A couple of days back, there was this thread going on:
http://forum.allaboutcircuits.com/showthread.php?t=63934
One of the reasons that I so heavily participated was that I actually in the process of developing a high-precision temperature controller for the past two and 1/2 weeks, and the material was fresh in my mind.
The controller supplies up to 20 watts to a small volume of air (about 1 1/2 gallons or so). I implemented a PID controller on a PIC 18F processor using a US Sensor PS502J2 5K@25C thermistor with 0.1C tolerance as the temperature sensor. The only other precision component in use is a 0.1% fixed resistor in series to form a voltage divider, the output of which goes to the PIC's A/D.
No other precision components, voltage or current sources were used.
Unfortunately, I don't have possession of any high accuracy and high precision thermometry equipment, at least not as good as the thermistor is supposed to be. So I decided to test at 37C (body temp), and went to Walgreens and got a fever thermometer. These thermometers supposedly are accurate to 1/10C, and have really good resolution (about 1C/cm), albeit with a limited range.
The volume took about 3 minutes to stabilize to 37C. I then stuck the thermometer in. It took about another 5 minutes for the thermometer to stabilize.
I expected at most an 0.1C error due to the tolerance of the thermistor, a 0.1C error due to the 0.1% precision resistor, and 0.05C error due to self-heating, for a total error budget of at most 0.25C without any calibration whatsoever.
Here is the result of the test:
Sorry, I cannot post additional details or photos of the circuit or setup. I just wanted to illustrate what's possible with very few precision parts.
http://forum.allaboutcircuits.com/showthread.php?t=63934
One of the reasons that I so heavily participated was that I actually in the process of developing a high-precision temperature controller for the past two and 1/2 weeks, and the material was fresh in my mind.
The controller supplies up to 20 watts to a small volume of air (about 1 1/2 gallons or so). I implemented a PID controller on a PIC 18F processor using a US Sensor PS502J2 5K@25C thermistor with 0.1C tolerance as the temperature sensor. The only other precision component in use is a 0.1% fixed resistor in series to form a voltage divider, the output of which goes to the PIC's A/D.
No other precision components, voltage or current sources were used.
Unfortunately, I don't have possession of any high accuracy and high precision thermometry equipment, at least not as good as the thermistor is supposed to be. So I decided to test at 37C (body temp), and went to Walgreens and got a fever thermometer. These thermometers supposedly are accurate to 1/10C, and have really good resolution (about 1C/cm), albeit with a limited range.
The volume took about 3 minutes to stabilize to 37C. I then stuck the thermometer in. It took about another 5 minutes for the thermometer to stabilize.
I expected at most an 0.1C error due to the tolerance of the thermistor, a 0.1C error due to the 0.1% precision resistor, and 0.05C error due to self-heating, for a total error budget of at most 0.25C without any calibration whatsoever.
Here is the result of the test:
Sorry, I cannot post additional details or photos of the circuit or setup. I just wanted to illustrate what's possible with very few precision parts.
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