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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

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  • \$\begingroup\$ 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. \$\endgroup\$ – Marcus Müller Aug 8 '19 at 15:14
  • \$\begingroup\$ 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. \$\endgroup\$ – Marcus Müller Aug 8 '19 at 15:20
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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.

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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

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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.

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