Talpadk’s Blog

April 20, 2014

Measuring printbed tempeartures on a RepRapPro Huxley

Filed under: 3D Printing — talpadk @ 4:13 pm
Tags: ,

I have finally gotten around to measuring the surface temperature of my Huxley.

Temperature as function of set-point

Temperature as function of set-point

Method and instruments used

For measuring the temperature a Agilent U1233A with a U11186A (k type thermocouple) has used.

The ambient temperature has measured by waiting for the display to settle and the taking a readout.

The heat bed temperatures has measured on top of the aluminium print surface with the polyarmide tape left in place.
The thermocouple was held in place by another piece of polyarmide tape.

The thermocouple was left on the print bed for 1 minute for the temperature to stabilize, the temperature was then measured on  the multimeter using the “avg” function after a 2 minute sampling period.

Measurements

The temperatures were measured at the centre and approximately 1cm from the edge.
The center temperature was measured an additional time at the end of the measurement cycle.
The print bed was in its forward position with the print head to the left at the end stop (cooling fan running)

The ambient temperature was measured as 22.1C at start of the surface scan, and 24.4C at the end.
The heat bed has maintained at 85C using the 3d printer firmware.

NA 71.2C 75.8C
77.6C 71.1C
76.1C
75.2C
 75.6C  77.1C  72.8C

After this the thermocouple was reapplied using a fresh piece of polyarmide tape at the centre of the print bed and left there.
The print bed set point was then reduced and the surface temperature measured.

Set point [C] Measured [C] Percentage
85 76.2 90
70 63.1 90
55 50.2 91
40 37.8 95

Notes

Some of the variances in the measurements across the bed might be related probe mounting relative to the surface and cooling to ambient.
Using a piece of foam or another insulator might improve this.
The lower measurement points may simply be caused by a bad thermal contact to the print bed.
Heat sink compound could perhaps have alliviated some of this as well (and made a lot of mess).

Also even though the measurements was taken as a 2 minute average, the temperature swings of the heat bed regulation may have contributed with some noise.

Also a thermal camera would have made this much easier and quicker, too bad they are so expensive.
(And that Fluke VT02/VT04 visual thermometers has such a bad resolution)

Conclusion

I would consider the bed temperature constant across the print bed within the uncertainty of my measurements.

At “higher” temperatures the surface temperature seems to be roughly 90% of the set point.

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1 Comment »

  1. I observed something rather similar on Huxley #710, although slightly better correlated with the set point. I used a Fluke 189 and RS 409-4908 type K thermocouple (and adapter for the multimeter), also with polyamide tape. To affix the sensor to the top of the bed also with polyamide tape. Don’t forget the reference sensor, inadequacies in the look up table and input channel calibration contribute to differences between the set point and achieved temperature. One of the contributions in later versions of Marlin is improved temperature measurement, a reference to your firmware revision would be handy.

    I don’t use polyamide tape anymore as I became quickly fed up of applying the stuff and upsetting the bed level.

    Check out http://northernhope.blogspot.co.uk/2013/01/the-futures-borosilicateperhaps.html
    With the glass, I have around a 20C offset.

    One thing people often forget with thermal cameras is to assess the correct correction (emissivity) for the surface being measured to allow a correct measurement.
    Your thermocouple can readily be checked with pure water as mushy ice and boiling point; you could even correct for height above sea level.

    Comment by Conseils — April 24, 2014 @ 10:28 pm | Reply


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