I’ve got a Sontex 789 heat meter and a Samsung heat pump with a Sika flow meter, I’m logging data from both. I’ve found they match very closely. The Sontex has a slightly more steady output:
Over the past month the Sontex recorded an average of 8.9L/min which the Sika reported 8L/min.
If there was any discrepancy I would be more included to believe the Sontex since the Sika is a vortex flow meter which is prone to being more effected by air bubbles than the fluid oscillation principal that the Sontex uses.
Temperature sensing is a different story, I’ve found significant inaccuracies in the onboard temperature sensing in my Samsung heat pump compared to the Sontex which uses wetted pocket temperature sensors. This results in the heatpumps inbuilt monitoring reporting 30% less heat delivered than the Sontex. The Sontex has Class 2 accuracy guarantee while the manufacture’s inbuilt monitoring has no accuracy guarantee, therefore I’m more inclined to believe the Sontex.