IPA specific gravity when too much resin

I would like to figure quantitatively when I should replace my IPA, rather than relying on murkiness or judging if parts are not washed well anymore. Clearly a hydrometer is used for that with Form Wash, but I am using a manual finishing kit. I thought of getting a hydrometer from elsewhere, but when I asked support what is the specific gravity of IPA above which IPA is not good anymore, they could not tell.

I looked on the website and they only give such threshold for TPM (1.0). The only hint is that they say that their hydrometer that comes with FormWash measures in the 0.716–0.881 range. So I suspect one should change IPA above 0.85 or something like that. Anyone has a better idea?

Edit: I am using >=99.5% IPA, this probably matters somewhat,

Also, if anyone knows the specific gravity of the resin, I could probably figure it out, as Formlabs give percentage-based recommendations for replacing IPA.

Never mind, specific gravities of resins are given in the SDS
based on that, I calculated specific gravities of IPA/resin mixtures of various resin concentrations for several resins. Maybe someone will find it useful.
Disclaimer: use at own risk, I may have made mistakes in calculation, or may have misunderstood something.

density resin concentration
IPA 0.786 0.0% 2.5% 5.0% 7.5% 10.0% 12.5% 15.0%
grey 1.08 0.786 0.791 0.797 0.802 0.808 0.814 0.819
rigid 1.26 0.786 0.793 0.801 0.809 0.817 0.825 0.833
elastic 1.02 0.786 0.791 0.795 0.800 0.804 0.809 0.814
tough 2000 1.11 0.786 0.792 0.798 0.804 0.810 0.816 0.822
biomed clear 1.09 0.786 0.792 0.797 0.803 0.809 0.814 0.820
biomed black 1.13 0.786 0.792 0.798 0.804 0.811 0.817 0.824
ceramic 1.6 0.786 0.796 0.807 0.817 0.828 0.839 0.851
PU rigid 1000 1.097 0.786 0.792 0.797 0.803 0.809 0.815 0.821

Hi @pawelkusmierek,

Thank you for sharing this information! I’m certain that this can be useful to other Finish Kit users.