PT100 sensors for 3D printing

I've been wanting to try polycarbonate filament for a while, but didn't have any means of reading the high hot-end temperatures required, so when I got my Duet Ethernet board from Tony at Duet3D.com, he was kind enough to supply a PT100 interface board to go with it.

I ordered some cheapo PT100 sensors from China (when will I ever learn?) via ebay and hooked it all up.  These were supposed to be good up to 450C.  Things were fine for a little while, but after about 2 weeks of use, printing mostly ABS, I started having weird temperature behavior in the hot-end.  First it was just some small, random temperature fluctuations, and then jumps to 2000C, and finally it started throwing heater faults.

I checked the connectors, the cables, the connections to the board, and everything looked fine.  Then I noticed a message on the Panel Due console (or was it the DWC?) that said there was a short between the temperature sensor and other electronics.  Hmmm.  I pulled the PT100 sensor out of the heater block and checked its resistance between the leads and between the leads and the metal casing.  I found a short between the one of the leads and the metal casing.  The printer's frame is grounded, which means the heater block is also grounded, hence the error message about a short.

The type of cheesy PT100 sensors I bought have teflon leads that go into the steel housing.  They are completely useless for printing ABS or any material that requires much more than about 230C because the teflon breaks down when you get much higher than that.

I did some shopping on Ali-express and found that many of the PT100 suppliers will say that the sensors are good to 400C or 450C, but they don't tell you that the wires are teflon insulated, so while the sensor itself may be fine at 400C, the wires connected to it will not be.  The only insulation I know of that is reliable at 400C is glass fibers.

E3D sells a PT100 sensor for about $20 that has glass fiber insulation on the leads.  This should be good for printing ABS, PC and other high temperature filaments.  The only other PT100 sensors I could find with glass fiber lead insulation were industrial parts that cost about $80-100.

TLDR: cheap PT100 sensors with Teflon insulated leads will self-destruct when used at high temperatures, so don't try to use them for hot-end temperature readings.