Can heavy rain dilute Eagle Ray readings significantly?

[Krzysztof Petrov]

After a heavy rainstorm, I sometimes notice slight changes in my test results. Does rainwater significantly dilute chemical levels in a pool?

 

[Afiq Tan]

Heavy rain can noticeably shift test results, especially if the rainfall raised the water level a lot or you had any overflow. The common pattern is sanitizer reading lower from dilution and from extra demand (dust/organics brought in by the storm), and pH can drift even when the water still looks fine. If I’m handling it, I run circulation 30–60 minutes first, clear debris, take an elbow-deep sample away from returns/skimmer, test, and then test again 12–24 hours later since the bigger drop sometimes shows up the next day.