WayneRATS 0 #1 June 2, 2003 just in interest i know that the needle moves up because there is a decrease in pressure .. but how do manual bog standard needle alti's physicaly work? has anyone ever had one apart? ------------------------ Can You Ere Me Now? Quote Share this post Link to post Share on other sites
quade 4 #2 June 2, 2003 Read THIS.quade - The World's Most Boring Skydiver Quote Share this post Link to post Share on other sites
nacmacfeegle 0 #3 June 3, 2003 hehehe, love that picture of a wee ballon turning the pointer. Which is more or less how they work, just grossly simplified.-------------------- He who receives an idea from me, receives instruction himself without lessening mine; as he who lights his taper at mine, receives light without darkening me. Thomas Jefferson Quote Share this post Link to post Share on other sites
WayneRATS 0 #4 June 3, 2003 great stuff cheers for that------------------------ Can You Ere Me Now? Quote Share this post Link to post Share on other sites
AndyMan 7 #5 June 3, 2003 When people ask me this and I don't have a pooter handy... Start with a standard Mason jar. Take a standard latex baloon, (or any other latex product....) and cut it up so that you can pull the latex over the top of the jar. Use a stiff rubber band to hold the latex tight over the jar. Now take a popsicle stick, tape it to the baloon so that it runs from the center of the baloon outwards. Place a ruler standing beside the jar, so that the popsicle stick points at the ruler. As the air preasure outside increases, the baloon will push down into the jar, and the popsicle stick will point up. As the air preasure outside decreases, the baloon will push up out of the jar, and the popsicle stick will point down. Turn your ruler upside down so that pointing down points to big numbers, and pointing up points to small numbers. Calibrate accordingly. Now you just need to figure out how to turn the linear scale of the ruler into a radial scale of the altimeter... This is explained on alti-2's website as "The mechanical movement transfers the linear motion of the capsule to a rotational motion of a pinion through a system of levers and gears. A pointer is attached to the pinion so the rotation of the pinion can be indicated." _Am__ You put the fun in "funnel" - craichead. Quote Share this post Link to post Share on other sites
Unstable 9 #7 June 3, 2003 take your apart and find out sds=========Shaun ========== Quote Share this post Link to post Share on other sites
JamesNahikian 0 #8 June 3, 2003 Done that, and observed a bit of lubrication solves most of your aneroid troubles. Not that I suffer from aneroid problems very often. D. James Nahikian CHICAGO Quote Share this post Link to post Share on other sites
nightjumps 1 #9 June 4, 2003 QuoteFM Paul, Thanks for the laugh. Haven't heard that since my Army days. Quote Share this post Link to post Share on other sites
stuartjl 0 #10 June 4, 2003 One of the problems of this type of barometer style equipment is that air pressure doesn't change uniformly with altitute. The atmosphere creates a loss of 2 degrees per thousand feet barometric, but in reality the temperature can be the same for several thousand feet in freefall which will falsify the alti reading so you need to either have an audible or keep an eye on the ground!! Quote Share this post Link to post Share on other sites
WayneRATS 0 #11 June 4, 2003 do audibles work on the same principal?------------------------ Can You Ere Me Now? Quote Share this post Link to post Share on other sites
SkydiveMonkey 0 #12 June 4, 2003 Why would an audible make a difference? Still works off air pressure. The only accurate reading would be GPS____________________ Say no to subliminal messages Quote Share this post Link to post Share on other sites
quade 4 #13 June 4, 2003 Normal variations in temperature beyond ISA do not represent a significant inaccuracy in simple barometeric pressure altimeters normally worn by skydivers. In fact, they do not represent an inaccuarcy large enough to be a cause of concern during instument approaches in airplanes where the standard acceptable deviation is plus or minus 75 feet. You -may- be confusing the concepts of barometeric pressure with the concept of density-altitude. Density-altitude is barometeric pressure corrected for temperature variation. More information HERE.quade - The World's Most Boring Skydiver Quote Share this post Link to post Share on other sites
quade 4 #14 June 4, 2003 QuoteThe only accurate reading would be GPS GPS has a number of issues with regard to altitude accuracy. Generally speaking, you'd currently be much safer with a barometeric altimeter than reliance on any consumer available GPS device that you might wear for the purposes of skydiving.quade - The World's Most Boring Skydiver Quote Share this post Link to post Share on other sites
quade 4 #15 June 4, 2003 Quotedo audibles work on the same principal? Not exactly. They use piezoelectric sensors and convert the pressure to electrical signals which are then interpreted and converted to altitudes. Pro-Tracks also contain temperature sensors in an attempt to figure out "Skydiver Airspeed" which is an L&B invention that attempts to normalize readings to a specific ISA equivalentt density-altitude of 4,000 feet MSL. The concept is that this would be helpful in matching fall rates.quade - The World's Most Boring Skydiver Quote Share this post Link to post Share on other sites
stuartjl 0 #16 June 4, 2003 I hope that you're ready for this... The altimeter works on the principle that the pressure within a column of air decreases in a known way with height. For the scientifically inclined, the relationship is z = (RT/gM).loge(po/p) where z is the height difference between the starting height and the measurement height, R is the gas constant, T is temperature of the air in degrees Kelvin, g is the acceleration due to gravity, M is the molar mass of the gas (in this case air), po is the atmospheric pressure at the starting height and p is the atmospheric pressure at the measurement height. Electronic altimeters have this relationship programmed into the chip. Happy? Quote Share this post Link to post Share on other sites
SkydiveMonkey 0 #17 June 4, 2003 Quote GPS has a number of issues with regard to altitude accuracy I'll take my ft-50 anyway, but my point is gps isn't affected by temperature /pressure variations etc____________________ Say no to subliminal messages Quote Share this post Link to post Share on other sites
WayneRATS 0 #18 June 4, 2003 wish i'd never asked ------------------------ Can You Ere Me Now? Quote Share this post Link to post Share on other sites
quade 4 #19 June 4, 2003 Among my points was that most GPS units that are available to consumers that skydivers might wear is that most (all?) of them have no idea how high they are over the terrain. They -do- have an idea of how high they are over the GPS spheroid, but that's not the same as AGL or MSL. Further, GPS requires at a minimum good signals from 4 satellites in order to display altitude information. While it is possible that you may have this at points during your freefall, it sure would suck if you didn't at a critical time. As a data gathering device for BirdMan flights -- rock on, but currently, I would not recommend GPS as a replacement for a barometeric altimeter.quade - The World's Most Boring Skydiver Quote Share this post Link to post Share on other sites
stuartjl 0 #20 June 4, 2003 The point is that to rely on just one piece of equipment is a risky business if for no other reason than because all equipment is liable to failure. My advice for what its worth is to use a mechanical alti and an audible for flexibility and safety. Belt and braces... By the way which Regt.? Quote Share this post Link to post Share on other sites
nacmacfeegle 0 #21 June 5, 2003 Instead of the "balloon" turning a needle via pinions, gears, wee rods, and stuff, it compresses a piezoelectric device. When you distort a piezoelectric device, its electronic characteristics change. If you can link a lever to a "balloon" in the altimeter described above, to a piezolectric device, you can change the amount of distortion of the peizoelectric device as the balloon expands and contracts with pressure change. You then take the change in electrical characteristics, measure it and use that to drive your display. This is how audibles and digital altimeters work. They still need a "balloon" as a pressure sensor, its just a different way of translating the movement of the "balloon" into something meaningful to your eye and brain. If you then take this altitude calculation and hook it up to a clock, you have a pretty good speed calculator, add on some logic circuitry and you have something that can alarm at various heights, or even actuate a small explosive charge which can be used to drive a piston and cut a closing loop. Et Voila, you have an AAD.-------------------- He who receives an idea from me, receives instruction himself without lessening mine; as he who lights his taper at mine, receives light without darkening me. Thomas Jefferson Quote Share this post Link to post Share on other sites
