indoor  air  conditions.  It  is  used  to  maintain  the environment  of  an  enclosure  at  any  required temperature,  humidity,  and  purity.  Simply  stated,  air conditioning   involves   the   cooling,   heating, dehumidifying, ventilating, and purifying of air. One of the chief purposes of air conditioning aboard ship  is  to  keep  the  crew  comfortable,  alert,  and physically fit. None of us can long maintain a high level of  efficiency  under  adverse  environmental  conditions. We have to maintain a variety of compartments at a prescribed  temperature  with  proper  circulation.  These compartments must have the proper moisture content, the correct proportion of oxygen, and an acceptable level of air contamination (dust, airborne dirt, etc.). We also  have  to  provide  mechanical  cooling  or  ventilation in  ammunition  spaces  to  prevent  deterioration  of ammunition  components.  We  have  to  provide  them  in gas storage spaces to prevent excessive pressure buildup in  containers  and  contamination  in  the  space  caused  by gas  leaks.  Finally,  we  must  provide  cooling  and ventilation in electrical/electronic equipment spaces. his  is  done  to  maintain  the  ambient  temperature  and humidity, as specified for the equipment. To properly air-condition a space, the humidity, heat of the air, temperature, body heat balance, the effect of air motion, and the sensation of comfort is considered. HEAT OF AIR The heat of air is considered from three standpoints– sensible, latent, and total heat. SENSIBLE HEAT is the amount of heat, which, when  added  to  or  removed  from  air,  changes  the temperature of the air. Sensible heat changes can be measured by the common (dry-bulb) thermometer, Air  always  contains  some  water  vapor.  Any  water vapor  in  the  air  contains  the  LATENT  HEAT  OF VAPORIZATION.  (The  amount  of  latent  heat  present has no effect on temperature and it cannot be measured with  a  dry-bulb  thermometer.) Any mixture of dry air and water vapor contains both sensible and latent heat. The sum of the sensible heat and the latent heat in any sample of air is called the TOTAL HEAT of the air. TEMPERATURES To  test  the  effectiveness  of  air-conditioning equipment and to check the humidity of a space, you must consider two different temperatures-the dry-bulb and  wet-bulb  temperature. Measurement of Temperatures The  DRY-BULB   TEMPERATURE   is  the temperature of sensible heat of the air, as measured by an ordinary thermometer. In air conditioning, such a thermometer  is  known  as  a  dry-bulb    thermometer because its sensing bulb is dry. The  WET-BULB   TEMPERATURE   is  best explained by a description of a wet-bulb thermometer. It is an ordinary thermometer with a loosely woven cloth sleeve or wick placed around its bulb and which is then wet with distilled water. The water in the sleeve or wick is evaporated by a current of air at high velocity (see next  paragraph).  This  evaporation  withdraws  heat  from the thermometer bulb, lowering the temperature by several  degrees.  The  difference  between  the  dry-bulb and  the  wet-bulb  temperatures  is  called  the  wet-bulb depression.  when  the  wet-bulb  temperature  is  the  same as the dry-bulb, the air is said to be saturated; that is, evaporation   cannot   take   place.   The   condition   of saturation   is   unusual,   however,   and   a   wet-bulb depression  is  normally  expected. The  wet-bulb  and  dry-bulb  thermometers  are usually mounted side by side on a frame that has a handle and a short chain attached. This allows the thermometers  to  be  whirled  in  the  air,  providing  a high-velocity air current to promote evaporation. Such a device is known as a SLING PSYCHROMETER (fig. 10- 13). When using the sling psychrometer, whirl it rapidly-at least four times per second. Observe the wet-bulb temperature at intervals. The Point at which there is no further drop in temperature is the wet-bulb temperature for that space. MOTORIZED  PSYCHROMETERS  (fig.  10-  14) are provided with a small motor-driven fan and dry-cell batteries.   Motorized   psychrometer   are   generally preferred   and   are   gradually   replacing   sling psychrometer. Relationships Between Temperatures You  should  clearly  understand  the  definite relationships  of  the  three  temperatures-dry-bulb, wet-bulb, and dew-point. When  air  contains  some  moisture  but  is  not saturated, the dewpoint temperature is lower than the dry-bulb temperature; the wet-bulb temperature lies between them. As the amount of moisture in the air increases,   the   difference   between   the   dry-bulb temperature and the wet-bulb temperature becomes less. 10-14