Meriam MANOMETERS User Manual
Instruction Sheet
F/N: 022C:440-14
E.O. # 5946
General Installation, Operation & Maintenance Instructions for
MERIAM MANOMETERS
1. Uncrating & Inspection
Carefully unpack the instrument. Make a thorough check for possible shipping damage. Remove all protective cushions or plugs. If
damaged, notify carrier and Meriam of damage. Be certain all items
on packing list are removed from container.
2. Mounting
Mount the instrument at a convenient height for reading vertically
on wall, panel, or table depending on mounting style. Be certain
instrument is level. Check with level on side and front of instrument
case. If instrument is furnished with a built-in level, use it to insure
level mounting. Table mounting stands are shipped separate from
instruments. Therefore attach the stand to the instrument.
3. Scale Adjustment (most models)
Position the scale at the center of the adjustment span.
4. Filling
U-Tube Manometers — Remove the top manometer head or fill
plug. Be sure instrument is properly vented on the low pressure
side. Slowly pour the selected manometer indicating fluid into the
glass tube until the indicating fluid level is at approximately the zero
graduation on the scale. Be sure all air bubbles are eliminated. Replace the head tightly. Adjust the scale for the correct zero position
in relation to the indicating fluid meniscus.
5. Connections to Process
It is necessary that the instrument be properly connected to the
system in order to obtain a significant and accurate indication. All
connections must be leak-free.
U-Tube Manometers
nected to either side of the instrument, provided that the difference
between them is within the indicating range of the instrument.
Well & Inclined Tube Manometers — Vertical and incline tube
manometers require more careful consideration. Basically, the
indicating fluid will move towards the low pressure and away
from the high pressure, until a hydrostatic balance has been
reached. In many cases it is possible that atmospheric pressure can
be considered as the high pressure. By noting the liquid level in
the filled manometer, it is possible to deduce whether the visible
level should be moved up or down to produce visible readings. By
knowing which of the two pressures is greater (as measured from
zero psia), the proper choice of meter connections can be made. It
may be noted that in flow metering work using orifice plates the
upstream pressure is greater than the downstream. For pitot tubes
the impact pressure is greater than the static pressure.
— Either high or low pressure may be con-
Well Type Manometers
sure instrument is properly vented on the low pressure side. Slowly
pour the selected manometer indicating fluid into the well until the
indicating fluid level is at approximately the zero graduation on the
scale. Be sure all air bubbles are eliminated. Replace the fill plug
tightly. Adjust the scale for the correct zero position in relation to
the indicating fluid meniscus.
Inclined Tube Manometers — Remove the fill plug. Be sure to
check the instrument scale before filling to be certain the correct
indicating fluid is being used. Most inclined tube manometers use
Meriam 827 Red Oil or Meriam 1000 Green Concentrate. For accurate measurements, it is critical to have the correct indicating fluid
in the instrument! Be sure instrument is properly vented on the low
pressure side. Slowly pour the selected manometer indicating fluid
into the well until the indicating fluid level is at approximately the
zero graduation on the scale. Be sure all air bubbles are eliminated.
Replace the fill plug tightly. Adjust the zero position by adjusting
the scale or the zero adjustment knob on the well, depending on the
model involved.
** THESE INSTRUMENTS MAY CONTAIN MERCURY.
PLEASE RECYCLE OR DISPOSE AS HAZARDOUS
WASTE ACCORDING TO LOCAL, STATE, AND FEDERAL
REGULATIONS. Consult Material Safety Data Sheets (MSDS)
for handling information and safety data.
— Remove the fill plug on the well. Be
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/sec2 (32.174 ft/sec2)
at sea level and 45.544 degrees latitude
Temperature: 0°C (32°F) density = 13.5951 g/cm
3
It is important to remember the levels in both legs of
U-tube manometers must be read and these readings
added together to obtain an actual indication.
A plane tangent to the fluid meniscus and at a right
angle to the tube bore intercepts the scale where it
should be read.
7. Theory
The fundamental relationship for pressure expressed
by a liquid column is:
p = P2 - P1 = ρgh
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
In the case of absolute manometers (barometers), P1
is equal to zero absolute pressure, simplifying the
equation to:
P =
ρgh
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.
Standard conditions for water used as a unit of pressure.
Gravity: 980.665 cm/sec2 (32.174 ft/sec2)
at sea level and 45.544 degrees latitude
Temperature: 4°C (39.2°F) density = 1 g/cm
3
Though it is recommended that the value of a water
column as a unit of pressure be at 4°C, its universal
acceptance has been slow. For instance in aeronautics
15°C (59°F) is used. The American Gas Association uses
15.56°C (60°F), and in orifice flowmeter work 20°C
(68°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.
Fluid Density Corrections
Manometers indicate the correct pressure at only one
temperature. This is due to the fact that the indicating
fluid density changes with temperature. If water is the
indicating fluid, an inch scale indicates one inch of water
at 4°C only. On the same scale mercury indicates one
inch of mercury at 0°C only. If a reading using water or
mercury is taken at 20°C (68°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.
A simple way of correcting for the temperature error is to
ratio the densities.
(Standard) ρogho = (Ambient) ρtgh
t
ho = the corrected height of the indicating fluid to standard
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
when read
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