You're not listening!!!!! Do not try to go cheap with a temp control. The end result will be unsatisfactory and could cost you much more. If you can relocate the controller closer to the thermocouple and run the wiring to it, that would be a better choice. I work with thermocouples every day. Mostly type K and we have some type J. We also have RTD's which are resistive units and with them you can use copper wire. Some of our runs from the point where the thermocouple is and to the point where the controller is are hundreds of feet.
Not to worry... I broke down and got 50' of type k extension wire. Relocating the controller isn't really an option for me.
It's all about resistance. The sensor converts resistance to display degrees. If you can keep the resistance exact you'll be OK. I suspect you will not be able to extend 25'!
Resistance has nothing to do with thermocouples (you may be thinking about thermistors)! It's all about a millivolt level voltage generated by the interaction of two dis-similar metals at the junction.
To extend a thermocouple, you can buy small male/female mating connectors specifically designed for thermocouples. These come in different colors for the different types of T/C's- if I remember correctly (but don't hold me to it), yellow connectors are for type K's, and black ones are for type J's. The connectors, as well as the extension cable MUST be of the same type as the thermocouple. If different types of connectors or extension cables are used, then each connection point becomes its own thermocouple junction, adding or subtracting the voltage it generates to the voltage generated by the thermocouple, inducing large measurement errors! I've worked on industrial installations where thermocople runs in excess of 100 (and even 200+) feet caused no problems or reduction in accuracy.
The different types of thermocouples (K, J, T, etc) are made by using different metal combinations, and all have different temerature ranges and voltage/temperature curves.
Hi I totally agree with Sue and Gary about having to use the special cable or you will have problems I would also suggest using the special plugs and sockets mentioned earlier to prevent any "additional" thermocouples being formed due to disimilar metals being in contact even the terminal screws were of different metals so don't mix them up. If I remember correctly these plugs and sockets are also polarised but can not remember which colour goes to which terminal. It is over 15 years since I used thermocouples. so I can't be of any more help, sorry. Maybe one of the others can remember the plug polarity.