Hydrometer floats are calibrated to indicate correctly only at 80 F temperature. It used at any other temperature, a correction factor must be applied. The reason for this lies in the fact that a liquid expands when it is heated and shrinks when it is cooled. This will cause a change in the density of the electrolyte solution which will raise or lower the specific gravity reading.

A thermometer is built into the temperature compensating-type hydrometer. The scale of this thermometer indicates the temperature of the solution. This reading should be used so that the proper temperature correction factor can be applied.

The table is based on an electrolyte temperature of 80°F. For other temperatures, correct the indicated reading by adding 4 points(.004) for each 10° above 80°F, and subtracting 4 points for each 10° that the electrolyte temperature is below 80°F.

TEMPERATURE CORRECTED HYDROMETER

For example: A specific gravity reading of 1.230 is obtained at a solution temperature of 10°F. If the electrolyte temperature is disregarded, the reading of 1.230 may be considered as low but acceptable. When the reading is temperature corrected, the true reading of 1.202(7*4=28 from 1.230) reveals that the battery is actually very low and definitely in need of charging.

A specific gravity reading of 1.235 is obtained at a solution temperature of 120°F. The reading itself may be interpreted as being rather low but when temperature corrected the reading is actually 1.251 (4*4=16 added to 1.235). This is specific gravity may be high enough for the battery to be restored to full charge by the car generator.

These examples indicate the importance of temperature correcting specific gravity readings to accurately interpret the true state of battery charge. To accurately test the true condition of the battery, a light load test or capacity test should be conducted after the specific gravity has been tested.