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.
Figure 5 - Relationship of Specific Gravity to
Transfer of Sulphate From Electrolyte
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 26.70C (800F) is assumed.
State of Charge
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. 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 nor 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
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.
Figure 6 - Correct Method of Reading
Hydrometer (Eye on Level With Liquid
Because there are many different types of battery
hydrometers available, always follow manufacturers
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 5.50C (100F) increment above 26.7°C (80°F) and
four points are subtracted for each 5.5°C (10°F)
increment below 26.70C (800F). This correction is
important at extremes of temperature because it can
become a substantial value.
CTS-4111B - Page 6