sous-vide cooking using the tmp36

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MakerDane
 
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Joined: Sun Mar 11, 2012 1:37 pm

sous-vide cooking using the tmp36

Post by MakerDane »

Hello all,

First the background: For my first project after discovering the arduino I decided to build a sous vide cooker. I've found many posts from those who have chosen to build one of these from scratch, but I figured it would be better to just use an arduino as a precision control on my slow cooker. It has been fun problem solving my way through issues with my LCD display and relay, but I keep getting hung up on the temperature sensor. I settled on the TMP 36 for ease of use, and bought an extra for good measure. Now they are both broken and I want to make sure my plan is solid before I buy more.

The plan: Arduino sous-vide cooker using a TMP 36 sensor housed in a waterproofed small glass bottle which will then float in the liquid contents of my slow cooker. I would like to limit the materials which will contact the liquids to either glass or stainless steel for home yogurt making.

The problem: My first TMP 36 melted after I wired it backwards. Good lesson there. My second one was working fine when I had it plugged directly into a breadboard, but started giving wildly fluctuating and inaccurate readings when I hooked it up to 3 foot 14 gauge doorbell wire. When I tried to de-solder it from the wire to see if it the wire was the problem I ended up snapping one of the leads off (combination of user and metal fatigue).

The questions:
1) Will the tmp 36 work when wired up to 3, 4, or 6 foot cables?
2) Should I put a resistor somewhere between the temperature sensor's analog voltage out pin and the arduino, and if so what size?
3) Should I just dump the tmp 36 and use something else? I've thought about using a thermocouple inserted between the slowcooker's ceramic bowl and the outer housing, but I don't think it will give me as precise a measurement.

Any help would be appreciated.

P.S. Thanks to everyone who have worked so hard to make the arduino a reality. Very cool beans!

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adafruit_support_bill
 
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Re: sous-vide cooking using the tmp36

Post by adafruit_support_bill »

A couple issues here. First: the glass bottle is not going to give you very good thermal conductivity between the liquid and the sensor. The thermal lag will make it difficult to achieve good temperature control. Second: since the TMP36 is an analog sensor, it is easily affected by electrical noise and fluctuations in the reference voltage. Anything involving switching of high current (such as for heating elements) is going to have both fluctuations and noise. If you want to use an analog sensor you will probably want to use twisted pair and/or shielded cable. The 3.3v analog reference on the Arduino is usually more stable than the 5v (see the TMP36 tutorial near the end for instructions on using the 3.3v reference).

The best sensor for a sous-vide application is this one: http://www.adafruit.com/products/642
It is already waterproofed and the sheath on the tip has good thermal conductivity for fast response. Moreover, it has it's own internal reference and uses digital communication to the Arduino, so it is much less susceptible to noise.

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philba
 
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Re: sous-vide cooking using the tmp36

Post by philba »

I'd spend some time perusing the datasheet http://www.analog.com/static/imported-f ... _36_37.pdf It does talk about driving long cables (though doesn't define long very well).

Some thoughts on using the TMP36:
  • use a bypass cap at the tmp36 end of the cable
  • avoid electrical interference (use shielded wire)
  • filter the results (either in SW or via an RC low pass filter)
  • use a voltage reference

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