As part of the Experimenting with Thermistors challenge, I'm learning all about thermistors and how to apply them for my hummingbird feeder heater/monitoring project.
In case you missed it, my first blog post for this challenge talks about the reasons I'm doing this.
After receiving the kit, I pulled out my trusty wire stripper and exposed the ends of all the sample thermistors in the kit. I don't have any matching plugs for the ring-terminal with micro-lock samples, so (sadly) I snipped off the ends and made them into pigtails as well. Given that it's a hot week here and my office temperature was up around 25 *C (which matches the thermistor rating temperature 
), I used my multimeter to check that all of them were in the right positions in the kit - they all read a little bit lower resistance than stated, which I think was because my office temperature had climbed a degree since I started the process.
On that note, I might add that all of the thermistors in the kit are NTC (Negative Temperature Coefficient) thermistors - ie, the resistance goes down as the temperature goes up. The parts are all labelled as such, but until recently I had no idea that was a thing. The other types are PTC (Positive Temperature Coefficient) thermistors - their resistance goes up with temperature. Interesting tidbit on learning there too.
One of the first things I did next, as part of understanding more about these thermistors, was to print out all of the data sheets. Molex provides all of the data neatly on their website here:
https://www.molex.com/molex/products/family/ntc_ring_temperature_sensor_cable_assemblies
If you follow the link to any of the specific thermistors, you will notice a download link for "Drawing", which is where you will find the technical drawing for the part, which includes a "resistance vs temperature" chart.
These values are crucially important to the project, as we will need to interpret the resistance values for the thermistor(s) to get the correct temperature values, in order to use them as our temperature sensors.
To get a better understanding of what the difference is between these thermistors, and why some are rated at 3 kOhm and others at 47 kOhm, I decided to input the values for the 2 epoxy coated thermistors at either end of the spectrum into a spreadsheet. This allowed me to graph their behaviour to get a nice visual of what is going on.
There are a LOT of numbers to input, so I stuck with just the 3 kOhm (item 4 in the kit) and the 47 kOhm (item 10) for now. I imagine the others will have curves that fit in-between. I'll probably be inputting those numbers soon, but I may just limit it to my roughly -20 to +30 range.
Here is what the graph looks like:
Interesting - the higher kOhm thermistor seems to have a lot more change in resistance across most of the range.
The other detail of note is that the higher kOhm thermistor also shows more resistance at the higher temperature range - for example, at about 30 *C the 3 kOhm thermistor line is already pretty flat with pretty small resistance values, where the 47 kOhm thermistor goes to about 85 *C for similar resistance values.
Zooming in to my desired monitoring range of -20 *C to +30 *C we see the following chart:
(We just had +32 yesterday... so I guess I need to include extra leeway!)
So then, what is my take-away from all this?
First, it looks like the higher value kOhm thermistors would be better for the higher temperature ranges.
Secondly, it looks like the choice of thermistor very much depends on the project and the desired temperature range being monitored.
Thirdly, it looks like I have more math and more learning to do in selecting the ideal value of fixed resistor for the voltage-divider circuit so that the analog input of the micro-controller can read the temperatures to a certain degree of accuracy - and it looks like each thermistor will need a fixed resistor tuned to it and to the desired sensing range.
Please correct me if I'm wrong on any of my assumptions - a big part of this challenge is for me to learn, and I'm sharing in the hopes that we can all learn together 
Next steps?
Well, it looks like I'm not off the hook for learning yet!
I need to figure out what this whole "voltage divider" thing is all about. I understand the concept of it - take the 3.3v voltage and split it using a thermistor and a fixed resistor so that an analog input (ADC) can assign it a value, which we can then interpret into a temperature.
The extra learning is about fine-tuning the fixed resistor in such a way that the -20 *C to +30 *C range gets very neatly assigned evenly along the 0-1023 range the ADCs of my Arduino Uno and ESP-based boards can provide. So the goal is to set up the voltage divider circuit in such a way that one end of the temperature range returns 0 and the other end returns 1023, and all the other values fall neatly in between with no major gaps.
To me that also seems to indicate that a less-curvy, more linear, graph for the desired temperature range is what would give the best overall resolution, and best accuracy in the reported temperatures.
Cheers,
-Nico
edit: Additional thought... I'm guess a higher resistance thermistor also reduces the amount of current used, which would be important for battery-based applications.
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