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 Some inclined manometers do not use water as the fluid, but instead, a
red gage oil is used. This has the advantage of easier readability. Also,
there is a problem using water because when the water evaporates, the solids
left behind can cloud the inside of the manometer tube. Manometers that use
red gage oil will not have an actual vertical rise that matches the numbers
on the gage. This is explained by the fact that the gage oil has a specific
gravity of 0.86 compared to that of water. The exact readings from the
water and red gage oil may not be used interchangeably. The correct fluid
to match the actual calibration of the manometer must be used.
When using inclined manometers, it is important that they be mounted in
a level position when taking readings. Many units are supplied with an
integral water level, and magnetic mounts or screw-type feet for attaining a
level position. Once leveled, the gage must be set to zero by adjusting the
amount of fluid allowed to remain in the reservoir. When using the inclined
manometer, it is important to anticipate which pressure sensing port will
see the greater pressure, and always have the greater pressure pushing the
fluid down the scale. Some popular ranges available in inclined manometers
are 0 to 0.25 inch, 0 to 0.50 inch, and 0 to 1.0 inch. For measurements
above 1.0 inch, a U-tube manometer may be used with acceptable accuracy.
NOTE: When hooked up backwards, the technician will face the embarrassing
task of cleaning up all the oil which has been either blown out of the gage
onto the floor, or sucked up into the duct. Also, it is important that the
pressure differential being measured does not exceed the range of the
manometer.
2-18 PRESSURE GAGES. Dial type pressure gages are also used in conjunction
with pitot tubes in the same way as inclined manometers. Figure 2-32 shows
an industry standard commonly referred to as a magnehelic gage. These gages
are available in all the same ranges as inclined manometers. They each have
a high pressure and a low pressure connection. When measuring a negative
static pressure, for example, you would attach the negative pressure sensed
by the pitot tube to the low pressure connection, and leave the high
pressure connection open to the atmosphere. Pressure gages have the
advantage of being easy to use, relatively rugged, and accurate. However,
only the manometer can be counted upon to never get out of calibration.
2-30
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