employed is a sensitive aircraft altimeter 

 (Fig. 9.17). A regular barometer is limited in 

 range. Altitude (pressure) changes are 

 caused by variations in oxygen or carbon 

 dioxide within the cabin; in DOWB (140 ft=» 

 internal volume) a one percent increase in 

 either will cause a decrease in altitude of 277 

 feet (15). Unfortunately, temperature and 

 relative humidity changes are also reflected 

 on the altimeter and a means for correcting 

 the altimeter for these changes must be pro- 

 vided if concentration of gasses is desired. 



Inadvertent or accidental pressure buildup 

 to the point where decompression would 

 have been required has never been reported; 

 in most cases the buildup is slight, but suffi- 

 cient to be noticed on the ear drums when 

 the hatch is opened following a 6- or 8-hour 

 dive. The possibility that such a buildup 

 might occur can bear on the methods used to 



Fig, 9 17 Cabin pressure indicator (Altimeter) (Mr A P lanuzzi. Naval Faalittes 

 Eng. Comm.) 



membrane clamped between a pair of alumi- 

 num retaining rings. A spring-loaded linkage 

 assembly is connected with the center of the 

 diaphragm, pulling it into a conical shape. 

 The other end of the linkage is connected to 

 the indicating pointer. Since the diaphragm 

 is sensitive to moisture in the surrounding 

 air, changes in humidity cause proportional 

 changes in its dimensions. Movement of the 

 apex of the diaphragm is transmitted to the 

 pointer through mechanical linkage. The 

 unit does not require maintenance or adjust- 

 ments. 



Cabin Pressure: 



Ascertaining the atmospheric pressure 

 within the cabin is important for crew com- 

 fort (a sudden decrease in pressure when the 

 hatch is opened may be quite painful) and 

 safety (undetected buildup of pressure might 

 reach proportions requiring decompression). 

 Additionally, the oxygen flow rate in many 

 vehicles is usually adjusted by noting 

 changes in cabin pressure. There are a num- 

 ber of devices which can be used to measure 

 pressure changes, but the one most widely 



Fig 



9 18 Two dillerent methods ol secunng a halcti cover a) DS'2UUU. D) 

 AQUARIUS I (a Westinghouse Corp ) 



436 



