Sensors Magazine Online - October 2001 - Manometer Basics








































 SENSOR 
 TECHNOLOGY
AND DESIGN 

ManometerBASICS








Manometers are both pressure measurement instruments
and calibration standards. They range from simple U-tubes and
wells filled with liquid to portable digital instruments with
a computer interface.
Manometers, devices for measuring
differential fluid pressures, come in two flavorsliquid-based and
digital. Knowing how each type works will help you make the best choice
for your application.
Dave Thomas and Rick
DÅ‚Angelo,Meriam Instrument
One of
the earliest pressure measuring instruments is still in wide use today
because of its inherent accuracy and simplicity of operation. Itłs the
U-tube manometer, which is a U-shaped glass tube partially filled with
liquid. This manometer has no moving parts and requires no calibration.
Manometry measurements are functions of gravity and the liquidłs density,
both physical properties that make the U-tube manometer a NIST standard
for accuracy.
As shown in Figure 1, with each leg of a U-tube manometer exposed to
the atmosphere, the height of liquid in the columns is equal. Using this
point as a reference and connecting each leg to an unknown pressure, the
difference in column heights indicates the difference in pressures (see
Figure 2).





Figure 1.
With both legs of a U-tube manometer open to the atmosphere or
subjected to the same pressure, the liquid maintains the same level
in each leg, establishing a zero reference.

Figure 2.
With a greater pressure applied to the left side of a U-tube
manometer, the liquid lowers in the left leg and rises in the right
leg. The liquid moves until the unit weight of the liquid, as
indicated by h, exactly balances the pressure.

The fundamental relationship for pressure expressed by a liquid column
is:




p = P2-P1 = gh
(1)
where:




p 
= differential pressure

P1
= pressure at the low-pressure connection

P2
= pressure at the high-pressure connection


= density of the indicating fluid (at a specific
temperature)

g
= acceleration of gravity (at a specific latitude and
elevation)

h
= difference in column heights
The resulting pressure is the difference between forces exerted per
unit of surface area of the liquid columns, with pounds per square inch
(psi) or newtons per square meter (pascals) as the units. The manometer is
so often used to measure pressure that the difference in column heights is
also a common unit. This is expressed in inches or centimeters of water or
mercury at a specific temperature, which can be changed to standard units
of pressure with a conversion table.
All pressure measurements are differential. The reference can be zero
absolute pressure (a total vacuum), atmospheric pressure (the barometric
pressure), or another pressure. With one leg of a manometer open to the
atmosphere (see Figure 3A), the measured pressure is that which exceeds
atmospheric pressure, which at sea level is 14.7 psi, 101.3 kPa, or 76
cmHg.





Figure 3.
Gauge pressure is a measurement relative to atmospheric pressure and
it varies with the barometric reading. A gauge pressure measurement
is positive when the unknown pressure exceeds atmospheric pressure
(A), and is negative when the unknown pressure is less than
atmospheric pressure (B).
This measurement is called gauge pressure, and the relationship for a
positive pressure is expressed by:




absolute pressure = atmospheric pressure + positive gauge
pressure
(2)
For a negative pressure (vacuum) measurement (see Figure 3B), the
column heights reverse and the relationship is expressed by:




absolute pressure = atmospheric pressure + negative gauge
pressure
(3)
These pressure relationships are shown in Figure 4.





Figure 4.
A graphical representation of positive and negative gauge pressure
shows the differential aspect of all pressure measurements, where
gauge pressure is the difference between absolute pressure and
atmospheric pressure.








Figure 5. In a sealed-tube manometer, the pressure
reference is a vacuum, or zero absolute pressure. The most
common form of a sealed-tube manometer is the conventional
mercury barometer used to measure atmospheric pressure.
A manometer
can be designed to directly measure absolute pressure. The manometer in
Figure 5 measures the pressure compared to zero absolute pressure in a
sealed leg above a mercury column. The most common form of this manometer
is the conventional mercuśy barometer used to measure atmospheric
pressure. With just one connection, this configuration can measure
pressures above and below atmospheric pressure.
Variations on the U-Tube
ManometerThe differential pressure is always the difference
in column heights, regardless of the size or shape of the tubes. As shown
in Figure 6A, the legs of both manometers are open to the atmosphere and
the indicating fluids are at the same level. Connecting the same pressure
to the left leg of each manometer causes its level to lower. Because of
the variation in volume in the manometer legs, the fluid in each column
moves a different distance. However, the difference between the fluid
levels in both manometers is identical (see Figure 6B).





Figure 6.
The pressure reading is always the difference between fluid heights,
regardless of the tube sizes. With both manometer legs open to the
atmosphere, the fluid levels are the same (A). With an equal
positive pressure applied to one leg of each manometer, the fluid
levels differ, but the distance between the fluid heights is the
same.








Figure 7. In a well-type manometer, the
cross-sectional area of one leg (the well) is much larger than
the other leg. When pressure is applied to the well, the fluid
lowers only slightly compared to the fluid rise in the other
leg.
Carrying
this variation in tube sizes further is the well-type (or reservoir)
manometer (see Figure 7). As pressure is applied to the well, the level
falls slightly as compared to the level rise in the column. By
compensating the columnłs scale graduations to correct for the well drop,
it is possible to make a direct reading of differential pressure. There
are connection guidelines placed on well-type manometers, compared to the
U-tube style:

Connect pressures higher than atmospheric to the well; connect
pressures lower than atmospheric to the tube.
For differential measurements, connect the higher pressure to the
well.
For raised-well manometers, the well connection can be used for
gauge and vacuum measurements.
A variation of the well-type manometer is the inclined-tube (or draft
gauge) manometer in Figure 8. With an inclined indicating tube, 1 in. of a
vertical rise is stretched over several inches of scale length. The
inclined-tube manometer has better sensitivity and resolution for low
pressures.





Figure 8.
Low pressure and low differentials are better handled with an
inclined-tube manometer, where 1 in. of vertical liquid height can
be stretched to 12 in. of scale length.

Indicating
FluidsLiquid manometers measure differential pressure by
balancing the weight of a liquid between two pressures. Light liquids such
as water can measure small pressure differences; mercury or other heavy
liquids are used for large pressure differences. For an indicating fluid 3
times heavier than water, the pressure measurement range is 3 times
greater, but the resolution is reduced.
Indicating fluids can be colored water, oil, benzenes, bromides, and
pure mercury. When selecting an indicating fluid, check the specifications
for specific gravity, operating temperature range, vapor pressure, and
flash point. Corrosive properties, solubility, and toxicity are also
considerations.
Digital
ManometersA liquid manometer has limitations. Glass tubing,
indicating fluids, and level mounting requirements are more suited to a
laboratory than the field. Also, it cannot be interfaced with a computer
or PLC. Such limitations can be overcome with digital manometers. These
microprocessor-based instruments are available in convenient, portable
sizes for ease of use in the field, or in panel or stand-alone mounting
styles, with outputs for controlling a process or transferring measurement
data.
Variations from standard conditions of density and gravity must be
compensated for manually when making pressure measurements with liquid
manometers. This is easier with digital manometers, because some of the
correction factors for liquid manometers can be ignored and others can be
compensated for in software.
With dual ports, swapping sensors is all that is needed to change among
differential, gauge and absolute pressure measurements.
Other common features of digital manometers include:

Onboard memory for data logging or storing min./max. readings
Averaging a number of readings to dampen pressure pulses
Higher accuracy digital manometers are used to calibrate pressure
transmitters and other pressure instrumentation in the field. Digital
calibrators are faster and simpler as they require no boxes, gas
cylinders, regulators, or weights to set up and have no special platforms
or critical leveling requirements. Further comparisons of liquid and
digital manometer specifications are shown in Table 1.


TABLE 1

Manometer Specifications

 

Liquid Manometers
Digital Manometers

U-tube
Well

Inclined
General Purpose
Calibrating

Range

100
in.
100
in.
20
in.
20-2000 in H2O,20-2000 psig,2000 mmHg

2000 in H2O,2000 psig,2000 mmHg


Accuracy

Ä…½ of
minor scale graduation
Ä…½ of
minor scale graduation
Ä…½ of
minor scale graduation
Ä…0.025-0.1% F.S.
Ä…0.025-0.1% F.S.

Wetted Partsor
MediaCompatibility
Cast
iron, stainless steel, PVC, glass, Viton
Stainless steel, glass, Viton
Acrylic, stainless steel, aluminum, glass, Viton

Clean, dry non-corrosive gases; liquids compatible with
stainless steel
Clean, dry non-corrosive gases; liquids compatible with
stainless steel

PressureRating

250
psig
250-500 psig
100-350 psig
2 ×
range
2 ×
range

Mounting

Wall,
table
Wall,
table, flush front, pipe
Wall,
table
Portable
Portable

Relative Cost

Low

Low/medium
Medium
Medium
High

For Further
ReadingMassey, B.S. 1989. Mechanics of Fluids, 6th
Ed., London: Van Nostrand Reinhold.
Meriam Instrument. 1997. Using Manometers to Precisely Measure
Pressure, Flow and Level, Cleveland: Meriam Instrument.
Meriam, J.B. 1938. The Manometer and Its Uses. 2nd Ed.,
Cleveland: Meriam Instrument.
Omega Engineering. 1999. Transactions in Measurement and Control:
Force-Related Measurements, 2nd Ed. Stamford, CT: Putnam Publishing
and Omega Press.
Yeager, John, and Hrusch-Tupta, M.A., Eds. 1998. Low Level
Measurements. 5th Ed. Cleveland: Keithley Instruments.
SIDEBAR:







Manometer Pressure and Accuracy
Glossary

Absolute
Pressure. A measurement referenced to zero pressure;
equals the sum of gauge pressure and atmospheric pressure.
Common units are pounds per square inch (psia), millimeters
mercury (mmHga), and inches mercury (in.Hga).

Accuracy. A measure of the closeness of agreement of
a reading to that of a standard. For absolute accuracy,
compare to a primary standard (one recognized by NIST).
Accuracies are usually specified as a plus or minus percent of
full scale. Calibration accuracies are often given as plus or
minus percent of reading with plus or minus counts.
Ambient Pressure. The pressure of the medium
surrounding a device. It varies from 29.92 in.Hg at sea level
to a few inches at high altitudes.
Atmospheric Pressure. The pressure of the atmosphere
on a unit surface. Also called barometric pressure. At sea
level it is 29.92 in.Hg absolute.
Count. The smallest increment of an A/D conversion
that is displayed.
Differential Pressure. The difference between two
measurement points. Common units are inches of water
(in.H2O), pounds per square inch (psi), and
millibars (mbar).
Display Resolution. The maximum number of digits on
a digital display. For example, a display resolution of 4½
digits reads a maximum of 19,999 counts; and a display
resolution of 5 significant digits reads a maximum of 99,999
counts.
Gauge Pressure. A measurement referenced to
atmospheric pressure. It varies with the barometric reading.
Also used to specify the maximum pressure rating of
manometers. Common units include pounds per square inch
(psig).
Range. The region between the lower and upper limits
of measurements.
Resolution. The smallest portion of a measurement
that can be detected.
Sensitivity. The smallest change in measurement that
can be detected.
Uncertainty. An estimate of the possible error in a
measurement. This is the opposite of accuracy.
Vacuum. Any pressure below atmospheric pressure.
When referenced to the atmosphere, it is called a vacuum (or
negative gauge) measurement. When referenced to zero pressure,
it is an absolute pressure measurement.
Zero Absolute Pressure. The complete absence of any
gas; a perfect vacuum.









For
further reading on this and related topics, see these
Sensors articles.
"DSSP-Based Pressure Sensors," January
2001"A Pressure Sensor for a Smart Barometer," May
2000"Fundamentals of Pressure Sensor Technology,"
November 1998"Choosing the Right Low-Pressure Sensor,"
September
1998






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Dave
Thomas is a Technical Service Manager and Rick DÅ‚Angelo is
General Manager, Meriam
Instrument, 10920 Madison Ave., Cleveland, OH 44102; 216-281-1100, fax
216-281-0228.











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