There's likely a correlation between volume, pressure, temperature, where if you know several you can determine the one.
freshman physics: pV = nRT
pressure, volume, temperature, a constant R, and n = number of molecules.
Problem is there will be molecules present also, yes? O2, N2, CO2, etc.
Being rainwater it is rather acidic depending on the wind direction so NOx is likely a problem although i use a limestone bed to mitigate this , PH is already monitored before ozone is used though.
Cheap ozone sensors are also responsive to NOx .
I am hoping that i can take a sample into a chamber with a head space and make a meaningful measurement with a cheap gas sensor.
It is complicated by the fact that ozone can have a short half life which depends upon organic contamination of the water.
This BTW also causes problems with my current measurement method which is a chemical , one shot , colourmetric/titration method.
My town (city) uses Ozone to purify water, comes out nice & soft (or maybe that's due to other minerals) so it suds up nice, the water tastes good, and there is no chorine smell. I don't know much more about how it's actually done.
Ozone does not materially affect water hardness although it can oxidise metals if present, it is of little use where there are large amounts of organic contamination though, water should filtered and sedimented first before use.
Rainwater although quite soft can be very filthy when collected from roofs , even glass ones.