Pressure Measuring Instruments: Dr. Fahad Rehman CUI, Lahore Campus

Pressure Measuring
Instruments
Dr. Fahad Rehman
CUI, Lahore Campus
Pressure Measurement
• Macondo Oil leak
• Leak through the riser
• Rate peaked at 62000 bbl/d
• So it is critical
lets start from the basic
Vapour pressure
Pressure units
Different type of pressure
Static Pressure
Dynamic Pressure
Barometric Pressure
Pressure Measuring devices
Barometer
U-tube Manometer
Inclined Tube Manometer

Bourdon Gauge
Bourdon Gauge
Diaphragm
Bellows
Gauge selection mounting
Gauge selection criteria
Pressure Transmitter
• A Pressure Transmitter is used where indication and/or record of
pressure is required at a location not adjacent to the primary element.
• A Pressure Transmitter is used for both indication and control of a
process.
• A Pressure Transmitter is used where overall high performance is
mandatory.
• Both Electronic and Pneumatic Transmitters are used.
• These can be either Gauge, Absolute or Differential Pressure
Transmitters.
Transmitter Measuring
Principles
• The diagram in the next slide shows an
electronic differential pressure sensor. This
particular type utilizes a two-wire capacitance
technique.
• Another common measuring technique is a

strain gauge.
• Process pressure is transmitted through

isolating diaphragms and silicone oil fill fluid to
a sensing diaphragm.
Principles
• The sensing diaphragm is a stretched spring
element that deflects in response to the
differential pressure across it.
• The displacement of the sensing diaphragm

is proportional to the differential pressure.
• The position of the sensing diaphragm is

detected by capacitor plates on both sides of
the sensing diaphragm.
Principles
• The differential capacitance between the
sensing diaphragm and the capacitor plates is
converted electronically to a 4–20 mA or 1-5
VDC signal.
• For a gauge pressure transmitter, the
lowpressure side is referenced to atmospheric
pressure.
The majority of industrial DP Transmitters
are fitted with diaphragm as the pressure
sensing element. This diaphragm is a
mechanical device. It is placed in between
the two pressure inlet ports. The diaphragm
will be deflected by the applied pressure.
This deflection is converted into an electrical

signal. This is normally done by the sensors.
The commonly used sensors are (a) Strain
Gauge (b) Differential Capacitance (c)
Vibrating wire. The sensor output is
proportional to the applied pressure.
Strain Gauages
If a strip of conductive metal is stretched, it will become skinnier and longer,

both changes resulting in an increase of electrical resistance end-to-end.
Conversely, if a strip of conductive metal is placed under compressive force

(without buckling), it will broaden and shorten.
If these stresses are kept within the elastic limit of the metal strip (so that the
strip does not permanently deform), the strip can be used as a measuring
element for physical force, the amount of applied force inferred from
measuring its resistance.
. • A strain gauge’s conductors are very thin: if made of round wire, about 1/1000
inch in diameter. Alternatively, strain gauge conductors may be thin strips of the
metallic film deposited on a nonconducting substrate material called the carrier.
The latter form of the strain gauge is represented in the previous illustration. The
name “bonded gauge” is given to strain gauges that are glued to a larger
structure under stress (called the test specimen).
• The task of bonding strain gauges to test specimens may appear to be very
simple, but it is not. “Gauging” is a craft in its own right, absolutely essential for
obtaining accurate, stable strain measurements. It is also possible to use an
unmounted gauge wire stretched between two mechanical points to measure
tension, but this technique has its limitations
Resistance Gauge Resistance
• Typical strain gauge resistances range from 30 Ω to 3 kΩ (unstressed).

This resistance may change only a fraction of a percent for the full
force range of the gauge, given the limitations imposed by the elastic
limits of the gauge material and of the test specimen. Forces great
enough to induce greater resistance changes would permanently
deform the test specimen and/or the gauge conductors themselves,
thus ruining the gauge as a measurement device. Thus, in order to
use the strain gauge as a practical instrument, we must measure
extremely small changes in resistance with high accuracy.
Quarter bridge circuit
Bridge Measurement Circuit
• Such demanding precision calls for a bridge measurement circuit.
Unlike the Wheatstone bridge shown in the last chapter using a null-
balance detector and a human operator to maintain a state of
balance, a strain gauge bridge circuit indicates measured strain by the
degree of imbalance, and uses a precision voltmeter in the center of
the bridge to provide an accurate measurement of that imbalance:
• Typically, the rheostat arm of the bridge (R2 in the diagram) is set at a
value equal to the strain gauge resistance with no force applied. The
two ratio arms of the bridge (R1 and R3) are set equal to each other.
Thus, with no force applied to the strain gauge, the bridge will be
symmetrically balanced and the voltmeter will indicate zero volts,
representing zero force on the strain gauge. As the strain gauge is
either compressed or tensed, its resistance will decrease or increase,
respectively, thus unbalancing the bridge and producing an indication
at the voltmeter. This arrangement, with a single element of the
bridge changing resistance in response to the measured variable
(mechanical force), is known as a quarter-bridge circuit.
There are unlimited industrial applications of Differential Pressure
Transmitters.
• Oil and Gas flow metering in onshore, offshore and subsea applications.
• Water and effluent treatment plants. It is largely used to monitor filters in
these plants.
• It is used to monitor Sprinkler Systems.
• Remote sensing of Heating Systems for Steam or Hot Water.
• Pressure drops across valves can be monitored.
• Pump control monitoring.
Differential pressure sensor/Transmitter
Differential pressure sensor/Transmitter
Vibrating wire
Vibrating wire sensors are well known for their long-term stability. The advantage of vibrating wire sensors over
more conventional types lies mainly in the sensor output, which is a frequency rather than a voltage.
Frequencies can be transmitted over long (>2000 m) cables without appreciable degradation of the signal
caused by variations in cable resistance, which can arise from water penetration, temperature fluctuations,
contact resistance or leakage to ground.
Installation
The following should be taken into consideration when installing pressure-sensing devices.
1. The distance between the sensor and the source should be kept to a minimum.
2. Sensors should be connected via valves for ease of replacement.
3. Overrange protection devices should be included at the sensor.
4. To eliminate errors due to trapped gas in sensing liquid pressures, the sensor should be located
below the source.
5. To eliminate errors due to trapped liquid in sensing gas pressures, the sensor should be located
above the source.
6. When measuring pressures in corrosive fluids and gases, an inert medium is necessary between
the sensor and source, or the sensor must be corrosion-resistant.
7. The weight of liquid in the connecting line of a liquid pressure sensing device located above or
below the source will cause errors at zero, and a correction must be made by the zero adjustment, or
otherwise compensated for in measurement systems.
8. Resistance and capacitance can be added to
Process Safety
• Pressure is monitored across, almost, all industrial equipment
• It has serious implications on process design, vessel design,

metallurgy control and operation
• At moderate pressure, (1 barg- 20 bar g) the overall investment may

be insensitive to pressure
• Beyond 2 bar, compressors are required. Increasing the cost an order

of magnitude higher.
• Safety precautions and control must be more robust
• Material of construction; e.g., SS304 and SS316 are approximately

three times more expensive. More expensive alloys such as Monel
and Titanium based are 5 to 10 time are more expensive
•
Pressure Regulator
• Regulates the pressure
• Specialized valves that interrupt flow

when the line upstream pressure drops
below the set point.
• It will shut off when the cylinder

pressure drains to the set point
pressure
• Two stage regulators are more common
and safe
•
Back Pressure Regulators
• a pressure regulator reduces
the supply pressure at the inlet
to a lower pressure at the
outlet, a back pressure
regulator throttles the flow
downstream to maintain the
inlet pressure.
• A needle valve or a back
pressure regulator is required
at the vent line to match the
pressure across the reactor,
process lines, and analytical
equipment.
Relief Valve
• Back pressure regulators provide steady-state control; relief valves
provide on-off protection from overpressure or vacuum conditions.
When the set pressure threshold is exceeded, the valve opens either
to atmosphere or to an auxiliary line (where the fluid may go to a
flare or storage tank or even to recycling).
• The difference in pressure between when the valve relieves and when
the valve reseats is referred to as the blowdown; it is typically 2–20%
lower than the relief pressure.
Relief Valve
Rupture Disk
Most pressure vessels require rupture disks for which designs
are specified international standards codes.
They protect from either overpressure or vacuum conditions.
It is composed of a membrane that will instantaneously fail

(within milliseconds) at a specified differential pressure.

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Pressure Measuring

• Leak through the riser

• Rate peaked at 62000 bbl/d

Inclined Tube Manometer

• Another common measuring technique is a

• Process pressure is transmitted through

• The displacement of the sensing diaphragm

• The position of the sensing diaphragm is

This deflection is converted into an electrical

If a strip of conductive metal is stretched, it will become skinnier and longer,

Conversely, if a strip of conductive metal is placed under compressive force

• Typical strain gauge resistances range from 30 Ω to 3 kΩ (unstressed).

3. Overrange protection devices should be included at the sensor.

• It has serious implications on process design, vessel design,

• At moderate pressure, (1 barg- 20 bar g) the overall investment may

• Beyond 2 bar, compressors are required. Increasing the cost an order

• Material of construction; e.g., SS304 and SS316 are approximately

• Specialized valves that interrupt flow

• It will shut off when the cylinder

They protect from either overpressure or vacuum conditions.

It is composed of a membrane that will instantaneously fail

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