A hydrometer is the instrument used to measure the
amount of active ingredients in the electrolyte of the
battery. The hydrometer measures the SPECIFIC
GRAVITY of the electrolyte. Specific gravity is the
ratio of the weight of the electrolyte to the weight of the
same volume of pure water, The active ingredient, such
as sulfuric acid or potassium hydroxide, is heavier than
water. Because the active ingredient is heavier than
water, the more active the ingredient in the electrolyte,
the heavier the electrolyte will be; the heavier the
electrolyte, the higher the specific gravity.
A hydrometer is a glass syringe with a float inside it.
The float is in a hollow, glass tube, weighted at one end
and sealed at both ends, with a scale calibrated in
specific gravity marked on the side. The electrolyte to
be tested is drawn into the hydrometer by the suction
bulb. Enough electrolyte should be drawn into the
hydrometer so that the float will rise. Hydrometers
should not be filled to the extent that the float rises into
the suction bulb. Since the weight of the float is at its
base, the float will rise to a point determined by the
weight of the electrolyte. If the electrolyte contains a
large concentration of the active ingredient, the float
will rise higher than if the electrolyte has a small
concentration of the active ingredient.
To read the hydrometer, hold it in a vertical position
and take the reading at the level of the electrolyte. Refer
to the manufacturers technical manual for battery
specifications to find the correct specific gravity ranges.
An example of what your manual may say about the
specific gravity is that for a fully charged battery the
specific gravity should be 1.280 ± 0.005. The manual
may tell you to recharge the battery if the specific
gravity is less than 1.250.
Always return the electrolyte in the hydrometer to
the cell of the battery from which it was taken.
NOTE: Hydrometers should be flushed with fresh
water after each use to prevent inaccurate readings.
Storage battery hydrometers must not be used for any
Perhaps it should be said that adding the active
ingredient (sulfuric acid, for example) to the electrolyte
of a discharged battery does not recharge the battery.
Adding the active ingredient only increases the specific
gravity of the electrolyte and does not convert the plates
back to active material, and so does not bring the battery
back to a charged condition. A charging current must be
passed through the battery to recharge it.
You must check the engine crankcase oil level
before operating the generator set. The engine dipstick
(fig. 3-1, view B) is the crankcase oil level gauge. The
dipstick in the generator engine is the shielded type,
which allows checking the oil level while the engine is
either stopped or running. The dipstick is stamped on
both sides to indicate the two different oil levels. The
engine running side is stamped as follows: "ADD,"
"FULL," and "RUNNING." The engine stopped side is
stamped as follows:
"ADD," "FULL," and
"STOPPED." Be sure to use the appropriate add and
full marks, depending on whether the engine is stopped
or running. Also, ensure that the appropriate side of the
dipstick is up when inserting it since the underside will
be wiped in the gauge tube when the dipstick is
removed, therefore, indicating a false oil level reading.
To check the oil level, first remove and wipe the oil
from the dipstick. Loosen and remove the oil filler cap
(fig. 3-1) to allow the pressure to escape. Reinsert the
dipstick (with the appropriate side up) and remove it to
observe the oil level. Add oil through the fill tube, as
required, to obtain the "full" level on the dipstick. Be
sure to use the proper grade of oil. A lubricant chart in
the instruction manual furnished with each generator
will show the proper grade of oil to use at the operating
Check that the level of coolant is within 2 inches (51
mm) of the top of the radiator.
Do not attempt to remove the radiator cap
until the radiator has cooled to a point where
there will be no built-up steam pressure.
Failure to observe this warning could result in
second- or third-degree bums.