Well I'm back with this thermoprobe project. I've gained some insight and I'm pretty close, but need some help. Let me reset the stage here and describe what I'm trying to do, where I am at, and what the challenge is.
I want to build a multiple-probe BBQ system that measures as many thermoprobes as I can. When I get the data, I can send it wirelessly or keep the data...I'll deal with that once I get this going. The thermoprobes are actually negative slope thermistors. Resistance decreases as temperature increases. I loosely calibrated a probe yesterday and plotted it. It turns out it is a logarithmic curve with the following trend line:
Tp = -32.228ln(Rp) + 323.17
Tp = probe temperature in °C Rp = probe measured resistance in Ohms
I've measured resistance from 15,500 Ohms at 15°C, down to 1,500 Ohms at 89.9°C. So essentially, if I can measure the resistance as an analog input on the Arduino, I can import math.h and use the log function and find out the temperature. Bingo.
I'm not sure how to hook up the probe (two terminals on the probe) so that I can do this. Basically it's a potentiometer, but with only two pins, not three. It functions as a resistor. Do I hook it up to 5V and A0? Not too sure about this circuitry.
Second issue is the association with the resistance measure to the map. I guess I have to calculate the resistance based upon the measured input of the voltage through the probe. But how do I calculate that? Let's say it's 500/1024 = 48.8% potential of the 5V feed voltage). R = I/V doesn't help me here, and not sure what to use.
Any insight appreciated.