I need to make a bunch of thermocouples but I don't want to spend a ton buying them pre-made. I only need short thermocouples (like 6-8" long) so if i were to buy them it would be wasting a lot of wire too. I know thermocouple wire is pretty cheap but I don't have a good way of connecting the ends. I need to measure temps about 200C so regular solder won't do. I don't have a welder either. Is there any other good way of connecting the ends? Would JB weld work or would that mess up the reading. What about a high temperature epoxy? Can the wire ends be twisted together or can they only touch in one place at the tip?
Wire ends can be twisted together without any problems.
I performed an experiment as part of my undergraduate coursework where we used twisted pair of copper and constantan wires (twisted part about 2cm in length) that were held in place mechanically. It worked quite well. The temperature range was about 100-300C, so that should be right in your ballpark.
When you twist the wires, you have to make sure they are twisted around each other, not one twisted around the other. The best way I found to achieve this is to bend each wire 45 degrees, so they make a 90 degree angle with respect to each other. Then just hold both wires and twist.
A thermocouple welder can be made using capacitor discharge energy. I made one for fine gauge thermocouples using a 12VDC (variable is good) power supply connected to a 3300E-6 F capacitor via a 1k resistor. The TC junction should be prepared by stripping a bit so the 2 wires can be tightly twisted and cut off cleanly. Connect the negative lead from the capacitor (polarity is important) to a carbon rod using a sufficiently large alligator clip. Connect the positive lead from the capacitor to the thermocouple wires (both leads); it is good if you can connect near the junction, especially for long leads, but for < 3m it is ok to connect to the end away from where you will weld. Wear goggles or safety glasses! Now, with the capacitor charged, touch the thermocouple twisted wire to the carbon rod and it will spark and melt a bead. Depending on wire gauge you may need to vary the voltage or the capacitance to get a good result, and it may take a few tries to get a good weld. Energy scales with V^2 so a bit more voltage delivers much more energy. Ideally the carbon rod would be enclosed in a small plastic tube flushed with Argon gas and the thermocouple tip would be inserted through a small hole in the side to touch the carbon rod and produce an oxygen-free weld.
It's actually quite easy, get the wires, twist them, then use an acetylene welding torch to melt the joint into a ball.
The excess material will drip off and leave you with a small ball at the tip of the wire pair.
I'd make up a bunch of twisted wire probes and take them to a mechanic or an other kind of machine shop with a welder and have them meld the ends for me.
You can make a spot welder from a sufficiently large transformer. I have a transformer out of an old battery charger whose core I wrapped a couple turns of 6-gauge wire around.. It'll heat things up in a hurry.
For fine wire, even a few amps should heat the wires to molten temperature. You could use a 100W soldering gun, which is a basically a low-voltage high-amperage transformer, with the tip removed.
Based on what you described the best way to terminate the wires to the thermocouples being used in a high heat environment is to crimp the wires together using barrel splices. I believe Raychem carries them. Solder could melt and welding heat could damage the thermocouple.
Nicrome wire from a heating element can also be used.
I think you can embed in solder, or anything else conductive without changing the reading. ( assuming the whole joint is isothermal in space ).
Welding is good because it holds until the couple melts, not so with solder.
The best way to do this without the proper equipment such as a TIG (aka Tungsten Inert Gas) arc welder, is to silver solder (braze) the wires. You can do that with a propane or MAPP torch.
Many of the alloys contain cadmium which has consequences beyond the scope of this answer, but they are not considered suitable for contact with potable water or food. Last time I personally picked up some of the solder was from a precious metal specialist and it involved some interesting security precautions. It might cost $50 or more an oz. for just a small quantity. Pick up the proper flux at the same time- it's a white paste, inexpensive and it lasts a long time.