Original URL: http://www.theregister.co.uk/2012/06/01/rehab_altimeter/
LOHAN sucks Reg reader's instrument to death
Strange doings in the shed at our mountaintop REHAB complex
As followers of our Low Orbit Helium Assisted Navigator (LOHAN) project will be aware, we've done quite a bit of head scratching as to how we're going to fire the rocket motor of our Vulture 2 spaceplane.
The Rocketry Experimental High Altitude Barosimulator (REHAB) experiment is a shed-built hypobaric chamber designed to perform said test, and as such, was built specifically with rocket ignition in mind.
However, we realised that we could get extra value from REHAB when reader Neil Barnes got in touch to say he'd built a barometric altimeter a few years back, and wondered if we'd like to give it a blast in the hypobaric chamber.
Rocket motor ignition trigged by barometric altimeter is among the proposals we're considering for LOHAN, so we jumped at the chance to see how one would work under simulated high-altitude conditions.
This is Neil's kit out of its box, and with the display (face down) disconnected from the main board:
The main components are, Neil explained, an Intersema MS5534 temperature compensated pressure sensor, Atmel Mega32 microcontroller, DS1307 real time clock, and Lascar Electronics SP 5-GFX1 display.
He added that there are "sundry other bits such as a switch mode power supply and level converters to sort out the different voltages everything wanted.
"The devices all talk with not-quite-compatible two wire serial data streams, which was a bit of a pain. These days, things work at a wider range of voltages, which makes things easier."
Neil told us: "The original development was largely as a learning experience; I wanted to try some designs for a small but usable variometer/altimeter for use when flying – commercial ones were all three times the size, or smaller but with no display. Up to about eight thousand feet – as high as I got with it – it was fine, but I never expected to meet the stratosphere!"
Here's the altimeter inside the REHAB chamber, indicating a pre-test altitude of 3,445ft (1,050m), which is about right for the Special Projects Bureau's mountaintop headquarters. The reading at the top is the elapsed time since power-up, and the one at the bottom is the rate of rise/fall:
The entire REHAB rig currently looks like this, after we got our hands on a decent vacuum pump from Applied Vacuum Engineering. The bricks are simply to prevent the chamber falling over*, should somebody knock into it...
It's fair to say that the test proved entertaining. After powering up the pump, I very, very slowly opened the isolation ball valve to begin evacuating the chamber.
The altitude reading rose slowly, peaking at 32,707ft (9,969m)...
... before dropping back to around 22,000ft (6,706m), where it hovered for a couple of minutes until I closed the isolation valve.
I'm pleased to report that our lovingly-moulded silicone seal works a treat, so there was no noticeable drop in pressure inside the chamber.**
Accordingly, you'd expect the indicated pressure to remain the same, but it then inexplicably started to rise over 15 minutes to 30,000ft (9,144m) before the altimeter decided it'd had enough and shut itself down.
We know the REHAB system draws a much greater vacuum than the equivalent of 32,707ft, so what happened? Neil said: "If it was completely non-tracking once it started descending, I suppose the only conclusion is that the pressure sensor has died the death. It's six or seven years since I built it, and I have no idea of the expected lifespan. Maybe the cell sprung a leak?"
Regarding the shut-down, Neil offered: "I wouldn't expect the batteries to have died, so perhaps there the display didn't like the pressure. A set of Duracells lasts a week continuously; it only eats a few milliamps. Or if the batteries died to the extent that the SMPSU got upset?"
We may never know. More investigation is is evidently needed into the possibility of using a barometric altimeter. Neil concluded: "It does suggest that this might be an approach which could be reengineered, if you can't find anything commercial. One of the posters pointed out a more recent Intersema package which might be of use..." ®
*That's not strictly true. They're also there to annoy my Spanish mate Eloy, who's been following LOHAN and recently protested that the same bricks keep popping up.
He may have a point...
...although I'm ignoring his plea to "use some different bricks for a change, for the love of all that's holy".
**We know this because I left the chamber for half an hour, and still couldn't prise the perspex lid off without relieving the vacuum.
Further LOHAN resources:
- New to LOHAN? Try this mission summary for enlightenment.
- You can find full LOHAN coverage right here.
- Join the expert LOHAN debate down at Reg forums.
- All the LOHAN and Paper Aircraft Released Into Space (PARIS) vids live on YouTube.
- For our SPB photo archive, proceed directly to Flickr.
- We sometimes indulge in light consensual tweeting, as you can see here.