Fig. 2 - Comparison of Column Height for Mercury and Water Manometers
The U-tube manometer is a primary measuring device indicating pressure or vacuum by the difference in the height of
two columns of fluid.
Connect the manometer to the source of pressure, vacuum or differential pressure. When the pressure is imposed, add
the number of inches one column of fluid travels up to the amount the other column travels down to obtain the pressure
(or vacuum) reading.
PRESSURE CONVERSION CHART
The height of a column of mercury is read differently than that of a column of water. Mercury does not wet the inside
surface; therefore, the top of the column has a convex meniscus (shape). Water wets the surface and therefore has a
concave meniscus. A mercury column is read by sighting horizontally between the top of the convex mercury surface
(Fig. 2) and the scale. A water manometer is read by sighting horizontally between the bottom of the concave water
surface and the scale.
Should one column of fluid travel further than the other column, due to minor variations in the inside diameter of the tube
or to the pressure imposed, the accuracy of the reading obtained is not impaired.
Refer to Table 7 to convert manometer readings into other units of measurement.
ENGINE ELECTRICAL GENERATING SYSTEM
Whenever trouble is indicated in the engine electrical generating system, the following quick checks can be made to
assist in localizing the cause.
A fully charged battery and low charging rate indicates normal alternator-regulator operation.
A low battery and high charging rate indicates normal alternator-regulator operation.
A fully charged battery and high charging rate condition usually indicates the voltage regulator is set too high or is not
limiting the alternator output. A high charging rate to a fully charged battery will damage the battery and other electrical
A low battery and low or no charging rate condition could be caused by: Loose connections or damaged wiring, defective
battery or alternator, or defective regulator or improper regulator setting.
Contact an authorizedDetroit Diesel Allison Service Outlet if more information is needed.