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HOW TO USE A HYDROMETER
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TM-5-4210-230-14P-1 Aerial Ladder Fire Fighting Truck Manual
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TEMPERATURE AFFECTS BATTERY OUTPUT CAPACITY (CRANKING POWER)
TRUCK SERVICE MANUAL TM 5-4210-230-14&P-1 Fig.  6  Correct Method of Reading Hydrometer (Eye on Level with Liquid Surface) Because there are many different types of battery hydrometers available, always follow manufacturer's instructions. TEMPERATURE CORRECTION Hydrometer floats are calibrated to give a true reading at one fixed  temperature  only.    A  correction  factor  must  be  applied for  any  specific  gravity  reading  made  when  the  electrolyte temperature is not 26.7° C (80°  F). A     temperature     correction     must     be     used     because     the electrolyte  will  expand  and  become  less  dense  when  heated. The float will sink lower in the less dense solutions and give a lower   specific   gravity   reading.      The   opposite   occurs   if   the electrolyte is cooled.  It will shrink in volume, becoming more dense.  The float will rise higher and read too high. A correction factor of .004 specific gravity sometimes referred to  as  4  "points"  of  "gravity")  is  used  for  each  5.5°  C  (10°   F) change  in  temperature.    Four  "points  of  gravity"  (.004)  are added   to   the   indicated   reading   for   each   5.5°    C   (10°    F) increment    above    26.7°    C    (80°    F)    and    four    points    are subtracted  for  each  5.5°  C  (10°   F)  increment  below  26.7°   C (80° F).        This    correction    is    important    at    extremes    of temperature because it can become a substantial value. Figure   7   illustrates   the   correction   for   hydrometer   readings when the acid temperature is above or below 26.7° C (80°  F). In  example  No.    1,  in  cold  weather,  a  partially  discharged battery   in   a   vehicle   at   6.7°   C   (+20°   F)   might   read   1.250 indicating  it  was  almost  fully  charged.    However,  when  the correction   factor   is   applied,   the   true   value   is   only   1.226. Example No.  2 could be encountered in a battery exposed to the  sun  in  hot  weather.    Also,  electrolyte  frequently  reaches 43° C (110° F) in service in warm weather.  The 1.235 specific gravity  reading  might  indicate  too  low  a  state  of  charge  to install  in  a  vehicle  or  that  there  is  a  problem  in  the  electrical system if the battery is in service.  However, the true reading of   1.247   may   not   be   unreasonably   low   depending   on   the length of storage of the battery or the type of service which it has been experiencing in the vehicle. Fig.  7  Specific Gravity Temperature Correction Values SPECIFIC GRAVITY DETERMINES ELECTROLYTE FREEZING POINT Specific gravity of the electrolyte determines the temperature at which a battery will be harmed or damaged by freezing.  A fully charged battery can be stored at sub-freezing temperatures without freezing the electrolyte.  However, as a battery  approaches  the  discharged  condition,  the  electrolyte will   freeze   at   higher   temperatures.      Table   2   shows   the approximate  freezing  points  of  electrolyte  at  various  specific gravities. CTS-2771  Page 7 PRINTED IN UNITED STATES OF AMERICA

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