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6. Reading
For consistent results, it is necessary that the fluid
meniscus always be observed in the same way. A
convex meniscus forms when mercury is used. In
this instance the fluid level should be observed from
the upper most point. For all other indicating fluids
a concave meniscus forms. The reading in this case
should be observed from the lowest point of the
meniscus. To duplicate factory calibration of incline
manometers, this technique needs to be followed.
Density is a function of temperature and gravity is
a function of latitude and elevation. Because of this
relationship some ambient conditions must be selected as
standard so that pressure bears a fixed definition.
Standard conditions for mercury used as a unit of pressure:
Gravity: 980.665 cm/sec
2
(32.174 ft/sec2)
at sea level and 45.544 degrees latitude
Temperature: 0o C (32o F) density = 13.5951 g/cm
3
It is important to remember the levels in both legs of
Standard conditions for water used as a unit of pressure.
U-tube manometers must be read and these readings
added together to obtain an actual indication.
Gravity: 980.665 cm/sec2 (32.174 ft/sec2)
at sea level and 45.544 degrees latitude
A plane tangent to the fluid meniscus and at a right
Temperature: 4
o
C (39.2o F) density = 1 g/cm
3
angle to the tube bore intercepts the scale where it
should be read.
Though it is recommended that the value of a water
column as a unit of pressure be at 4o°C, its universal
acceptance has been slow. For instance in aeronautics
15o°C (59o°F) is used. The American Gas Association uses
15.56o°C (60o°F), and in orifice flowmeter work 20o°C
(68o°F) is commonly used.
Recognizing a manometer may be read outside, standard
temperature and gravity corrections can be applied to
improve the accuracy of a manometer reading at any given
conditions.
7. Theory
The fundamental relationship for pressure expressed
by a liquid column is:
Fluid Density Corrections
Manometers indicate the correct pressure at only one
temperature. This is due to the fact that the indicating
p = P
- P1 = ρgh
2
fluid density changes with temperature. If water is the
indicating fluid, an inch scale indicates one inch of water
p = differential pressure
P1 = pressure applied to one liquid surface
P2 = pressure applied to the other liquid surface
ρ = mass density of the liquid (specific gravity)
g = acceleration of gravity
h = height of the liquid column
at 4o°C only. On the same scale mercury indicates one
inch of mercury at 0o°C only. If a reading using water or
mercury is taken at 20o°C (68o°F) then the reading is not
an accurate reading. The error introduced is about 0.4% of
reading for mercury and about 0.2% of reading for water.
Since most manometers are read at temperatures well
above the standard temperature, corrections are needed.
In the case of absolute manometers (barometers), P1
is equal to zero absolute pressure, simplifying the
A simple way of correcting for the temperature error is to
ratio the densities.
equation to:
(Standard) ρogho = (Ambient) ρtgh
t
P = ρgh
ho = the corrected height of the indicating fluid to standard
As simple as manometry is, certain aspects are often
overlooked. Manometry incorporates both a value for
density and gravity. These two values are not constant.
temperature
ht = height of the indicating fluid at the temperature
when read
ρo = density of the indicating fluid at standard temperature
ρt = density of the indicating fluid at the temperature
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when read
Using this method is very accurate when density/
temperature relationships are known. Data is readily
available for water and mercury.
Density (g/cm
3
) as a function of temperature (oC°) for
mercury:
= 13.556786 (1-0.0001818 (T- 15.5556) )
Density (g/cm3) as a function of temperature (oC°) for
water:
= 0.9998395 + 6.7982999 x 10
-5
(T)
-9.1060255 x 10-6 (T2) + 1.0052729 x 10-7 (T3)
-1.1267135 x 10
-9 (T4
) + 6.5917956 x lO
-12
(T5)
Gravity at a known latitude is:
gx = 980.616 (1 - .0026373cos2x + .0000059cos22x)
gx = gravity value at latitude x, sea level (cm/s2)
x = latitude (degrees)
The relationship for inland values of gravity at elevations
above sea level is:
gt = gx - 0.000094H+ 0.00003408 (H-H1) (cm/sec2)
H = elevation (feet) above mean sea level
1
H
= average elevation (feet) of the general terrain within a
radius of 100 miles of the point
For other fluids, manometer scales and fluid densities
may be formulated to read inches of water or mercury at
a set temperature. This temperature is usually ambient
temperature. This decreases the error due to the temperature
change, because most manometers are used at or close to
ambient temperature. In some work direct readings close
to design temperature are accurate enough. The manometer
still only reads correctly at one temperature, and for precise
work the temperature corrections can not be overlooked.
Gravity Corrections
The gravity correction arises because gravity at the location
of the instrument governs the weight of the liquid column.
Like the fluid density correction, the gravity correction is a
ratio.
(Standard) ρ
gtρ
ho = x h
go = standard gravity - 980.665 cm/s
goρ
= (Ambient) ρtgtht
ogoho
t
o
t
2
(45.54° N latitude & sea level)
gt = gravity at the instruments location
1
The second term may be eliminated when H
is unknown, but
the accuracy of the gravity determination will decrease. The
degree of inaccuracy is determined by how far H
1
varies from
H. In mountainous terrain this error could be large.
For every different set of conditions the significance of the
corrections vary. Whether these corrections can be overlooked
or not depends upon the accuracy requirements.
Other corrections do exist, but are generally outside the
resolution of the instrument. For information on these
considerations consult Meriam.
8. Accuracy
A manometer, when used properly, is a very accurate
instrument. NIST recognizes the U-type manometer as a
primary standard due to its inherent accuracy and simplicity
of operation. The manometer has no moving parts subject
to wear, age, or fatigue. It is simply a determination of the
height of a liquid column. When the previously mentioned
corrections are accounted for the uncertainty of a manometer
reading is ± 1/2 the smallest graduation. This is due to your
eyes ability to interpolate between the graduations.
A 10° change in latitude at sea level will introduce
approximately 0.1% error in reading. At the Equator (0°) the
error is approximately 0.25%, An increase in elevation of
5000 ft will introduce an error of approximately 0.05%.
9. Trouble Shooting
When properly selected and used, the manometer is a very
accurate standard. It can, however, present what appears
to be incorrect indications. These are generally the result
of misapplied or improperly used pressures, or incorrectly
Gravity values have been determined by the U.S. Coast and
evaluated indication.
Geodetic Survey at many points in the United States. Using
these values, the U.S. Geodetic Survey may interpolate and
obtain a gravity value sufficient for most work. To obtain
a gravity report, the instrument’s latitude, longitude, and
elevation are needed. For precise work you must have the
value of the gravity measured at the instrument’s location.
No Reading — Pressure is not being applied to either, or both,
sides of the instrument. Check for plugged or leaking pressure
lines. Check internal passages of the meter for foreign
particles, which may be blocking flow. Be sure atmospheric
pressure connection is vented to atmosphere. Check for proper
zero setting.
Where a high degree of accuracy is not necessary and the
value of local gravity has not been determined, local gravity
can be calculated.
High Reading — Specific gravity of the indicating fluid may
be too low; check scale legend and provide correct indicating
fluid. With tank gauges, tank contents fluid density may be too
great. If the low pressure line is above atmospheric pressure
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or the high pressure line is below atmospheric pressure
they should be checked for leaks or plugs. Check for
proper zero setting.
Low Reading — Specific gravity of indicating fluid may
be too great: check scale legend and provide correct
indicating fluid. Check for leaks or plugs in high pressure
line, if it is above atmospheric pressure, and or in low
pressure line, if it is below atmospheric pressure. If meter
has a valve by-pass line, this valve must be leak free.
Check vent when used. Check for proper zero setting.
11. Parts
For replacement and/or spare parts request parts
drawing specifying the following information.
Model number
Serial number
Range
12. Accessories
To broaden the application and simplify operation
of your instrument, Meriam provides the following
accessories:
10. Maintenance
Normal maintenance necessary with Meriam manometers
is the occasional cleaning of the tubes. Some manometer
indicating fluids become oxidized with use. Others, over
long periods of time, react with gases and fluids with
which they are in contact, leaving a deposit on the surface
of the fluid, and, in turn, on the tube.
Prior to cleaning the glass tube, a manometer’s indicating
fluid must be drained using the steam flow drain valve on
well type manometers or by carefully tipping a U-tube
manometer on its side. As the fluid in the manometer
may be classified as a SARA Title III Section 313 Toxic
Chemical and may be harmful to your health if handled
improperly, please consult the Material Safety Data Sheet
(MSDS) sheet supplied with this product for detailed
health and safety information and proper handling
information. Dispose of used indicating fluids and used
cleaning materials according to local, state, and federal
regulations.
The method of cleaning the various models is similar,
but, there are some differences. It is generally more
convenient to clean from the upper end of the manometer.
On installations where accessories or piping is connected
to the upper end of the manometer and it is inconvenient
to remove equipment, the tube can often be cleaned
through the drain plug connection in the lower channel
end block. It is recommended that wherever possible the
instrument be removed to a convenient table where it can
be laid horizontally to afford greatest care in performing
the various operations.
Return Wells— for over-range protection
Fluid Check Valves — for over-range protection
Line Traps — to keep instruments clean by trapping
dirt from the process.
Moisture & Sediment Traps — to keep instruments
clean by trapping dirt from the process
Pressure Vacuum Variator.— static pressure/vacuum
pressure
Valves & Piping Manifolds — to simplify different
pressure measurements
Sight Feed Bubblers — for metering purge gasses
Table Mount Stands
Cleanout Bore Brushes
Seal Pots — for isolating process fluid from the
instrumentation
Orifice Plates & Flanges
Meriam Instrumentation has pioneered the
development and application of manometry since
1911. The highest standards of accuracy and
practical, high quality design have made the words
MERIAM and MANOMETERS synonymous to
the engineering and research world. We welcome
your inquiries pertaining to manometry, and will be
pleased to help in any way we can.
Meriam Cleaning Fluid
A clean manometer is essential for readability
and accuracy. Meriam No. 915E cleaning fluid is
recommended for use in all Meriam manometers. It is
available in gallon containers only.
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