both leads MUST NOT have cold junctions--and the reading at the control system end will be the thermocouple reading, regardless of the temperature (high or low) in the junction box. You can twist the two positive leads together and the two negative leads together and put them under separate terminals, or you can use set-screw wire nuts to put the two positive leads and the two negative leads together--either method eliminates the cold junction for that circuit.
But, if there is a cold junction on <b>EITHER</b> the positive <b>OR</b> the negative lead of the thermocouple circuit in a junction box, any ambient temperature experienced in that junction box (high or low) will have an impact on the reading at the control system end of the thermocouple circuit.
D
d
excellent description of the problem faced and how to get proper readings
O
otised
Just a clarification on the condition of both thermocouple leads having cold junctions. The cold junctions will exactly cancel each other out only if they are at the same temperature, which, if they are in the same junction box is generally the case. My practice, in this case, has always been to locate them on adjacent terminals on the same terminal block.
> If <b>BOTH</b> thermocouple leads have cold junctions in a junction box, then
> any ambient temperature the junctions are exposed to in the junction box will
> be canceled out by each other (the two cold junctions), and the reading
> provided at the other end of the thermocouple extension wire will be the
> temperature measured at the thermocouple.
> If <b>BOTH</b> thermocouple leads have cold junctions in a junction box, then
> any ambient temperature the junctions are exposed to in the junction box will
> be canceled out by each other (the two cold junctions), and the reading
> provided at the other end of the thermocouple extension wire will be the
> temperature measured at the thermocouple.
C
Curt Wuollet
If the junctions are similar metals, say thermocouple wire and the proper extension wire, yes it should. If there are dissimilar metals, you have more thermocouples and it won't.
Regards
cww
Regards
cww
R
RAJESH
good answer
>> if the connections are made properly and proper thermocouple extension wiring
>> and terminal blocks are used then yes.
> To just expand a little on what d wrote, any time a thermocouple is joined
> to thermocouple extension wire in a junction box care must be taken to ensure <b>EIHER:</b>
----- rest of quote snipped by moderator. To read answer go to http://www.control.com/thread/1404678876#1404756699 -----
>> if the connections are made properly and proper thermocouple extension wiring
>> and terminal blocks are used then yes.
> To just expand a little on what d wrote, any time a thermocouple is joined
> to thermocouple extension wire in a junction box care must be taken to ensure <b>EIHER:</b>
----- rest of quote snipped by moderator. To read answer go to http://www.control.com/thread/1404678876#1404756699 -----
