I drained the radiator and fill in then 3 litres of Castrol Antifreeze, 2 litres of water and 1.5 litres must have remained in the system as the Haynes manual says 6.5 litres with external expansion tank.
So the concentration of antifreeze should be at least 46% (3 l antifreeze of 6.5 l in total), but the tester says repeatedly less than -12°C.
What's this?
Car was driving about 3 miles before testing the coolant.
Two other cars show a concentration for -18°C resp. -22°C with this tester, but I haven't filled up the system and don't know the real concentration in there.