the difference in pressure when that pressure difference falls to a first threshold value which is equal to a first setpoint of the pressure switch 100 the switch 100 acts to turn on the blower 32 and fill the air reservoir 80 when enough air has been pumped into that reservoir the pressure difference rises to a second threshold value equal to a second set point of the pressure switch 100 and the switch 100 acts to turn off the blower 32 the setpoints of the pressure switch 100 may be discovered from the same arguments that were previously offered in a discussion of differential pressures important to the apparatus when the diver has exhausted the air in the air reservoir 80 his her next attempt to inhale will cause the pressure in the air reservoir 80 to fall below the pressure in the ambient water by as much as 20 30 cm of water e g approximately 2500 newtons per square meter thus the pressure switch 100 may be set to turn on when the pressure in the air reservoir is at least 20 cm of water e g approximately 2000 newtons per square meter less than the ambient the blower 32 is to be shut off before the pressure in the air reservoir rises high enough to unseat the flaps or poppets in the blower outlet valve 46 and in the exhaust check valve 52 and to exhaust air into the water through exhaust check valve 52 since exhaust check valve 52 is preferably built into the mouthpiece 50 and the mouthpiece is approximately 10 cm above a chest mounted air tank when the diver is swimming in a normal horizontal position the maximum differential pressure supplied by the.