Pressure is defined as a force per
unit area - and the most accurate way to measure low air
pressure is to balance a column of liquid of known weight
against it and measure the height of the liquid column so
balanced. The units of measure commonly used are inches
of mercury (in. Hg), using mercury as the fluid and inches
of water (in. w.c.), using water or oil as the fluid.
Fig.
2-1. In its simplest form the manometer is a U-tube
about half filled with liquid. With both ends of the tube
open, the liquid is at the same height in each leg.
Fig.
2-2. When positive pressure is applied to one leg,
the liquid is forced down in that leg and up in the other.
The difference in height, "h," which is the sum
of the readings above and below zero, indicates the pressure.
Fig.
2-3. When a vacuum is applied to one leg, the liquid
rises in that leg and falls in the other. The difference
in height, "h," which is the sum of the readings
above and below zero, indicates the amount of vacuum.
Instruments employing this principle are called manometers.
The simplest form is the basic and well-known U-tube manometer.
(Fig. 2-1). This device indicates the difference between
two pressures (differential pressure), or between a single
pressure and atmosphere (gage pressure), when one side is
open to atmosphere. If a U-tube is filled to the half way
point with water and air pressure is exerted on one of the
columns, the fluid will be displaced. Thus one leg of water
column will rise and the other falls. The difference in
height "h" which is the sum of the readings above
and below the half way point, indicates the pressure in
inches of water column.
Fig.
2-4. At left, equal pressure is imposed on the fluid
in the well and in the indicating tube. Reading is zero.
At the right, a positive pressure has been imposed on the
liquid in the well causing the level to go down very slightly.
Liquid level in indicating tube has risen substantially.
Reading is taken directly from scale at liquid level in
indicating tube. The scale has been compensated for the
drop in level in the well.
The U-tube manometer
is a primary standard because the difference in height between
the two columns is always a true indication of the pressure
regardless of variations in the internal diameter of the
tubing. This principle makes even the Dwyer® Slack Tube®
roll-up manometer as accurate as a laboratory instrument.
This provides a real convenience to the person who might
otherwise have to board an airplane carrying a 60"
long rigid glass U-tube manometer.
VARIATIONS
IN MANOMETER DESIGN
To overcome the U-tube requirement of readings at two different
places, the well-type manometer was developed. See Fig.
2-4. The reservoir (well) may be made large enough so that
the change of level in the reservoir is negligible, or the
scale may be compensated for the change in reservoir liquid
level. For purposes of a more practical instrument the Dwyer®
well-type manometer uses a precision bored well that requires
approximately a 10% scale correction for well drop effect,
thus avoiding an overly large and bulky reservoir.
Fig.
3-1. At left, equal pressure is imposed on the liquid
in the well and the indicating tube. Reading is zero. At
the right a positive pressure has been imposed on the liquid
in the indicating tube pushing it down to a point on the
scale equal to the pressure. Liquid level in the well rises
proportionately. Inclining the indicating tube has opened
up the scale to permit more precise reading of the pressure.
To improve and expand
readability, certain Dwyer® U-type and well-type manometers
are available with a .826 sp. gr. red oil indicating fluid,
and scales compensated to read pressure directly in inches
of water. To further increase readability and sensitivity
the well-type manometer indicating tube is inclined, as
in Fig. 3-1, to cause a greater linear movement along the
tube for a given pressure difference. The inclined manometer
is frequently called a Draft Gage because it is widely used
for determining the over-fired draft in boiler uptakes and
flues.
For an inclined
manometer to be a primary device, the inclined tube must
be straight and uniform. Dwyer's precision machined solid
plastic construction has been applied to a basic line of
rugged manometers, inclined and inclined-vertical, which
are industry accepted as primary instruments. See
discussion.
Fig.
3-2. At left with equal pressure on liquid in well
and indicating tube, reading is zero. When positive pressure
is imposed on liquid in indicating tube, liquid level is
depressed in tube and rises slightly in well. Reading is
direct since scale is compensated for change of level in
well.
The combination of an
inclined and a vertical manometer is very useful in air
movement determination. See Fig. 3-2. For air velocity measurement,
an inclined scale, generally up to 1" w.c. is used
(1" w.c. velocity pressure = 4000 fpm). In the Dwyer®
Durablock® inclined-vertical instrument, this scale
is combined with a vertical section allowing readings of
high pressures, usually 1" w.c. to 5 to 10" w.c.,
to be taken. There vertical section is used primarily for
determining static pressure above the range of the inclined
section. Many special purpose types of manometers exist.
Examples are the Dwyer® Hook Gage and Microtector®
gage. These are simply U-tube manometers modified so the
liquid level can be read with a micrometer, yet retaining
the basic "Physics" of the hydrostatic U-tube
primary standard. Readings accurate to ±.001"
w.c. in a range of differential pressures from 0-24"
w.c. are accomplished with Dwyer Model No. 1425-24 Hook
Gage. The Model 1430 Microtector® gage incorporates
modern electronics to increase the accuracy of readings
to ±.00025" w.c. on a 2" w.c. scale.
FACTORS
AFFECTING MANOMETER PERFORMANCE AND USAGE
