How does compensation cable used with thermocouple, and can someone brief to me how does a smart transmitter compensate the unwanted voltage in thermocouple system? if T type used with copper wire to connect the t/c with the tx, how does the smart tx eliminate the unwanted voltage? Need help here ASAP!

GTDBB- going to die brain burst!

 

the transmitter will usually have cold junction compensation built in. It's usually done with some kind of temperature sensitive device that adds a voltage to the thermocouple to compensate for the junction temperature. a
lot of companies use a diode junction scaled appropriately.

Rick Daniel
Intelligent Instrumentation
[email protected]

 

No, No, NO! You never use copper wire to connect thermocouples to a transmitter. Normally, you just connect the thermocouple wire itself to the proper terminals of the temperature transmitter. If the transmitter must be located at a distance from the thermocouple, you MUST use thermocouple extension wire made from the same metals as the thermocouple - take care in matching like metals.

Smart transmitters only convert thermocouple voltages to digital values in temperature units. They cannot detect or compensate for copper wire used to extend thermocouples to the transmitter.

Thermocouples generate an electrical potential between the junction where the two metals are welded together, and the measuring point called the reference. The electrical potential is non-linearly proportional to the temperature difference between the junction and reference. Transmitters measure the electrical potential and the temperature at the reference and convert it to temperature at the junction.

Dick Caro
============================================
Richard H. Caro, CEO
CMC Associates
2 Beth Circle, Acton, MA 01720
Tel: +1.978.635.9449 Mobile: +1.978.764.4728
Fax: +1.978.246.1270
E-mail: [email protected]
Web: http://www.CMC.us
============================================

 

I'll try :^)

Extension cable works by using the same metals as the thermocouple does. If it's the same or close on the thermomelectric series, you aren't creating new thermocouples at each junction, which is why copper doesn't work well with the most common TCs. Copper will work with copper/copper alloy TCs. And the smart transmitter has cold junction compensation for the TC type. It's all about only having one dissimilar junction at an unknown temp, and that's the temp being measured. If you know the temp of all the other dissimilar junctions by virtue of having them occur at the transmitter, you can simply sum in a voltage to cancel them out. The problems are almost always because there are extra dissimilar junctions which also generate voltage proportional to their temperature. If that temperature is different than the cjc temp, it can't compensate.

Hope this makes sense

I believe R&T are copper/copper alloy types, are not standardized and would use alloy specific CJC. But I could be mistaken as I don't have my reference handy and my memory fades with time. The rest of the blather I'm pretty solid on.

Regards

cww

 

Join the forum. Do not remain anonymous.
Simplistically, extension cable is low grade thermocouple cable. For example, thermocouple cable is normally 2% accurate or better. Extension cable is typically about 10% accurate.
A transducer (does not have to be smart if it is a thermocouple transducer of the correct type) has an inbuilt thermocouple to compensate for ambient temperature. IT CANNOT COMPENSATE FOR FALSE READINGS FROM USING THE INCORRECT TYPE OF CABLE.

Copper cable should not be used for thermocouples, only RTDs.

For thermocouples you must use thermocouple cable for the full length or extension cable from the TC to the transducer. I always prefer to use proper thermocouple cable and not extension cable as it is more accurate, particularly if the cable is travelling through high and low temperature zones. Some quite large errors can be introduced.

 

Compensation cables for the thermocouples are of the same type as the thermocouple. They are a little bit smaller in size, depending of the distance to be covered and a X suffix. So, a T type thermocouple extension wire is TX cable.

A smart transmitter compensates for the thermocouple created by the junction of the extension wire to the connection screws by measuring the ambient temperature and adding or subtracting the corresponding millivolts in the smart transmitter software running in the microprocessor.

The smart transmitter cannot compensate for a copper extension wire, it's not a current practice.

If you have to install terminal blocks in a thermocouple junction box, you have to use special compression type terminals (Entrelec manufacture them) which compress the thermocouple wire with the extension wire, so no cold junction generating voltage is created.

If you have a lot of thermocouple to send to a remote site and in order to lower the extension wire costs you can use copper wires. But, here's the trick. If you look at thermocouple table, you can see that all thermocouple types generates zero millivolts at zero degrees centigrade. The first solution (which is not practical) is to freeze at zero the interior of your junction box. The result is zero millivolts generated at the cold junction. The other solution is to heat the interior of the junction box at a known temperature over the maximum reachable ambiant temperature and compensate for the known error millivolt generated. It's a solution I have once seen but we had to determine the error generated by copper to type K thermocouple. The junction box was heated at 150 degrees Fahrenheit.

If you have any further question, don't hesitate to ask at [email protected]

Hope this will help

 

Do you recommend Junction box for connecting the external thermocouple (J type).

regards,
Ananth

 

> Do you recommend Junction box for connecting the external thermocouple (J type).

It's not the box. You need a terminal strip of the correct type to match the T/C and extension wire.

Don't forget, if you need high accuracy the calibration curve needs to be in the device used for measurement and you need high quality extension wire.

 

Also, don't forget that the thermocouple termination strip AND the extension wire must all be made of the same materials as the thermocouple. This is often more costly to install and maintain than to terminate the thermocouple in the field in a thermocouple transmitter and then to use copper wire to connect to a DCS or other instruments in the control room.

Dick Caro

 

The thermocouple law of intermediate metals says that junction terminal strips for thermocouples need not be made of thermocouple metals, as long as the terminals are isothermal and have no temperature difference between the terminals.

Don't believe me, Google law of intermediate metal

 

If you send me your email address I can send you a lot of information on TCs. From connections to extension wires. Just keep in mind the Pos and Neg wires are different metals so while using a terminal block the spade connectors, screws, and terminals need to be the same metal as the Pos and Neg wires. I am sure you are aware of this I just wanted to post this reminder. Thanks.

Email address is Jr.MarcoFerreira at Gmail.com Send me an email and I will send you all the information you could need for TC's.