A zeroing adjustment screw, or knob, on the face of the  instrument is used to center the pointer exactly on zero while  the aircraft is on the ground.  [Figure 10-38] As the aircraft climbs, the unrestricted air pressure in  the diaphragm lowers as the air becomes less dense. The  case air pressure surrounding the diaphragm lowers more  slowly, having to pass through the restriction created by the  orifice. This causes unequal pressure inside and outside the  diaphragm, which in turn causes the diaphragm to contract  a bit and the pointer indicates a climb. The process works in  reverse for an aircraft in a descent. If a steady climb or descent  is maintained, a steady pressure differential is established  between the diaphragm and case pressure surrounding  it, resulting in an accurate indication of the rate of climb  via graduations on the instrument face.  [Figure 10-39]  A shortcoming of the rate-of-climb mechanism as described  is that there is a lag of six to nine seconds before a stable  differential pressure can be established that indicates the  actual climb or descent rate of the aircraft. An instantaneous  vertical speed indicator (IVSI) has a built-in mechanism to  reduce this lag. A small, lightly sprung dashpot, or piston,  reacts to the direction change of an abrupt climb or descent.  As this small accelerometer does so, it pumps air into or  out of the diaphragm, hastening the establishment of the  pressure differential that causes the appropriate indication.  [Figure 10-40] Gliders and lighter-than-air aircraft often make use of a  variometer. This is a differential VSI that compares static  pressure with a known pressure. It is very sensitive and gives  an instantaneous indication. It uses a rotating vane with a  pointer attached to it. The vane separates two chambers.  One is connected to the aircraft’s static vent or is open to  the atmosphere. The other is connected to a small reservoir  inside the instrument that is filled to a known pressure. As  static air pressure increases, the pressure in the static air  chamber increases and pushes against the vane. This rotates  the vane and pointer, indicating a descent since the static  pressure is now greater than the set amount in the chamber  with reservoir pressure. During a climb, the reservoir pressure  Figure 10-38.  A typical vertical speed indicator.  Figure 10-39.  The VSI is a differential pressure gauge that compares free-flowing static air pressure in the diaphragm with restricted  static air pressure around the diaphragm in the instrument case. 10-23 3 4 I 0 2 2 I 3 VERTICAL SPEED THOUSAND FT PER MIN UP DOWN .5 .5 Zeroing adjustment screw 3 4 I 0 2 2 I 3 VERTICAL SPEED THOUSAND FT PER MIN UP DOWN .5 .5 3 4 I 0 2 2 I 3 VERTICAL SPEED THOUSAND FT PER MIN UP DOWN Restricted passage Free flow out Diaphragm Restricted flow Climb Descent Static port connection 