Peter H
There is a bit more info in the design specification of Airbus pitot tubes systems in the ATSB Guam report AO-2009-065 (involving the preferred Goodrich probe).
but again it just mentions monitoring current against thresholds. To justify the term Probe Heat Computer I trust it measures true AC (115V 400Hz?) rms power using V*I*cos(phi), but that used to be done in simple hardware devices. There should be a measurable change in heater resistance with heater coil temperature, but guess not simple to compensate for 'wind chill effect' of different humidity air streams - its also possible external ice build up creates a thermal insulating layer. If you just want to detect icing conditions use an ice sensor- melting point close to 0 deg C despite air mass at -40C at cruise altitude?