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.
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.