Hi,
Thermal time is analogous to an RC time constant.
Tau = RC
Tau (thermal) = Thermal Mass x Thermal Resistance
The sensor from Farnell, manufactured by Carel, is a cylinder of mostly plastic attached to wire. The sensor will have a thermal mass dominated by that amount of plastic.
The sensor has a time constant of 75 seconds in air (assume natural convection).
If you have forced air, the thermal resistance will be reduced by a 5x and the thermal time constant will be proportionally shorter.
This is independent of the manufacture, any similar sized blob of plastic will have a similar time constant.
A metal-cased sensor will have a higher thermal mass than a plastic sensor, and therefore a longer time constant.
The beta and the resistance at 25C define the resistance versus temperature curve.
Jay_Diddy_B