Got this from the German K-bike technical site ( http://k-technik.pytalhost.com/pdf/wassertemp.pdf ). I thought it is useful to reproduce the information here in the BMW K100 forum for future reference.



First line in the Blue region is #1; Last line in the Red region is #8. The following resistances will deflect the needle on the temperature gauge to the given indicator line.

Indicator Line	Resistance (Ohm)	Notes
1	200	First line in the Blue region
2	150
3	100
4	70	Reg's White dot
5	50	Reg's Red dot
6	40	Cooling fan should engage
7	30
8	25	Last line in the Red region

The aproximate resistance values measured at the water temperature thermocoupler on the water pump for certain engine conditions are as follows:
Engine temperature = 10 Celcius ~ 1.3 Kohm
Engine temperature = 20 Celcius ~ 250 Ohm
Normal Engine Operating temperature ~ 70 Ohm

I am uncertain whether the thermocoupler is linear or non-linear in its response (which may match a linear or non-linear response in the gauge) However, with this technical information it should be possible to calibrate a non-BMW temperature gauge to the BMW thermocoupler sensor. A suitable test bed for such a calibration will look like this (taken from the document above):