NTC question

Hello, I'm working with picaxe some while, but to measure temperatures I always used the DS18B20. Now I would like to have a more accurate measurement for reading the temperature of an egg incubator.
Now I was wondering which NTC to use. It has to be very lineair around 37.5 degrees since this will be the required temperature. I was thinking about this one (the first or second 10k): http://www2.produktinfo.conrad.com/datenblaetter/450000-474999/467235-da-02-ml-Heissleiter_B57350-G103-256_de-en.pdf
Will this do for a such a temperature? And then a 100k resistor to make the circuit?

Stick with the DS18B20.
Thermistors are not that linear and a lot
harder to set up and use accurately with a picaxe.
Most incubators only have a Bi-metal switch
or diaphram type which can give differing
results, unless you have a good digital design type.

The thing is, I'm making the incubator myself, and to get good readings I want an accuracy of 0.1 degrees, and a ds18b20 only has an accuracy of 0.5 degrees. That's why I rather use an NTC, I know they're not lineair, but they can be more precise. The lineair range only has to be a few degrees, say from 35 to 39...
Thx for the help

Try a 10k or 47K NTC and a 10 k resistor.
You will have to calibrate it some how
fick off the ADC values that suit you best
and work with them.

I used a DS18B20 in my own incubator with
2 x 40W light globes for both Chickens and
Bearded Dragons without any probs.

Wow. 0.1oC. Thats quite a spec.
How are you going to calibrate it?
Not with a 2 quid Beijing temp LCD module I hope?

Just out of interest have a look at this:
http://www.farnell.com/datasheets/72430.pdf
(merely food for thought.)

It looks like it could give good results over the required range - with calibration!

Perhaps there's some fancy calibrated digital sensor on the market?

I've got a 25k laying around, I'll try that one, if that doesn't work, I'll use a ds18b20, if that works for you michael, it should work for me too.
Thx for the help guys, hope it works! Grtz

Ideally your source impedance to the PICAXE ADC input should be 10K ohms or less - this is close with your existing thermistor and a series reistor equal to the thermistor resistance at 37deg - if the thermistor is 20K at that temperature and you put 20K in series you are in the range - if you want more sensitivity ad an OP amp. You don't need to worry about non-liniarity when working across a small temperature variation. Remember that the series resistor should be quality with low drift and temperature coefficient [metal film is great].

Your best bet REALLY IS the DS18B20.
It may only have an absolute accuracy of 0.5C but it has a repeatable resolution of 0.06C using readtemp12. Your biggest problem (as already mentioned) will be calibration. It is possible to hire calibrated 4 digit thermometers for a small fortune if you really do need such accuracy.
If you take the NTC route, how will you keep everything calibrated and within spec while things such as ambient change considerably? If you change the leads or other parts, you will need to calibrate again.
I have used several DS18B20s over many years and never seen any spec drift that has exceeded the its own resolution.

Stick with the ds18b20 and don't worry about calibration. Opinions on what the temperature should be for incubators vary by more than 0.5 degs. Place sensor at top of eggs. Remember your picaxe can only regulate the temp of the sensor, it is rest of incubator design that influences how constant that temperature will be throughout the incubator chamber. Build it and hatch some eggs and be guided by your hatch outcome what you need to do. Chicken eggs take 21 days to hatch, if yours are little late is usually a bit to cool but if early probably afraction warm. If you must have 0.1 precision you can program the Thigh and Tlow limits for each sensor you have calibrated and use/read them in control program rather than read.temperature value.