Mity Eltu
Member
OK, using 18F4580 and Type T thermocouple (TC) to make a thermometer to be used in laboratory setting. I have the thermocouple interfaced with a difference amplifier using KA4558 and a gain of about 400. The output voltages are working nicely. I will be using a 16-bit a/d givig me a bit/voltage step of about 76uV. The reason for the 400 gain is for my max temp of about 250°C. With type T, the TC output will be 12.013mV. With 400 gain, the voltage to the A/D will be close to 5V.
Now the question. Everything I read tells me I need cold junction (CJ) compensation for the TC. I'm not sure how to accomplish this. Assuming I use a thermistor or other such device, I will be measureing 2 temperatures. One will being coming in as an A/D result from the 16bit chip and the other read directly by the PIC. OK, so now what do I do? Do I convert both ressults to temperature and then subtract the CJ temp? What is the result if I ignore the CJ temp? If I scale then result from the A/D and ignore the CJ temp, can't I still get the right temp using firmware? I mean, if I assume that 65535 from the A/D = 5V = 250°C, then the lookup table would direct me to 250°C when I get that reult, right? So, why do I need the CJ temp?
Does that make any sense?
Now the question. Everything I read tells me I need cold junction (CJ) compensation for the TC. I'm not sure how to accomplish this. Assuming I use a thermistor or other such device, I will be measureing 2 temperatures. One will being coming in as an A/D result from the 16bit chip and the other read directly by the PIC. OK, so now what do I do? Do I convert both ressults to temperature and then subtract the CJ temp? What is the result if I ignore the CJ temp? If I scale then result from the A/D and ignore the CJ temp, can't I still get the right temp using firmware? I mean, if I assume that 65535 from the A/D = 5V = 250°C, then the lookup table would direct me to 250°C when I get that reult, right? So, why do I need the CJ temp?
Does that make any sense?