I'm making a pressurized bottle rocket out of a plastic bottle, vinegar, baking soda and a cork. When the vinegar & baking soda react in the closed bottle, the pressure builds up but i want to measure how high this pressure is. Ideally, i'd like to use a small, light weight gauge so that it does not affect the rocket's launch dynamics significantly. I hope that i can also mount the instrument to the inside of the bottle so that i can measure the pressure entirely from within the bottle. Is there a small, lightweight instrument i can use to measure the pressure build-up from inside the bottle?
Why not mount the pressure gauge to the cork itself,so that the gauge remains on the ground with the cork? (i.e. gauge inserted in a hole in the cork or similar setup)
If you were hoping to measure pressure over time as the rocket rises, you can get a pretty decent estimate as follows. This method does assume that nearly all of the gas is generated prior to launch.
1) Measure the pressure prior to launch. Also measure the gas (air) volume in the container prior to launch.
2) record as best you can the time from launch until the fluid is fully ejected.
Assuming a more or less linear expulsion rate, you know the gas volume has gone from its starting value to the full-container value in that time. PV=nRT and you're done.
I agree that this is not a high-precision operation, but it's cheap and simple.
Use a Manometer
A manometer can be mounted to your launch platform and stick into your rocket. The manometer can be made of a simple transparent tube affixed to a vertical rod with dimension tick marks along it. Measure the density of your vinegar (you could just use water density and probably be fine to a close approximation), and then use standard pressure formula using density gravity and height of fluid above datum.
Discaimer - I haven't actually done this.
Put an ordinary balloon in the bottle with the mouth folded over the mouth of the bottle. Add the vinegar and baking soda to the empty balloon and cork. The size of the balloon (reduction of original air volume) should give you a reasonable estimate.
Firs of all, I would measure pressure only during [lots of] ground tests. What's the actual need for making these measurements in-flight? Does it justify the difficulties of miniaturizing your instrumentation?
I would use a pressure sensor that can work with liquid (such as this Honeywell MLH series).
I would use a digital storage oscilloscope for data acquisition. It will show you the time history for the pressure. Most of today's oscilloscopes can export data to file, which allows you to process it later.