Thermocouple and Temperature vs Time curve

[ruzfactor]

I need help on the following idea:

I want a thermocouple to get temperature after certain interval (say 1 min) until the temperature reaches a steady value. These temperature and time data will be used to plot Temp. Vs. time curve simultaneously.
How do I approach this project? Can it be done using matlabs pc interfacing or pc interfacing using C? Is it possible to show the simultaneous curve on a lcd display (using MCU)? I think it would be better using a lcd rather than a PC.

Thanks

 

[Sceadwian]

What's your temperature range? Thermistors are easier to work with than thermocouples, but don't have nearly the range a thermocouple does.

To measure the temperature of a thermocouple you need the thermocouple, a low noise instrumentation op amp to boost that voltage to a readable level, and a thermistor on the reading side for 'cold junction' calibration as thermocouple produce a relational voltage not an absolute one based on temperature.

Building one from scratch is not excessively easy. An MCU is a good option as most have ADC's and some have internal temperature sensors, so all you would need is the instrumentation amp and to set it within your useable range. Many multimeters have thermocouple inputs, and if you look around you may be able to find one that can read a thermocouple and has an RS232 output to hook up to a computer or MCU which takes all the hastle of building the thermocouple module yourself out of the picture.

 

[ruzfactor]

I want to use it on a refrigerated space. So the temperature can be below 0 deg.

 

[Sceadwian]

A thermistor will work fine. It's a variable resistor that has a non-linear relation of resistance to temperature. Thermocouples are generally used to measure high temperatures, as they can be made to measure temps where most common materials melt. Reading a thermistor on an MCU is as simple as choosing another fixed resistor to act as a voltage divider in the temperature range you're interested in and feeding that into an MCU to read as an analog voltage. If you want more details you'll have to be more precises about EXACTLY what you're trying to do.

 

[ruzfactor]

Thnx 4 replying
What I am trying to do is that I want to measure the change in temperature of a refrigerated space after certain intervals. A refrigerated space would have a temperature close to the ambient temperature at first and after certain time the temperature would decrease to zero or less (depends on load). So I want to measure the temperature change in every minute and simultaneously this temp. would be used to plot a temp vs. time curve. I don't have a clue on how to start this project?

 

[blueroomelectronics]

An ibutton thermocron can do this.

 

[ruzfactor]

I've never used a thermocron. How does it work?

 

[blueroomelectronics]

Well this fellow wears one on his head...

Look at **broken link removed** and Welcome to thermochron.com for more information.
**broken link removed**

 

[ruzfactor]

What is the problem using a thermistor as Sceadwian said? Where can I get some basics of converting temp to digital signal for a PC or MCU?

 

[blueroomelectronics]

No problem, you didn't indicate you wanted to build it yourself.

 

[ruzfactor]

yes I want to build it myself. But how do I start(have no clue!)??

 

[Sceadwian]

Personally the ibutton looks very nice. Calibrated no hassle, relatively inexpensive. A thermistor is relatively easy to use as well, but they're non-linear so they require a software look-up table to get a real reading out of them. You use a fixed resistor of your choosing (and the choice needs to be a careful one) with the thermistor and an applied voltage to form a voltage divider which you can feed into the ADC of a micro controller of your choice.

 

[3v0]

I use the dallas 1 wire sensing for my home control network. They come as ibuttons or as bare parts. Both work.

You can hang quite a few off a single twisted pair.

I have dallas 1 wire based sensors for temperature, barometic (sp?) pressure, and humidity. I plan to construct soild moisture probes soon.

Currenty my network mostly controls my small greenhouse and weather data. I plan to extend it to include the refridgerators and freezers but have not done so. About a month ago the door on the upright freezer was not fully closed and it cost me some food. The sensors would have been much less expensive.

You can find 1 wire code for many processors.

 

[ruzfactor]

I guess it would be better to use a thermocron. But I don know anything about it and how it works. How I'm gonna generate a curve simultaneously with the data? Will it be easier to plot the curve on matlab using serial or parallel port? or use a lcd panel?

 

[MrAl]

Hi,

You can also use a computer interface digital meter to measure the thermistor
voltage when placed in a voltage divider circuit. The meter reads the voltage
and transmits to the computer, then a small computer program converts the
voltage into temperature data. The temperature data is then logged with
time so you know when the temperature changed.
I did this many times with NiMH cell charging, where i wanted to measure the
temperature of the cell while it is being charged.
The circuit is easy to set up but you do need a digital meter that has a computer
interface, and if you want to write the software yourself you have to know
the protocol of the meter so that you can have your program read it and log
the values.
If you have never done anything like this before however it may be faster to
buy something already made that does this already as the other posts i think
have suggested.