Grapes contain a high level of tartaric acid which makes them quite acidic - I would guess that you should see values around 3.0 - You can use those pH strips, but I would only rely on them if your desired pH range isnt too important...A ph meter is really the best way to go about it - You can get a decent one these days for under $50 - It will pay off in the long run both for accuracy and peace of mind