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#### Simon

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• Did that just blow up? No? might work after all !!
« on: June 30, 2014, 08:57:27 am »
Attached is the datasheet in question, there are tables on page 2 that give values and some formula's to calculate temperature at any given temperature but I'm a bit lost for understanding them. How do i use the figures given to work out the resistance for a given temperature.

I think I'm being asked to divide the difference between my  new temperature and 25 by the new temperature (delta B/B) and then to i add the coefficient given in the table ?

#### Kremmen

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« Reply #1 on: June 30, 2014, 12:07:03 pm »
Not exactly. Thermistors behave in an exponential manner so the curve is nonlinear all the way. The coefficients give a close approximation to the curve and they are used as follows:
- firstly, you have the nominal resistance at 25C (=298.15 K). This is given in the table. Lets call that Rn
- you have the beta multiplier (B[K]) used in the formula, also given in the table. Lets call that b.

Using this info, you can calculate R at any temperature using the formula:
-First take the reciprocal, in Kelvin, of the nominal and the target temperatures, 1/T1 and 1/T2*
-then put all the numbers in this formula:

R = Rn / e(b * (1/T1 - 1/T2)), i.e divide the nominal resistance by the exponential of beta times the difference of reciprocal temps.

* In case you forget, you get Kelvin from Celsius just by adding 273,15. This makes the temps absolute which is usually required in physics calculations.

P.S. there is also the alpha coefficient given. That one indicates the percentage change of resistance per degree (Kelvin) of temperature. NOTE: The alpha is nonlinear; the magnitude changes over the temp range. That's why it is given at a specific temp. So be careful that your calculations make sense if you use the alpha coefficient.
« Last Edit: June 30, 2014, 12:15:54 pm by Kremmen »
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