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Project Help and Ideas » Meat Thermometer

June 18, 2009
by ddavies
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Hi,

I was psyched to see the new meat thermometer video tutorial. I am trying to build something like that as well. However, I'm trying to use a regular thermometer probe from my store bought meat thermometer. The probe is completely passive, I think. It's got a pointed piece of metal that you stick in the meat and some heat proof wire that extends to a two conductor sub miniature male plug.

Does anyone know how a probe like that works? I assume you put electricity on one conductor and ground the other and that as the temperature of the probed meat rises there would be a corresponding (linear?) change of either voltage or current coming back on the ground side. Can any one confirm that or clarify how it actually works?

I'd like to use this probe since it's heat proof, lead free and neat.

Thanks, Derek

June 18, 2009
by mcai8sh4
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@derek - I would guess it uses a thermocouple, I'm currently wanting to use a thermocouple for a function at work, so whatever I find out, I'll let you know (I think it's similar to the LM34 sensor). My guess is two wires of different materials produce small voltage when temperature is applied - but not yet sure.

@YouLovelyReaders(yeahYou) - quick question, kinda related so not hijacking this thread too much, I've just built the temp sensor circuit, then loaded this prog into it, on the graph I get a relatively steady blue line (actual) rising slowly, but the green line (estimate) is just constantly jumping up and down around the blue one - not like the vid. Any ideas as to what I've done wrong?

June 18, 2009
by mcai8sh4
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hmmm - it might just be that the temp is only changing slightly (me holding it with my fingers)... time to play around!

June 18, 2009
by mrobbins
(NerdKits Staff)

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Hi Derek,

The thermocouple actually develops a tiny voltage (millivolts to tens of millivolts) across it with temperature. The voltage changes by roughly 40 to 50 microvolts (10^-6 volts) per degree Celsius change -- very tiny. To get this into the microcontroller, you'll need some amplification, probably in the form of an op-amp. Because the voltages are so tiny and because the DC value matters, you have to specifically look for low input-offset-voltage amplifiers, that have small offsets and small offset drifts. But yes, this would be neat. (For the record, we used lead free solder for our meat thermometer system -- just like lead-free solder is used for copper plumbing in your home.)

As for the blue/green line jumping issue, I think this may just be a scale issue. As we mention in the video, you do amplify the noise by taking this derivative. When you're looking at big changes (room temperature to a burger), the noise is relatively small. But if you're just looking at it measure room temperature, yes, the green line (the estimate) will appear relatively noisy (plus/minus a few degrees F). Does that match what you're seeing?

Mike

June 18, 2009
by mcai8sh4
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Mike - yeah, that sounds right. I've only really been going in the region of 10 - 15 deg C variation.

Even the blue line seems to go up/down in steps, I've tried a heated screwdriver (~25C) to ice (in a bag) but I stll don't think the range is enough. I've just been reading about the tiny voltages used for the thermocouple idea. The temps I want to deal with are from room temp, 19C to ~450C, so I don't think the LM34 will cope with this.

BTW, thanks for the graph program... a great way to see the effect of what we're doing.

June 18, 2009
by ddavies
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Thanks for the info!

@mbobbins: Is there a particular op amp that would work well for this that you could point me to? I no nothing at all about op amps!

June 18, 2009
by Ethanal
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I tried to run the python graph program, but the Terminal said:

ImportError: No module named serial

I wonder why it didn't work...

Also, does pygame have to be installed to run this? I noticed something about pygame in the code.

June 18, 2009
by mcai8sh4
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@ddavis - I think it depends on the type of thermocouple. If you check the wikipedia entry for thermocouple, it gives you a list of the main types. Also have a look at THIS it might point you in the right direction

@Ethanal - you will need to install the serial library (and any other that you don't have from the import lines at the top of the program). If you are using Ubuntu, I can help, if not - sorry I don't know python well enough to be of more help.

June 18, 2009
by hevans
(NerdKits Staff)

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You do need to install py-game and py-serial for our live temperature graph to work. Both of these modules have windows installers.

pygame pyserial

Humberto

June 20, 2009
by ddavies
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I went to RadioScrap and got the two op amps they had: a 741, which seems to be the canonical op amp, and a 'TLO82 BiFET' op amp. The BiFET was only two bucks so I got it, but I'm thinking the 741 might be the one that actually works here.

Does anyone know if the 741 is a good fit for this project?

Thanks!

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