I measured my water ph and it is changing every few hours

Water company says it should be 7

str8 from tap: 4
10 hours later: to low to tell
24 hours later. 6
48 hours later to high to tell

wtf is happening :/

when is the correct time to measure ph?

I would say your meter needs calibrating. if water company says 7 than why would it be 4 out of the tap? many water supplies do add in buffers to stabilize the ph but they are not that strong and should not cause those fluctuations

Its what you are using to measure it. Its not accurate

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-- just a thought--