You're right. My DMM falls in the ordinary grouping with the 200 ohm scale having a +/- 0.5% + 10 count tolerance. Throws away an order of magnitude. The 2K scale resolving to tenths of an ohm has a +/- 0.3% +1 count tolerance.
4-20mA current loop. Old industrial standard derived from an even older one; it was hard for me to get my head around the idea at first but I think I at least did that now. Too simple! Works well. Say 12-24vdc source followed by a simple LM317L arranged as a current regulator, output resistor 60 ohms to limit the current to 20mA. Zero output is at 4mA so you need to use the 'L' version; the normal LM317 won't go that low.
This now goes through the 4-20mA module for the PT-100, and a 250 ohm resistor, then to ground. The module almost certainly contains a bridge and op amp. Two adjusting pots for zero and range. The PT-100 connects through this and has 800 microamps through it. That's it!
After I check my PT-100 probe for offset at freezing and boiling I'll calibrate the two together. Apply the offset correction in software.
I'll check a trustworthy source here in town about a 0.1% 250 ohm resistor. Otherwise it's to taobao, our online shopping place. Shops will reopen over the next few days.
My head was full of a jumble of things related to this so I went to play with two young kids for a while to help sort it all out. The sensors themselves limit the accuracy to a couple of degrees so the high resolution of the output voltage across the precision resistor and the ADC don't much matter (i.e. they won't add to the basic inaccuracy). Whether I use a lookup table or run the equation for each temperature determination doesn't matter. My 4 1/2 digit DMM is pretty good so I've probably got the precision resistor to 0.1%. It'll be OK.
If I'm going to do something with a guide for others I'll have to learn a lot more first. But maybe I'm getting a start.
From the script you used writing Happy New Year about the Lunar New Year looks like you're in the south of Asia. I'm in the north of China, on the east side. I use green beans from Yunnan Province in the south.
Do you have a simple way to offset the 1v floor (from the 4mA minimum) of the 4-20mA current loop output to 0 volts? Just for fun (for some reason I don't see the Agtron as a fun project).