Thermocouple Calibration: Isothermal Block

Verily it is written:

  • The man with one thermometer knoweth the temperature
  • The man with many thermometers knoweth not the temperature
Drilling the isothermal block
Drilling the isothermal block

Given the five thermocouples and their meters shown there, plus the Thing-O-Matic’s thermocouple, I had six different temperatures. They’re close, but we can do better than that.

The general idea is to put all the thermocouple beads in close proximity so they share the same temperature, record their opinions to various temperatures, then figure out an equation that adjusts their disparate opinions to reflect consensus reality.

I cranked out an isothermal block on the Sherline mill, using EMC2’s exceedingly handy polar coordinate notation to get a nice hexagon. Touch off XYZ=0 at the middle of the block, then center-drill and drill:

G0 Z3
G0 @5 ^0
G83 Z-5 R3 Q1 F100
G0 ^60
G83 Z-5 R3 Q1 F100
G0 ^120
... etc ..

For lack of anything better, 3000 rpm with a drill matching the ID of the brass tubes, plus dripping cutting fluid as needed.

Thermocouples in block
Thermocouples in block

I used a 6 Ω 50 W resistor (the adult version of the resistors on the Thing-O-Matic / MK5 head) as a heat source, clamping the block to the resistors with plastic clamps to provide mechanical force and thermal isolation. Good idea, bad implementation: as you’ll see, those little red tips melt at a rather low temperature.

The TOM thermocouple bead will fit into the empty hole.

Next step: numbers!