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Old 12-14-2013, 05:14 PM   #1
jumpkickpunch
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Default New here looking for help with my temp sensor.

Hey everyone,

I don't brew beer but I've been working on a electrical project and nearly every time I search google for information I'm brought to this forum so I thought I'd post here and see if anyone could point me in a direction.

I'm starting a project that seems to work very much like a RIMS controller (I think). I ultimately want to use a PID controller to hold water temperature in the ~140F range +/- 15F with as high a precision as reasonably possible. I know it's a bold goal but I'm shooting for .05C error. I doubt I'll get there but if I get close, I'll be very happy.

This is the beginning stages of the project and right now since the PID is entirely dependent on the precision of the temp input data, I'm just trying to focus on creating the best temperature sensing circuit I can. Once I'm sure I have quality input data, then I'll move on with the rest of the circuit.

Like I said, I'm new but from what I've gathered, my narrow temp range would be best suited for a thermistor. Also I haven't decided if I should be using the on board adc of my MCU. It's been suggested for the sake of my tolerances I consider a seperate ADC all together to minimize interference. Thoughts???
Also, to make the most of the 10 or 12 bit ADC I'll most likely be using, I'll need to amplify the signal somehow. Any thoughts on the best way to do this with minimizing interference?

I've been using the search function but I'm feeling a bit overwhelmed with the steep learning curve. Can anyone point me in the direction of a thread with a temp probe that fits my project goals or give me advice on component selection??


Thanks!

EDIT: Also, does anyone have any information, anecdotal or otherwise, about what kind of difference in precision I should expect between my temp input data and what an average pid can control?

In other words, if I want to control temperature to .1C is there an expected precision of my thermometer?

EDIT 2: I forgot, I expect to have access to some pretty quality lab equipment at a local universities medical research center so I should be able to overcome the non-linear quality of the thermistor with proper calibration. So unless someone has a better solution, I don't think this will be an issue.

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Last edited by jumpkickpunch; 12-14-2013 at 05:20 PM. Reason: forgot something
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Old 12-15-2013, 06:16 PM   #2
crane
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Check out the arduino playground. There are 4 examples of thermistor setups. #4 will give you the best readings. I have found that using that circuit/code that I can get at least 0.1F accuracy without any calibration. I just used the curves from the thermistor datasheets. I could be even closer than 0.1F but I don't have any way to tell with my equipment.

Unless the micro you are using is complete crap the built in ADC is probably good enough for temperature measurement. That is why I suggest implementing it on an arduino as I suggest above any try it out before you start over complicating things only to get a marginal increase in performance.

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Old 03-06-2014, 01:54 AM   #3
jlhaynes
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One option you might look into is a Dallas/Maxim DS18B20 which is a 9 to 12-bit programmable resolution 1-wire digital thermometer. It provides up to .0625°C resolution at 12 bits with a range of +/- 125°C. I like to interface to these using a DS2482 I2C-to-1wire bridge so I don't have to mess with the tight 1-Wire timing.

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Old 03-06-2014, 02:08 AM   #4
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Try this...

http://learn.adafruit.com/sous-vide-...uino/sous-vide

It's a bit much for a first build. But it is programmed to do exactly what you want.

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