Step 1) Take a can of Campbell's beef broth. Poke a small hole in the bottom of the can and drain all the broth out. Now grab a nice piece of tubing. Attach one end of the tube to the hole in the can and the other end to a vacuum cleaner. Turn on the vacuum cleaner and watch the can crumple into the shape of an accordion. Remove the tube from the vacuum cleaner. Note how the can remains somewhat squashed and accordion-looking. Got it so far?
Step 2) Build an air-tight case around your soup can, making certain that the tube attached to the can goes through to the outside of the case. Drill a very, very, very tiny hole in the case, a hole so small that it only allows a few particles of air per second the pass into or out of the case. This little hole we will call a "diffuser", or "calibrated leak". Grab yet another length of tubing and attach one end to this diffuser.
Step 3) Attach the free ends of both tubes to your airplane's static port. You now have the inner workings of a VSI!
As you start to climb, the airplane is moving upward into the atmosphere. As you move upward, the atmospheric pressure decreases. Since there are no restrictions to the pressure within your soup can, the pressure within the can decreases at the same rate as the atmospheric pressure outside your airplane.
The case surrounding the can, however, is vented to the atmospheric pressure through this diffuser, or calibrated leak. Therefore, the pressure within the case decreases at a slower rate than the pressure within the soup can. Having less pressure inside it than outside, the soup can "accordions" inward, making it shorter. If we had some mechanical device on the can to measure it, it would show that the can is no longer as tall (or....that we are climbing!).
Once you stabilize at a specific altitude (pressure), the pressure within the case slowly equalizes to that outside the airplane and the soup can once again resumes its normal semi-accordion shape. Our measuring device would show that the can has returned to its normal height (or...that we are no longer climbing).
As you start a descent, you are moving toward a lower altitude...into greater atmospheric pressure. The increasing pressure moves unrestricted into the soup can, but it moves more slowly into the case surrounding the soup can. With a greater pressure inside than out, the can "accordions" outward, increasing its length during the descent. Again, a measuring device attached to the can would tell us that the can has gotten longer (or...that we are descending).
Now you know the physical/mechanical aspects of how a VSI works!
The instant your airplane begins to climb or descend, the pressure transfers start. Therefore the needle on your VSI will move immediately toward the climb or descent. It takes some time (normally about 6-9 seconds) for the pressure differential between the inside of the can and the case to stabilize, though, so the “rate” of climb or descent is not immediately accurate.
Once stabilized in a climb or descent, if you change that rate of climb or descent, the needle will show an immediate change in the direction you moved. It will not, however, show an accurate indication of your new rate of climb or descent until you have been stabilized again for another 6-9 seconds.
Things to remember about VSI indications:
-It shows an IMMEDIATE indication of TREND (initiating or changing your rate of climb or descent)
-It LAGS in reference to accurate RATE information, requiring 6-9 seconds of stabilize climb, descent, or level flight to indicate accurately.