Thanks to all that replied.

In summary, the most that I am concerned about is the repeatability of the
Schmitt trigger.  Good accuracy can be achieved but you need to be careful.

Some ideas are:

1. Regulate the power supply.  This may reduce the variability of the
Schmitt trigger and improve the voltage consistency for charging the cap.

2. Check for noise on the thermistor lines if long lines are required.
Twist/shield if required.

3. Use a counter with sufficient bits to ensure that you get good
resolution.

4. Some suggested linearizing the thermistor somewhat by using a parallel
resistor of average thermistor resistance.  In my case the thermistor does
not have that large a temperature coefficient and is already moderately
linear.

5. Use a lookup table.

6. Maybe a non-Schmitt trigger port should be used for more consistent
results.

7. Average reading results to get more stability if required.

8. Keep sources of noise to a minimum.

9. Polycarbonate and Dipped Polypropylene have pretty flat capacitance
change vs temp curves in the region you are looking at.  Polycarbonate caps
should vary less than 1/2% over the -5 to 20C range.  Polystyrene should be
similar.  Avoid ceramics with dielectrics such as x7r, y5v, z5U,  and all
electrolytics. Some of these can vary 40% over the temperature range!!!!

Regards,

Gordon Williams

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