How to hookup pt100 with pi 3 without max31865 for temperature logging

Question

Absolutely noon on pi here.

I wanted to make a temperature logger with my raspberry Pi 3 and a "pt100 2 wire k type" thermocouple. I spent the evening googling about it and learned that one of the best way to do this would be through a max13865 board that communicates between the sensor and the board. But I'm for India and the board is exorbitantly costly to import and fakes on Amazon site has very poor review.

I also noticed there's a simpler approach discussed in raspberry help for DS18B20, which is a 3 wire configuration.

My question: 1. Is it possible to wire my 2 wire pt 100 to the pi3 without the max13865 in the middle? If yes can anyone help me with the wiring diagram.. 2. I'm trying to measure temperature between -70°c to 100°c. So will the waterproof DS18B20 work in such range?

Thanks.

Answer 1

Connecting a PT100 directly

Well, the PT100 is simply a resistor, reading it requires driving a constant current through it and measuring the voltage accross it. To do so one typically uses an ADC - analogue-to-digital converter - which the Pi does not have included.

While there is a way to read a resistor with the digital input/output of the Pi - as explained here: pigpio - Example LDR - I doubt it'll be as accurate as is required to properly read a PT100 resistance thermometer. At 20°C the change in resistance is about 2 Ohms / 5 K (see here); whereas the change of resistance of the LDR in the example is multiple orders of magnitue (1 MOhm to 2-4 KOhm).

_{^{Note that the above is with respect to a PT100 resistor not a type K thermocouple. See here for differences between resistance thermometers and thermocouples.}}

DS18B20

According to this datasheet the DS18B20 measures temperatures from -55°C to +125°C. So that would not be in the specified range. Note that an accuracy of ±0.5°C in the range from -10°C to +85°C is listed - which suggests that accuracy will be worse outside that range.