# Beta value of the thermistor

Why is that there are two different Beta constants for a same thermistor even though Beta constant depends on the material?

For example: Murata's NCU15XH103D60RC has three different beta constants:

• at B25/80: 3428,
• at B25/85: 3434,
• and at B25/100: 3455

• can you link to the datasheet in question (and point to the specific page etc) where it specifies three different Beta constants? Can't find that. – Marcus Müller Aug 8 '19 at 15:14
• ah! Um, what do you thing the "25/80", "25/85" and "25/100" mean? They have units in the datasheet, you know, and you don't need to think very hard when you realize what these units are measuring. – Marcus Müller Aug 8 '19 at 15:20

The classic approach when dealing with RTD's is the Steinhart-Hart equation

$$\\frac{1}{T} = a b\cdot ln(R) + c\cdot ln^2(R)\$$

This is a very accurate model of the characteristic change with respect to temperature.

This however is quite "complex" to deal with in real-time (if accurate temperature is require).

The $$\ \beta model\$$ is a special/reduce case of the Steinhart-Hart equation

$$\ R = R_0 e^{\beta ( \frac{1}{T} - \frac{1}{T_0})} \$$

such that:

$$\a = \frac{1}{T_0} - \frac{1}{B}ln R_0\$$

$$\b = \frac{1}{B} \$$

c = 0

It essentially sets the operating point around a given temperature. The further you drift away from this temperature, the less accurate the calculated temperature.

By providing the beta at different T0, it provides the designer the opportunity to tailor their accuracy around the temperature of interest. If you need an over-temperature warning at 85C then the beta at this temperature is of interest

The beta model is just that -- a model. You will get different answers for beta when you use different temperature points to estimate beta.

B25/80 is likely beta when calculated using data at 25 deg C and 80 deg C.

All those betas are closer than 1%. If you need better, consider using a thermistor quantified with Steinhart-Hart coefficients.