i would like to use a thermistor to measure the temp of an engine bay. Ive bought one of each of these from maplin.

R at 100C R at 0C
R at 25C B(K) (approx) (approx)
4k7 3977 3184[ch937] 1528k[ch937]
15k 3740 119k[ch937] 4513k[ch937]

However ive been trying to calculate the resistances at a range of temperatures using this calculation
where R25C is the thermistor's nominal resistance at room temperature, ch946 is the thermistor's material constant in K, and T is the thermistor's actual temperature in Celsius.

the problem i am having is that the values supplied by maplin for R at 100C and R at 0C do not tally up with what im calculating. Can anyone see where im going wrong?

Hi , like you I am having issues with Maplins data having just bought a 15K thermistor from them. I think that Maplin may have got the 0 - 100 R figures wrong.

Have a look at the farnell web site for product order code 1187027. The data sheet link takes you the manufacturer data. ( the 25oC and beta values seem to match. )

That said, I am only working with a thermistor at present whilst I wait for a LM35 to arrive. This seems a bit easier to integrate with the Arduino analog in as it is meant to have a linear 10mV per degC output.

In the mean time I will manually calibrate my output and build a array lookup table for the temperature range I will be working with.

Further checking on the data sheet shows that the Beta value is only valid between 25oC and 85oC.
The data sheet provides Steinhart-Hart coefficient info to calculate resistances out side this range.

I gave up the unequal struggle trying to get temperatures from thermistors and bought some Dallas DS18S20 'one wire' digital thermometers. You could just get one and use it to make a lookup table for the thermistors. You can have a lot of them all connected to just one digital pin. A bit more expensive than thermistors but can be had for reasonable money from ebay.