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LOW MAINTENANCE BATTERIES
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TM-5-4210-230-14P-1 Aerial Ladder Fire Fighting Truck Manual
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TEMPERATURE CORRECTION
TRUCK SERVICE MANUAL TM 5-4210-230-14&P-1 a   specific   gravity   of   1.835.      Water   has   arbitrarily   been assigned   a   value   of   1.000.      Therefore,   electrolyte   With   a specific gravity of 1.265 means it is 1.265 times heavier than pure water. The  state-of-charge  of  a  battery  can  be  determined  by  the specific gravity of the electrolyte.  The specific gravity can be measured     directly     with     a     hydrometer     (Figure     4).          A hydrometer is a bulb-type syringe which will extract electrolyte from   the   cell.      A   glass   float   in   the   hydrometer   barrel   is calibrated to read in terms of specific gravity.  The lower the float sinks in the electrolyte, the lower its specific gravity. Fig.  4  Battery Hydrometer Figure    5    graphically    illustrates    the    relationship    between specific  gravity  readings  and  the  combination  of  the  sulfate from the acid with the positive and negative plates for various states of charge.  The black dots represent the sulfate radical. A  fully  charged  battery  has  all  of  the  sulfate  in  the  acid.    As the  battery  discharges,  some  of  the  sulfate  begins  to  appear on the plates.  The acid becomes more dilute and its specific gravity drops as water replaces some of the sulfuric acid.  A fully discharged battery has more sulfate in the plates than in the electrolyte.  Note that the hydrometer float sank lower and lower in the electrolyte as the specific gravity became lower. Table  1  illustrates  typical  specific  gravity  values  for  a  cell  in various  stages  of  charge.    A  fully  charged  specific  gravity  of 1.265 corrected to 26.7° C (80° F) is assumed. TABLE 1 Specific State of Gravity Charge 1.265 100 % charged 1.225 75 % charged 1.190 50 % charged 1.155 25 % charged 1.120 Discharged Fig.  5  Relationship of Specific Gravity To Transfer of Sulfate From Electrolyte To Plates. HOW TO USE A HYDROMETER Figure     6     illustrates     the     correct     method     of     reading     a hydrometer.  The barrel must be held vertically so the float is not  rubbing  against  the  side  of  it.    The  electrolyte  should  be drawn in and out of the hydrometer barrel a few times to bring the  temperature  of  the  hydrometer  float  and  barrel  to  that  of the acid in the cell.  Draw an amount of acid into the barrel so that  with  the  bulb  fully  expanded,  the  float  will  be  lifted  free, touching neither the side, top or bottom stopper of the barrel. When reading the hydrometer, your eye should be on a level with   the   surface   of   the   liquid   in   the   hydrometer   barrel. Disregard  the  curvature  of  the  liquid  where  the  surface  rises against  the  float  stem  and  the  barrel  due  to  surface  tension. Keep the float clean.  Make certain it is not cracked. Never  take  a  hydrometer  reading  immediately  after  water  is added  to  the  cell.    The  water  must  be  thoroughly  mixed  with the   underlying   electrolyte,   by   charging,   before   hydrometer readings are reliable.  If a reading is being taken immediately after the battery has been subjected to prolonged cranking, it will  be  higher  than  the  true  value.    The  water  formed  in  the plates during the rapid discharge has not had time to mix with the higher specific gravity acid above the plates. CTS-2771  Page 6 PRINTED IN UNITED STATES OF AMERICA

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