12.36.12(b) . errors affecting the VSI. 

VSI Errors
Lag
– main cause of error in a pressure operated VSI
– to get accurate rate of cliimb measurement the VSI must be allowed to settle after the aircraft attitude is changed

Position Error
– occurs through fluctuations in pressure sensing at the static vent
– even if the aircraft is level
– inherent limitation of the instrument

12.36.12(a) . basic principle of operation; 

– provides a direct readout of the rate of change of altitude
– pressure instrument that converts a rate of change of static pressure to a rate of change of altitude
– expressed in hundreds or thousands of fett per minute (fpm)
–