<<Wouter wrote:
 The general idea is certainly doable, there was an article in CircuitCellar
 some time ago about a similar project that used an acceleration sensor.
 Gotcha: he got periodic misreadings, I think due to the current consumed by
 his eeprom chip during writing (influences the AD, maybe due to bad wiring
 or battery impedance or too small decoupling elco). Are you sure the change
 in pressure due to the velocity (the moving air gives less pressure thing)
 won't give you trouble? I guess yes, but not at the highest point in flight.

Dave wrote:
There's also a pressure spike around mach that can cause problems.
If you're just logging, it's not such a problem, but many designs detected
max altitude by indicated baro pressure, or zero acceleration (as opposed
to negative during coast phase) and got fooled.>>

I hadn't taken that into consideration. Do you think it would have affect the
data considerably? Considerably being +/- 15 feet at max alt. The spike at
mach I'd thought about, since my friend does intend to fire off a rocket that
busts mach before topping out above 3000ft. Possibly by installing an
accelerometer and comparing its data to the transducer's I could see where
problems lie in each one. The first few flights will have to be experiments
anyways.

<< There are a few free C compilers around, browse the list. FAIK the HiTech
is
 for 16x84 only, but at least BKD is for all PIC but limited to 1K. But when
 you just write the lowest pressure to eeprom you could easily stick with
 MPASM (free) or my Jal compiler (equally free). For all beginers: stick to
 FLASH pics, unless you absolutely have to use a non-flash. If you can live
 with the physical size take a 16F877, otherwise a 16F84. And for the
 calculatons: what good does it do te let the poor PIC do arithemetic when
 you will use a PC anyway to read out the data? Just store the raw AD value
 and let the PC handle the brainwork.>>

Thanks for the info. I had seen quite a few C compilers on the PIC beginners'
checklist but wasn't sure which one to use. If the HiTech is just for the
16x84 that's fine by me since that's all I'm using for now. I've been sure to
verify that when filling out an order form for PICs, I was getting the flash
vesion. On the next one I'll include a 16F877. As for arithmetic in the
computer, I think you're right. At first I figured since the PIC was probably
going to have little to do (read serial output from the ADC, send it to the
EEPROM) so why not have it do a few calculations. But the more I thought
about it, the more I realized it was just a waste of code and work for the
PIC. Not to mention the memory chip; straight output would take 12 bits each
reading, where-as computed data would take 14 bits for 0-10000 ft.


-Tony

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.