Hello again,
Eric:
Yes that's interesting too, and you'll notice i left off the 'C' constant because it is usually not needed, but even more to the point is it rarely shows up on data sheets for thermistors. So i figure the shorter version is just as good
Also makes solving for the constants a little easier.
Alec:
You're welcome, and if your users cant find the data for their thermistor (the two constants Ro and B) they can be calculated from two measurements taken at two different temperatures Tc1 and Tc2 (which yield two resistances Rx1 and Rx2):
B=((Tc1+273)*(To+273)*(ln(Rx1)-lnRo))/(To-Tc1)
(To is usually taken to be 20 or 25 degrees C)
and:
lnRo=((ln(Rx2)*Tc2-ln(Rx1)*Tc1+273*ln(Rx2)-273*ln(Rx1))*To+
((ln(Rx1)-ln(Rx2))*Tc1+273*ln(Rx1))*Tc2-273*ln(Rx2)*Tc1)/((Tc2-Tc1)*To+
273*Tc2-273*Tc1)
and so:
Ro=e^(lnRo)
The formula for lnRo is much simpler if we use Kelvin and ln(R) for all the resistances instead of just R:
lnRo=-(((R2-R1)*T1-R2*To)*T2+R1*To*T1)/(To*T2-To*T1)
where R1 and R2 are in units of natural log Ohms, and T1, T2, and To are in Kelvin,
but probably nobody likes doing it that way