Computing stuff tied to the physical world

Conquering the thermocouple

In Hardware, Software on Nov 1, 2010 at 00:01

(No Halloween stuff on this side of the pond – I’ll defer to Seth Godin for some comments on that…)

A while back, I had to shelve my experiments with the reflow controller, because I couldn’t get a reliable temperature reading from the Thermo Plug when using a thermcouple.

Or rather, sometimes it worked, sometimes it didn’t: the physical computerer’s equivalent of a nightmare!

The thermocouple circuit is very sensitive to ground currents, apparently. The effect was that my setup would work fine on batteries, but jump all over the place when attached to the USB port. Not very convenient for development, obviously.

It still has some unexplained behavior, but I’ve been able to narrow it down, so there are two new pieces of good news: 1) it only works badly while data is being transferred over the USB port, and 2) with some averaging, the readout is actually rock solid, both on batteries and on USB. I still see a difference in readout when data is transferred over USB, but since this is a JeeNode, I can work around that in the final version: go wireless!.

Here’s the readout code which produces good readings – all remaining jitter is now in 1/10’s of degrees Celsius:

Screen Shot 2010 10 31 at 18.48.10

The output is in 1/100’s of °C, because I’m trying to avoid floating point math in this sketch.

And here is the measuring side of my new reflow setup:

Dsc 2200

The thermocouple is taped to the thin aluminium insert in the grill using heat-resistant Kapton tape. When I turn on the heater, I now see a clear rise in temperature within seconds – perfect!

Note that I’m using a 4x AA pack i.s.o. 3x AA, because the AD597 needs at least 2V more on its supply line than the highest output voltage it is going to report. With 4x 1.2V (worst case, i.e. near-empty eneloops), the range will be at least 4.8 – 2 / 0.010 = 280°C, i.e. plenty!

And indeed, I’ve verified that at 250°, it reports valid temperatures on the attached LCD Plug w/ display.

The other plug you see in the lower left is a Blink Plug, with two pushbuttons and two LEDs.

Let’s see if this time around we can get the whole thing going properly!

  1. I’m glad to hear you have some more reliable readings! I know kapton tape is good for solder temperatures in the short term, but does it hold up longer term?

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