Flow - and - Pressure Measurement by Manometer
Flow - and - Pressure Measurement by Manometer
Flow - and - Pressure Measurement by Manometer
PRESSURE
HOW TO MEASURE
FLOW
by
G.J.Matthews
PS
PS
If the disc is stationary the pressure acting on the two surfaces will be the
same providing the presence of the disc does not disturb the flow.
PS
PS
NO DISTURBANCE
However if the stationary disc is turned through 90 its front face will be
subjected to not only static pressure, but also an additional dynamic
pressure equal to the velocity pressure in incompressible flow, due to the
impact.
PS
P D = x r x V
V
PD
The velocity pressure is due entirely to the motion of the flow and depends
upon the velocity and the density of the fluid.
3
The algebraic sum of the static pressure and the velocity pressure is called
the total pressure.
MEASURING TOTAL AND STATIC PRESSURE.
A tube placed in a duct facing into the direction of the flow will measure the
total pressure in the duct. If frictional losses are neglected, the mean total
pressure at any cross section throughout the duct system is constant.
Static pressure can only be determined accurately by measuring it in a
manner such that the velocity pressure has no influence on the
measurement at all. This is carried out by measuring it through a small hole
at the wall of the duct; or a series of holes positioned at right angles to the
flow in a surface lying parallel to the lines of flow. The pitot static tube is an
example of this. Figure 1 shows the principle of the pitot static tube.
Figure 1. Principle of the pitot static tube.
TUBE MANOMETER.
Although probably the oldest method of measuring low pressures, the simple
U tube has much to commend it.
If a U shaped glass tube is half filled with a liquid, e.g. water, and a
pressure is applied to one end of the limb, the other being open to
atmosphere, the liquid will move to balance the pressure. The weight of
liquid so displaced will be proportional to the pressure applied. As the
difference in height of the two columns of liquid and the density are known
the pressure can be calculated. Each millimetre height difference of water
column represents approximately 10 Pascals.
Alternatively with zero taken at the centre point the scale length is halved
with subsequent loss of resolution.
[2]. LIQUID - IN GLASS MANOMETERS.
The disadvantages of the simple U tube manometer are overcome and other
advantages incorporated in single limb industrial manometers in which it is
only necessary to read one liquid level.
In one such design, one of the limbs of the U tube is replaced by a reservoir,
thus substantially increasing the surface area. A pressure applied to this
reservoir causes the level of fluid to move a small, calculable amount. The
same volume of fluid displaced in the glass limb produces a considerable
change in the level. This nearly doubles the resolution compared with the U
tube manometer for vertical instruments and gives much greater
magnification when the limb is inclined.
The manometer fluid may be plain water, but problems can arise from algae
growing in the tube causing the density of the fluid to alter. Special blends of
paraffin are often used and these have several advantages: a free moving
meniscus, no staining of the tube, and expanded scales due to low relative
densities. Where higher pressures are required, denser fluids are used, of
which mercury is often used. For very low pressures the manometer limb is
inclined to improve the resolution further.
More precision can be achieved with adjustable - range limbs and it is
possible to achieve from 0 - 125 Pa. to 0 - 5000 Pa. with only two limbs.
Figure 4: Examples of Single Limb Manometers
The piezo resistive pressure sensor contains a silicon chip with an integral
sensing diaphragm and four piezo-resistors; pressure applied on the
diaphragm causes it to flex changing the resistance; this causes a low level
output voltage proportional to pressure.
A pressure transducer - based instrument allows continuous monitoring
using a recorder, or input to electronic storage or control equipment.
Figure 6. Pocket Manometer DB2.
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or
The volume flow rate in a system can be measured at the entrance to the
system, at the exit from the system, or somewhere within the system itself.
This could involve measuring the total flow rate or the flow rate in a portion
of the system. Wherever it is measured, it should be a prerequisite that the
flow should be swirl - free.
There are several methods with which the flow rate can be measured.
[1]. IN -
BS1042 Part 1 : 1990 describes such devices as the venturi nozzle, the
orifice plate, and the conical inlet. The venturi nozzle and the orifice plate
may be used at the inlet to or the outlet from a system as well as between
two sections of an airway. The conical inlet draws air from a free space at
the entrance to a system.
With regard to measuring the volume flow rate of general purpose fans, the
requirements of BS1042 with regard to the lengths of straight duct upstream
of the flow meter is reduced and BS848 Part 1 : 1980 details these changes
together with the associated uncertainty of measurement.
The general equation for these differential flowmeters is:
qm
where
qm
a
e
d
ru
DP
=
=
=
=
=
=
a e [ p d / 4 ] 2 ru DP
It is not proposed to go into too much detail with these devices as both
BS1042 and BS848 are complete documents and should be referred to.
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[2]. PITOT -
1.291 pv
(m/s)
=
=
=
=
=
=
air velocity ( m / s )
velocity pressure ( Pa.)
atmospheric pressure ( Pa.)
static pressure ( Pa.)
absolute temperature ( K ) (= t + 273)
airstream temperature ( C )
13
Figure 9. Measuring points for circular ducts. Log linear rule for traverse
points on 3 diameters.
14
15
20.00
15.00
10.00
400mm DIA. DUCT
5.00
0.00
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
16
19
20
The holes in the tubes are positioned such that an acceptable sampling
system is utilised.
The output signal can be compared to the mean duct velocity pressure that
would be present in the duct without the grid being present. This is
commonly termed the magnification factor.
The output signal can also be plotted against the volume flow rate in the
duct. This is possible because each Wilson flow grid is manufactured to suit
a given duct size.
It is possible to use a single double tube in a duct section, but because
there is no effective manifold the reading can be slightly erratic. It should
also be noted that the single tube type only senses the flow rate in one plane
which is not ideal.
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In the table below, I have tried to include all of the most popular
combinations, with the preferred S.I. unit shown in blue
VOLUME FLOW RATE.
1 m/s
= 999.97 l/s
= 3600 m/h
= 2118.9 cfm
= 60 m/min
1 l/s
= 3.60 m/h
= 2.119 cfm
= 0.06 m/min
= 0.001 m/s
1 m/h
= 0.5886 cfm
1 cfm
= 1.699 m/h
1 m/min
= 0.01667 m/s
= 35.315 cfm
= 16.67 l/s
= 60 m/h
= 0.2778 l/s
VELOCITY.
1 m/s
= 196.85ft/min
1 ft/min
= 0.00508 m/s
PRESSURE.
1 Pa
= 0.01 mbar
1 mbar
= 0.4015 in.wg
= 100 Pa.
1 in.wg.
= 25.4 mm.wg
= 0.0361 psi.
= 249.089 Pa.
= 2.49089 mbar.
1 mm.wg
= 0.00142 psi.
= 9.80665 Pa.
= 0.09807 mbar
= 0.03937 in.wg.
1 psi.
= 6894.76 Pa.
= 68.9476 mbar
= 27.68 in.wg.
= 703.07 mm.wg.
kilopascal
= kPa
millibar
= mbar
litre per second = l/s
=
=
=
1000Pa.
100 Pa.
10-3 m3/s
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