Need help with adjusting S02 and how PH effects it

[strongarm]

I have 50 liters of cabernet aging in a barrel. I believe I want my free S02 to be around 25ppm. I am confused on how the PH effects the S02 levels. Say my PH is at 3.9 and my S02 is at 0......do I want to achieve 25ppm or 100ppm based on my PH level?

I have used the calculator below but changing the PH doesn't seem to adjust the S02 needed.

http://winemakermag.com/1301-sulfite-calculator

 

[DoctorCAD]

I just tried it and it does change. At 3.26, recommended is 23ppm and at 3.90 recommended is 99ppm.

That pH seems off to me?? Should be down around 3.5 maybe?

 

[strongarm]

I see that now in the note section. I have a calibrated PH tester and got a solid 3.9 reading. It's an older tester and I have a new one coming in tomorrow to confirm. Is 3.9 out of the norm for a cab?


Sent from my iPhone using Home Brew

 

[strongarm]

I just got a Vinmetrica 300 and did another ph test....3.75. So it looks like I want my free S02 to be around 72.5ppm.