attract   moisture,   which   evaporates   when   the thermometer is ventilated. Evaporation will yield false low readings. A daily cleaning of the thermometers may be necessary in this situation. Changing the wick on wet-bulb thermometers should be done weekly aboard ship and monthly ashore. The procedure is detailed in NAVAIR 50-30FR-518. ALCOHOL  THERMOMETERS Alcohol  thermometers  may  be  used  to  measure temperatures  from  -115°C  (freezing  point  of  alcohol)  to 785°C  (boiling  point  of  alcohol).  The  standard thermometer  for  environmental  measurements  need only cover the range -20°F to 120°F, or about -30°C to 50°C. Because alcohol is a volatile fluid, the column in the thermometer   frequently   becomes   separated   by mechanical  shock.  The  column  may  be  reunited  by dropping the thermometer on a wood surface that is covered  with  several  sheets  of  paper. Hold the thermometer in a vertical position, bulb end down, 4 to 6 inches over the wood surface covered with paper, and then drop. After the thermometer bulb strikes the paper, catch the thermometer with your other hand to prevent breakage. Small amounts of alcohol may cling to the portion  of  the  capillary  tube  above  the  alcohol  column. Heating the high-temperature end of the thermometer under  an  incandescent  light  may  force  the  alcohol  back to the column. Allowing the thermometer to stand in a vertical position overnight will also allow the alcohol to drain down to the column. Do not use a thermometer with  a  separated  fluid  column;  it  will  give  you inaccurate   readings. MERCURY THERMOMETERS Recently,   several   Naval   Headquarters   have recommended  that  alcohol  thermometers  be  used  in place  of  mercury  thermometers  when  possible. NAVSEAINST   5100.3,   Mercury,   Mercury Compounds, and Components Containing Mercury or Mercury Compounds; Control  of, identifies mercury as a  toxic  substance  that  requires  special  handling  and control.  Overboard  discharge  of  any  amount  is prohibited. This instruction also defines mercury-spill decontamination  requirements  and  mercury-handling requirements. WARNING Liquid or vaporous mercury is a hazardous material. It is toxic to humans and most forms of  marine  life,  and  is  highly  corrosive  to electronic   components. The   use   of   mercury   thermometers   in   sling psychrometers is strongly discouraged, since users of sling psychrometers sometimes strike objects during the  spinning  operation,  breaking  the  glass  and  releasing mercury.  The  release  of  even  a  small  amount  of mercury from a broken thermometer is a "mercury spill," which must be handled in accordance with NAVSEAINST   5100.3.   Unbroken   mercury thermometers, either in use or in storage, are classified "functional mercury" and are subject to shipboard mercury   inventory   reporting   as   directed   by NAVSEAINST  5100.3. Separated  mercury  columns  in  thermometers should be rejoined by slowly heating the thermometer bulb  under  an  incandescent  lamp.  Withdraw  the thermometer from the heat source as the mercury approaches the top of the thermometer. Then, carefully control the heating so that the mercury rises slowly in the column and just reaches the end of the thermometer. Overheating at this point will cause the mercury to expand until it ruptures the thermometer, resulting in a spill. Allow the thermometer to cool; the columns of mercury should rejoin as the mercury recedes. MAXlMUM AND MINIMUM THERMOMETERS Maximum   and   minimum   thermometers   are alcohol-filled  thermometers  mounted  on  a  Townsend support   inside   an   instrument   shelter.   These thermometers  are  strictly  backup  equipment  for  the maximum  and  minimum  temperature  function  of automatic  and  semiautomatic  observation  systems. Proper use and care are detailed in NA 50-3OFR-518. Figure  2-15  shows  the  instruments  mounted  on  the Townsend support, and figure 2-16 shows the detail of the Townsend support. Figure 2-16.—ML-54 Townsend support details. 2-11