I believe the Raven uses the Intersema ms5541c which is an all-in-one pressure sensor & ADC all temperature compensated. I just skimmed the online Raven doc and it says 0.1mbar accuracy (as does the ms5541c data sheet). The Raven doc mentions the Wyoming balloon data and as Shawn points out, you could always get your own pressure info on the way down. You'll need temperature to go along with it in order to get density info so there's a reason to dig out one of your Arduino boards.
But it's interesting that Adrian says you can't trust the baro data during powered flight--that just tells you how tricky this stuff can be because I hadn't gotten to the point of even considering the effects of acceleration on the baro sensor. But it makes sense because to get 0.1 mbar resolution means having a sensor that responds to very little force. I can certainly see how a few Gs (or more) would overwhelm something that sensitive.
To Shawn's question of obtaining air density from descent rate for a given parachute & mass: the Cd of a given parachute varies dramatically during descent because of the oscillation (like a pendulum) and because of the lateral movement. Descent times are notoriously difficult to predict because of that. So the short answer to the question is "no". Richard's site has some info on this (thus bringing it back to sugar rocketry :-)) which can be found at http://www.nakka-rocketry.net/paracon.html
--SteveOn 02/23/2015 12:05 PM, Michael Monteith (Redacted sender michael_r_monteith@xxxxxxxxx for DMARC) wrote:
Steve, I actually didn't want to post it here as it was kind of off topic in the sense it wasn't sugar propellants in the strict sense but it is in the sense of measuring results. I forgot to take out the group email and put Richards in it so we could discuss his findings. But that's okay as long as others don't mind. I hate you can't un-send. lol But glad I did in a way too. I think your last statement sums it up. It's fun and we learn a lot from it. Hey, if I don't like the limitations of the instrumentation I can build bigger and better. Been there, done that too. Guess if nothing else it will give me an excuse to work more with my Arduino boards. I'm used to digging through datasheets as sometimes that's the best way to find what is ideal for your specific use. Guess I'll have to find out from the 2 altimeters I'm looking at which specific parts they use. For this part going in I just needed more of personal experience and what others have already done to at least start off with and an altimeter setup that would suffice. I love the fact EggTimer TRS has gps and radio for tracking and for that price I get altimeter and tracking. Except I won't have accelerometer. Then with the raven I have baro and accel but lose gps and radio. If I had the money I'd just go with the Altus but I love my marriage thanks. I'm much better off with a little here and there. Hey, if I have to do a tethered balloon to obtain comparison data it might be the way to go actually. I'd like to see that report you're talking about. Discouraged? No Way. Challenged? Great! Never know what might become of it.. Nothing if I don't try. Michael
