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Archive for January 10th, 2011

Thing-O-Matic: Thermal Core Instrumentation

Extruder head with thermocouple mounts - epoxy curing

Extruder head with thermocouple mounts - epoxy curing

As I described there, the resistors on the Thing-O-Matic MK5 Extruder Thermal Core operate in impossible conditions. To summarize, each resistor is rated to dissipate 10 W at 25 °C, but is actually dissipating nearly 30 W at well over 225 °C. Ouch!

I wanted to figure out just what was going on inside the Extruder Head, which means some instrumentation was in order, which meant I had to figure out how to attach a set of thermocouples to the Core. This picture shows one approach: epoxy a set of small brass tubes to various parts of the MK5 Extruder Head.

JB Industro-Weld epoxy is rated to 500 °F = 260 °C, which is barely adequate for the job at hand.

The general idea is that each tube provides an isothermal mount for a thermocouple bead, without the inconvenience of drilling holes in various metal bits and messing with high-temperature thermal compound. I am assuming that putting the beads inside the tubes, heating the Core, then waiting for the temperature to stabilize will produce meaningful results.

I have a motley assortment of meters that allegedly read temperature from Type K thermocouple beads. The business end of those thermocouples looks like this:

Thermocouple beads

Thermocouple beads

The twisted one in the middle has a completely non-standard red-black insulation color code, but as long as the meter it came with is happy, I’m happy. The two on the right have industrial-strength wires, as befits the fact that they plug into a Fluke 52 dual-thermocouple meter; them, I trust.

I skinned down the insulation a bit so they’d all reach into the middle of the tubes and filed down the bead on the right just a smidge.

It shouldn’t come as any surprise that each of the five thermocouples reported a different number for the ambient temperature, which meant a calibration run was in order.

Up next: an isothermal block.

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