TM 5-3895-346-14
ENGINE OVERHAUL
TROUBLESHOOTING (ENGINE)
The difference between the two readings, with and without the air cleaner and ducting, is the actual restriction
caused by the air cleaner and ducting. Check the normal air inlet vacuum at various speeds (at no-load) and
compare the results with the Engine Operating Conditions Specifications. PROPER USE OF MANOMETER
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.
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 3 to convert the manometer reading into other units of measurement.
Figure 2. Comparison of Column Height for Mercury and Water Manometers
TABLE 3
PRESSURE CONVERSION CHART
1 inch water
=
0.0735 inches of
mercury
1 inch water
=
0.0361 psi
1 inch mercury =
13.6000 inches of
water
1 inch mercury =
0.4910 psi
1 psi
=
27.7000 inches of
water
1 psi
=
2.0360 inches of
mercury
1 psi
=
6.895 kPa
1 kPa
=
0.145 psi
445
