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using a RTD, the norms give you dissipation constants in ohm per ohm per degree C, I dont understand as this should simplify to simply degrees C? what is the meaning of this unit?

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Thanks!

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  • \$\begingroup\$ For each degree C, the value, ohms per ohm, is unitless... a ratio vs the nominal (or instantaneous?) resistance \$\endgroup\$
    – Pete W
    Mar 10 at 4:26
  • \$\begingroup\$ In fact even that is a simplification. For metrological uses the equation is quite complex since it's not completely linear and there are also other effects coming into play. The number is simply "how many ohms it varies for each degree of temperature change" \$\endgroup\$ Mar 10 at 7:02
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That is not "dissipation constant", it is the temperature coefficient of resistance.

What they are saying is that if you have a 100 ohm RTD it will be 100 ohms at 0°C and 138.5 Ohms at 100°C (using the most common DIN 43760 standard with \$\alpha\$= 0.003850).

So the average temperature change per degree C from 0°C to 100°C is 0.385 ohms, and that is 0.00385\$\Omega/\Omega/°C\$ (base resistance taken at 0°C).

Similarly, a 1000\$\Omega\$ RTD with \$\alpha\$=0.00385 will change from 1000 ohms at 0°C to 1385.0 ohms at 100°C.

RTDs are nonlinear so different temperatures and ranges (other than the commonly used 0°C/100°C) will result in slightly different numbers for the same RTD.

The three different numbers you have represent slightly different platinum alloys or made with different construction (thin film vs. wirewound on pure alumina etc).

You can use \$°C^{-1}\$ as the unit, the dimensionless \$\Omega/\Omega\$ are to remind you of what it represents.

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  • \$\begingroup\$ very good answer, thank you, I completely understand! \$\endgroup\$
    – JCSB
    Mar 11 at 16:54
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Fractional change in resistance per degree celsius.

For example, a value of 0.01 would mean it changes 1% per degree.

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