Opinions sought on Sp'Rocketry Article: DIY Altitude Chamber

You know, I have a dead RRC2 that spent many months out in a rain-soaked airframe. I'd thought about dropping it into a beaker half full of water to find its volume, but couldn't bring myself to do it. Kind of a respect-for-the-dead kinda thing I guess. Maybe I'm still holding out hope that cryonics can someday bring it back to life or something . After submitting the Sp'Rocketry article, I started thinking some more about sources of error (other than my guesstimate of the altimeter volume): (1) There has to be some error in the altimeter's pressure-to-altitude calc and in the A-to-D conversion. I have no idea how much and haven't looked into it at all (besides I barely speak sparky language anyway...you know, peckerfarads and all), (2) I threw Z out of the PV=znRT claiming that it was so close to 1 that it was insignificant. This would normally be a good assumption; however, in working with equations with coefficients on the order of 10^(-09), maybe a really close-to-1 compressibility term isn't so negligible after all. Hmmmm, sensitivity analysis... .

Anyway, getting my analysis to agree within 50-100 feet of the altimeter is a complete success in my book, especially considering the unknowns and assumptions that I made. The goal was to come up with a way to tell if an altimeter's altitude circuit was healthy, and this little piston-based chamber will certainly do that. However, if you do some follow-on maturation of the chamber calculation/correlation, I'd certainly be interested in seeing what you did.

Chuck

Well, before you go any father down that path... I'd like to point out that if you just want to know if the alltimiter is busted or fliight worthy, you could just suck on the static port. OTOH, if you actualy want to "do some science", the first thing you should do is use a U tube manometer to measure air pressure. It's just a few feet of aquarium tubing, a meter stick, some tape, and a drop of dye. THe second thing is to add a thermometer to your test chamber, just you"ll know. Finally, drop all the curve fit nonsence from your article and use the U tube to calibrate the syringe calibrations to the measured presssure and curve fit P vs. syringe position directly. For extra credit you can run a flight sim to find P vs. t for a nominal flight, then try and pull your syringe to simulate the same pressure history as the flight. If you want to engage your geekyness, you can devise a mechanism that will pull the syringe for you, and perhaps record data well.

Alan