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?
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$\begingroup$ Most gauges use a bourdon tube with atmospheric pressure acting on the outside, if you put the gauge inside the bottle and leave the connection to the pressure gauge open to the atmosphere then it will show a reading butyou will have to re-scale it. Or use a pressure sensor but you will have to feed the wires out etc. $\endgroup$– Solar MikeAug 14, 2017 at 17:18
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$\begingroup$ Maybe using some simple method to calculate the height that the rocket goes to might help to back-calculate the pressure inside? $\endgroup$– Eashaan GodboleAug 14, 2017 at 17:42
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1$\begingroup$ @EashaanGodbole would the shape ie aerodynamics also have an effect? $\endgroup$– Solar MikeAug 14, 2017 at 17:48
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$\begingroup$ @SolarMike The stuff inside the rocket would generate CO2, which would be responsible for the pressure. At some point, the cork would pop off, and the CO2, vinegar and whatever is ejectable would be ejected rapidly, propelling the rocket in the other direction. The shape of the rocket would determine how high it would go. It might also determine if it turns - it depends on the wind conditions too. So, what you're saying is that my suggestion would not be that accurate. Am I right? $\endgroup$– Eashaan GodboleAug 14, 2017 at 17:52
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1$\begingroup$ @EashaanGodbole would you think that the dimensions of the orifice (jet) will also have an effect? $\endgroup$– Solar MikeAug 14, 2017 at 17:56
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4 Answers
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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.
answered Aug 14, 2017 at 18:33
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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.
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$\begingroup$ The 2 litre PET bottles can withstand 8 bar. I don't know what pressure the vinegar / soda mix will generate but if you're using water in the manometer you might need a tall ladder to read it. $\endgroup$ Aug 19, 2017 at 20:57
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$\begingroup$ The pressure in the bottle is highly unlikely to be 8 bar; probably somewhere closer to 1 or 2. However, you are right that it would be a significant height since 1atm of pressure would be about 33 feet of water column. Better to use a digital manometer then! $\endgroup$ Sep 14, 2017 at 15:38
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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.
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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.
answered Sep 3, 2017 at 3:46