Hi,
at least I think a thermistor is linear
No, they are not linear, but if you are using only a small range of scale there are ways to make the line fairly straight using an op amp circuit. (See "Analog Interfacing To Embedded Microprocessors," chapter 3, by Stuart Ball)
Panic mode gave you some equations, but you need the constants from the manufacturer to use them.
The reason for using resistive sensors, NTC thermistors, in particular, is that they are fast. They are not as fast as a thermocouple but can respond in less than five seconds, where it may take a silicon sensor up to thirty seconds or more. Thermistors also have much greater range than silicon, and I think Dan wants too measure temperature 'from 100 to 250F' which are beyond the range of silicon ICs. (top ~150F) They are also more accurate than silicon ICs.
RTDs are fast and accurate, but not as durable, and more expensive, and they also require linearizing, although with a 2nd degree polynomial, rather than 3rd degree like the NTC.
If you want to spend the time to develop the application, NTCs can give you within 0.1 deg C., but less fine results should be easy to get, without a big investment in time.
theyve been good enough for car people for years maybe its ok.
Yes, of course! I would stick with what you have; if you persist you will get there, and will have another tool under your belt by the end of it all.
It sounds like you are measuring the temperature of water? Also, if you wished to go with a nice IC that gives you a neat digital value, you would need to wrap it as they are not immune to water.
Just my 2 cents
Robert