I got about 5 good launches this past Saturday. Since I don't have a altimeter, I wrote down the amount of water used and the pressure on a piece of paper and ran a simulation once I got home. Here is the data I got.
Launch # PSI H2O (litres) Altitude (feet)
1 90 1.3 258.1
2 100 1.3 305.3
3 120 1.3 401.5
4 125 1.3 425.8
5 125 1.3 425.8
I plan on posting pictures ASAP.
What brand of beverage are you using for your bottles? We were talking to a local scoutmaster, and he was saying they were having a lot of bottles failing recently at 120PSI. He insisted that they were making bottles less durable. We have weighed some different size bottles and it looks like the never ones are 10 grams less in weight. It seems like they are lighter due to a more consistent wall thickness at the bottom of the bottle. That really thick part of the bottle at the center of the base is much thinner than the older bottles.
At a guess, we estimate that the preforms are made with a varying wall thickness to save plastic in the areas where the bottles are not expanded so much. That would account for the thinner walls at the base, and could mean less pressure in an unmodified bottle.