Archive for May 17th, 2013

Thermocouple Ensemble Comparison

Although I don’t have a good way to put thermocouples in a known temperature environment (ie, yes, I can freeze and boil water, but I doubt the trustworthiness of any measurements made therein), I can compare the results from several different thermocouples held at the same (unknown) temperature: if they all agree to within a reasonable tolerance, I assume that they’re all reporting the correct temperature. Even better, if some of the temperatures come from different thermocouple amplifiers, then the electronics cross-check each other.

With that in mind, I attached a 4 Ω 25 W aluminum-body power resistor to the back of the same isothermal block I built for the Thing-O-Matic extruder tests, atop a dab of heatsink compound for good measure:

Isothermal block with 25 W power resistor

Isothermal block with 25 W power resistor

Then I tucked four sensors into the drilled holes:

Sensors in heated aluminum block

Sensors in heated aluminum block

Clockwise from the front left corner:

  • T1: Fluke meter, eBay TC (black sheath)
  • T2: Fluke meter, Fluke TC (brown sheath)
  • TM5: TC4+Arduino, eBay 100 kΩ thermistor (invisibly fine bare wire leads)
  • TC1: TC4+Arduino, eBay TC (black sheath)

The TC4 Shield handles both thermocouple and thermistor sensors, so I added a thermistor from my collection just to see how it worked. The datasheet gives these parameters:

  • 100 kΩ resistance at 25 °C
  • beta = 3950 (from 25 °C to 50 °C)
  • beta = 3990 (from 25 °C to 85 °C)

Unfortunately, there’s no way to include the eBay thermocouple epoxied to the nozzle in this test, but it’s from the same lot as the two in this test, so I assume it’ll produce the same results. The consistency in earlier tests suggests they’re all really Type K thermocouples and produce the same results as the Fluke thermocouple and meter that I assume produces accurate readings.

The closed-cell foam insulating the block from the vice jaws seemed like a good idea at the time.

I connected the resistor to the bench power supply, channeled the true spirit of DIY 3D printing (ie, ignored the power derating curves), and fired it up:

Multiple Sensor Calibration - vs time

Multiple Sensor Calibration – vs time

The successive steps correspond to power levels of 0 W (the ambient temperature), 1 W, 2 W, 3W, 4W, and 6 W. The last point established that the foam melts at slightly over 100 °C, whereupon the test terminated. Eyeballometrically, the time constant of the resistor + block is on the order of 20 minutes, so these few points represent a rather tedious Basement Laboratory session.

Plotting the temperatures against the reading for T2, the Fluke thermocouple connected to the Fluke meter, and thinning the lines makes the results a bit more obvious:

Multiple Sensor Calibration - vs T2

Multiple Sensor Calibration – vs T2

The three thermocouples and Fluke meter / TC4 Shield pretty much overlay each other, with the thermistor reporting a somewhat higher temperature. Given that TM5 is an eBay thermistor, I’ll let you judge whether the beta value I got from its listing matches the beta of the actual thermistor.

In any event, I’d say the thermocouples report a temperature within at most a degree or two of the actual temperature. Plus, I didn’t get a steam burn in the process…

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