tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 1, posted (5 months 1 week 5 days 17 hours ago) and read 1105 times:
Quoting Novice (Thread starter): What is the difference between a VSI instrument capsule, case and metering unit?
Here's what a VSI looks like on the inside (at least in concept):
The capsule is the thing labelled diaphragm, the case is the case, and the metering unit is the "calibrated leak".
A VSI works by connecting the current static pressure to an airtight capsule inside a case. The pressure inside the capsule is the outside static pressure. The case is connected to outside static pressure via a calibrated leak. If the plane is level, the leak equalizes the pressure inside the case with the pressure inside the capsule and the VSI reads zero. If the plane is climbing, the pressure in the capsule drops faster than the pressure in the case (because the case pressure drop is contrained by the calibrated leak). The pressure inside the capsule is lower than the pressure in the case and the VSI reads positive. The reverse happens in a descent.
If the static port plugs, then the pressure inside and outside the capsule are always the same and the VSI always reads zero regardless of your altitude change.