I asked the same question 10 posts down - also have a 3c diff. No answers yet...

Quoted from "boswell"

maybe they will like me more and answer it   hehe

They'll think you'll bring the angels ;-)

Hi!

Maybe the temprature sensor in the pump sit near by the motor. The motor produces secondary temperature too and u have this 3 degrees C more on the display.
Maybe ur secondary sensor is not directly in contact with metal/liquid to garant a resistance free transfer of the heat.

I hope my english was good enough to understand what I mean....I´m german :o)

I don't think that water that is moving with that normal speed the xt ultra produces, could have an temperatur diffrent of 3°C.
I think that it's:
1. sensor inexactness: We dont have any high end labor sensors
2. normal diffrent becaus the pump-sensor is heatn up a little bit more becaus of the engine, and not cool down by the water as fast as the engine it heats it up.

Pumps add heat. That's a general (and I'm not trying to be a smartass) rule of watercooling.

It is usually best to put the pump directly BEFORE the radiator in a cooling loop, and push the maximal amount of heat out of the rad.

The temp sensor for the pump is indeed outside the water flow (there is no thermal probe in direct contact with the water). This has the advantage of getting you a "close enough" water temp, while providing a more accurate temp of the pump internals (to gauge whether you've cranked the freq up too high).

Why put no probe in contact with the water inside the pump? Because these Aquastream XT's are based on Eheim pumps. Putting a conductive thermal probe inside the pump housing can drastically screw up the electric motor (magnetic stator) that spins the pump impeller assembly. Additionally, AC would have to "re-invent" the wheel of the Eheim pumps. Why fix it if it ain't broke?

This issue pissed me off too, but considering that XT's can be applied to older pumps, the additional control improvements, and Aquaero/Aquasuite integration, the tradeoff is worth it.