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 shape. Water wets the sur-
face and therefore has a concave shape.
A mercury column is read by sighting
horizontally between the top of the con-
vex mercury surface (see Figure 4A-4)
and the scale. A water manometer is
r e a d b y sighting horizontally between
t h e bottom o f t h e c o n c a v e w a t e r s u r f a c e
and the scale.
Should one column of fluid travel further
than the diameter of the tube or to the
Figure 4A-4. Comparision of Column
pressure imposed, the accuracy of the
H e i g h t for Mercury and Water
reading obtained is not impaired.
To convert manometer readings into oth-
P R O P E R USE OF MANOMETER
er units of measure, use the follow val-
ues:
The U-tube manometer is a primary mea-
suring device indicating pressure or
vacuum by the difference in the height
PRESSURE CONVERSION CHART
of two columns of fluid.
1"
Water
=
0.0735"
Mercury
Connect the manometer to the source of
1"
Water
=
0.0361
psi
pressure,
vacuum or differential pres-
1" Mercury
=
13.6000"
Water
sure.
When the pressure is imposed,
1" Mercury
=
0.4910
psi
add the number of inches one column of
1
psi
=
27.7000"
Water
fluid travels up to the amount the other
1
psi
=
2.0360"
Mercury
column travels down to obtain the pres-
1
psi
=
6.8950
kPa
sure (or vacuum) reading.
1
kPa
=
0.1450
psi
4A-6