hydro test

With a phy of 10.4 you would have a high concentration of alkaline compounds in the water. A temperature of 48 deg in an arid area a lot of water could evaporate leaving a higher ph.

The following is said because of experience with dolomitic water and not as a chemical expert. The alternative possibility must be considered.

The alkalinity of the water caused by CaCO3 and MgCO3 does not show in a normal ph test. It has been noticed that the ph of water rises over time exceeding the possible rise due to concentration. It could be attributed either or both. Reaction with air loosing CO2 or O2, or the precipitation of a calcium film on the container.

In both cases I suspect the excessive alkalinity to be converted to something else having an effect on the ph reading.

It would be wise to inspect the whetted stainless steel for signs of deposits.

A remote possibility, but were the lines cleaned and flushed before test? What chemicals were used for cleaning and flushing? And as Hendrik points out, at what temperature?