•The (VSI) indicates whether the aircraft is climbing, descending, or in level flight

 

•The rate of climb or descent is indicated in feet per minute

 

•If properly calibrated, this indicator will register zero in level flight

 

•Inside the case, a diaphragm is attached directly to a static line and connected to linkage (this linkage is how you get the reading)

 

•As the aircraft climbs,  the pressure in the diaphragm drops (high alt, low pressure)

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•The sealed case (housing) has only a slow leak and remains at the higher pressure longer (the lower altitude you were at, had a higher pressure)

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•The diaphragm contracts resulting in climb indications

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•If level flight is resumed, pressure equalizes in the case and diaphragm within 6 to 9 seconds and the pointer returns to zero rate of climb