A covered thermocouple can either be for a higher intended temperature range, or a averaged out thermal response (more mass takes more time to heat / cool)
In the case of a 3D printer hotend, having a hard metal casing that you can use grubscrews to get good surface coupling can outweigh the disadvantages of the higher thermal mass,
For the bare thermocoupes, you need some way to ensure tight coupling to the thermal block, otherwise it can sit in the center of the cavity with weak or no coupling to the end junction and rapidly swing in temperature while not actually measuring the real temperature of the block
Also unless you directly short a thermocouple wire to a higher voltage, e.g. 12V, it should never be able to damage the main controller, shorting to ground, or shorting between the pins should not cause any significant effect (what is a thermocouple but a hard short that is only able to make a differential due to the high impedance of the measuring system, and its still only in millivolts)
I purchased a cheap (<$200 USD Shipped) Prusa i3 clone out of China. I'm developing a new product and wanted to be able to do rapid prototyping. I knew this wouldn't come close to commercial 3D printing, but I was hoping it would be good enough for the early stages of product development. I was very, very disappointed in the results. The Plexiglass frame was about as stiff as a wet noodle, flopping all over the place as the print head shuttled around. This was easily taken care of by stiffening the frame with various bits of aluminium. That helped keep a tighter geometry, but it was still awful because the extruder temp. was schizophrenic. It would get within a couple of C° of the set point relatively quickly, but would then take forever to get the last few °. Once there and the print started, it would be too hot, then too cold, dropping low enough to stop the extruder in it's tracks ruining the print (which take close to 7 hours to print each half). I finally removed the Kapton tape that was around the extruder and found a plain thermocouple bead hanging there. The tape wasn't keeping the thermocouple tight against the head
. Without tight coupling between the thermocouple and the aluminium extruder the temp. swings were huge. I ditched the POS thermocouple that relied on tape and looked for a better alternative which turned out to be 2 styles. 1 was the metal cased type that look like a generic tubular tuning fork 'crystal' for RTC's that gets inserted into a hole in the block then a set screw pushes it against the side of the hole. The second was made of brass and designed to thread into the block. I preferred the screw-in type so I drilled and tapped my head to accept it (it was designed for the can style). Night and day difference. It now tracks the temp. accurately and I'm getting really good prints that rival those coming off printers costing 10x as much. The added thermal mass of the thermocouple body is tiny compared to the extruder negating any added time constant due to it's larger size vs. the bare bead.
If I wanted to use a thermocouple with it's hot junction exposed , how could I thermally bond the thermocouple to whatever surface I want to measure the temperature of.
If I wanted to use a thermocouple with it's hot junction exposed , how could I thermally bond the thermocouple to whatever surface I want to measure the temperature of.
Heat sink grease.
Thermally conductive epoxy.
Normal epoxy.
Some Kapton tape.
It depends on what you are doing and how permanent you want the mounting to be.