One of our last posts was all about altitude. In it we discussed terminal velocity: what it is, how long it takes to reach it, and why body position/composition matters.
Since we have a finite amount of time in freefall, we thought that we would spend this post examining how an altimeter works.
Let’s start with the basics…what is an altimeter?
An altimeter is an instrument that measures the altitude of an object (you, the skydiver) above a fixed level (where you set it to zero).
As shocking as it may be, altimeters have not always been used in skydiving. Go talk to the oldest member at your DZ and you may just hear about the use of stopwatches. They would start their stopwatch and then 30-45 seconds later they would deploy, depending on the altitude they jumped at.
Today, we invest in altimeters…so how do they work?
Altimeters measure atmospheric pressure: as altitude goes up, atmospheric pressure goes down. The altimeters are calibrated in such away that assumes that the pressure will drop at a standard rate as altitude is gained. You can find the entire expected altitude-pressure readings in the FAA document that deals with altimeter calibration and testing tolerances, but we’ve included the portion of the table from 0-14,000 feet.
While there are some problems with the assumption that pressure changes are linear (layers of hot and cold air will lead to a nonuniform pressure change and density altitude changes can muck things up too), it works well enough. So inside our analog altimeters is a mechanical pressure sensor that is connected via a gear assembly to the display needle: as the pressure changes the needle moves to indicate the (approximate) altitude.
As an aside, there is an interesting article on how one group built a crude altimeter out of plastic tubing and water in order to find treasure on a mountainside.
But, back to what we were talking about:
Once you have calibrated your pressure-sensing instrument then you can slap a dial on the front…and we have just made an analog altimeter.
Interestingly, the original design for the Altimaster II (a super common student altimeter) was set up in such a way to mimic an analog clock. Since most people are/were familiar with a clock face (or a stopwatch), the Altimaster’s face was set up from 0-12,000′.
Well, that’s just dandy, but I have a digital altimeter. How does that work?
Once again we need something that will sense the pressure. This time, instead of being mechanical, the pressure sensor is electronic. Inside the sensor, there is a reference cell that is evacuated to an almost perfect vacuum. There is a thin diaphragm that covers this reference cell so that any change to the diaphragm is due to a change in the ambient pressure. A strain gauge can then be employed to convert these physical movements into electrical/digital outputs. If you are looking for more than a summary check out this newsletter from a model rocket company.