Archive for January 16th, 2011
This test determines the effect of thermal compound between the resistor and the Thermal Core on the MK5 Extruder head. The setup is essentially the same as before, with cotton fabric insulation wrapped around the Core.
I applied a thin layer of Thermalloy Thermalcote II from a small bottle that I’ve had since the days when you could actually use trichloroethylene as a solvent. It’s rated to 200 °C, so it won’t last long at full throttle, but it’s not nearly as permanent as epoxy.
That’s the thin blue line around the base of the resistor. You can actually have too much of the stuff, so I applied this by rubbing a dab from a scrap of paper onto the resistor’s base and squooshing it in place.
The instrumentation is the same as the last time around:
|TOM||MK5 t-couple||Top of core|
|T2||Fluke 52||Core edge adjacent to resistor|
|CA||Craftsman A||Bottom of core|
|CB||Craftsman B||not used|
|MPJA||MPJA meter||not used|
After once again wrapping the core up in cotton cloth, I skipped directly to the higher power levels and sampled the data at 20-minute intervals.
The adjusted temperature readings:
Notice that the T2 reading on the block starts out a bit higher than the T1 reading on the resistor; I didn’t wait quite long enough for the heat of my hands to settle out inside that insulating blanket.
The corresponding temperature differences:
|Power||R – Edge||Top – Bot||Edge – Top||Edge – Bot||R – Amb||Edge – Amb|
And now for the long-awaited and much anticipated thermal coefficients of the insulated and greased Thermal Core:
|R – Edge||Top – Bot||Edge – Top||Edge – Bot||R – Amb||Edge – Amb|
The grease reduces the thermal coefficient by about 20%, although I admit the numbers going into that calculation are getting pretty close to the limits of my instrumentation. Assuming the value remains the same at 30 W, the resistors will rise about 24 °C above the Thermal Core temperature to 250 °C, their maximum rated temperature. At that temperature, remember, their maximum rated dissipation is 10% of their 25 °C value: a whopping 1 W.
The R – Ambient and Edge – Ambient coefficients show that the insulation has about the same effect as before, which is comforting.
Now, to mull all this over for a bit…