module 8 pressure measurement

Energy Systems Engineering Technology

Pressure Module

College of Technology

Instrumentation and Control

Module # 8 Pressure Measurement

Document Intent:

The intent of this document is to provide an example of how a subject matter expert might teach

Pressure Measurement. This approach is what Idaho State University College of Technology is

using to teach its Energy Systems Instrumentation and Control curriculum for Pressure

Measurement. The approach is based on a Systematic Approach to Training where training is

developed and delivered in a two step process. This document depicts the two step approach

with knowledge objectives being presented first followed by skill objectives. Step one teaches

essential knowledge objectives to prepare students for the application of that knowledge. Step

two is to let students apply what they have learned with actual hands on experiences in a

controlled laboratory setting.

Examples used are equivalent to equipment and resources available to instructional staff

members at Idaho State University.

Pressure Measurement Introduction:

This module covers aspects of pressure measurement as used in process instrumentation and

control. Pressure measurement addresses essential knowledge and skill elements associated with

measuring pressure. Students will be taught the fundamentals of positive and negative pressure

measurement using classroom instruction, demonstration, and laboratory exercises to

demonstrate knowledge and skill mastery of pressure measurement. Completion of this module

will allow students to demonstrate mastery of knowledge and skill objectives by completing a

series of tasks using calibration/test equipment, pressure indicating, and pressure transmitting

devices.


Pressure Module

References

This document includes knowledge and skill sections with objectives, information, and examples

of how pressure measurement could be taught in a vocational or industry setting. This document

has been developed by Idaho State University’s College of Technology. Reference material used

includes information from:

1. American Technical Publication – Instrumentation, Fourth Edition, by Franklyn W. Kirk,

Thomas A Weedon, and Philip Kirk, ISBN 979-0-8269-3423-9 Chapter 3

2. Department of Energy Fundamentals Handbook, Instrumentation and Control, DOE-

HDBK-1013/1-92 JUNE 1992, Re-Distributed by http://www.tpub.com


Pressure Module

STEP ONE

Pressure Measurement Course Knowledge Objectives

Knowledge Terminal Objective (KTO)

KTO 2. Given examples, EVALUATE pressure measurement fundamentals as they apply to

measuring positive and negative process pressure variables to determine advantages

and disadvantages associated with different types of devices used to indicate,

measure, and transmit pressure.

Knowledge Enabling Objectives (KEO)

KEO 2. 1 DEFINE Pressure

KEO 2. 2 DEFINE Fluid/Liquid Pressure

KEO 2. 3 DEFINE Atmospheric Pressure

KEO 2. 4 DEFINE Head Pressure

KEO 2. 5 DEFINE Hydrostatic Pressure

KEO 2. 6 DEFINE Mechanical Pressure

KEO 2. 7 DEFINE Pascal’s Law

KEO 2. 8 DESCRIBE four common pressure scales:

a. Absolute

b. Gauge

c. Vacuum

d. Differential

KEO 2. 9 CONVERT Pressure Equivalents

KEO 2. 10 EXPLAIN how manometers are used to measure pressure


Pressure Module

KEO 2. 11 DESCRIBE four types of manometers

a. U-Tube

b. Inclined

c. Well

d. Barometer

KEO 2. 12 EXPLAIN how a mechanical pressure diaphragm device detects and measures

pressure.

KEO 2. 13 EXPLAIN how a mechanical pressure capsule device detects and measures

pressure.

KEO 2. 14 EXPLAIN how a mechanical pressure spring device detects and measures

pressure.

KEO 2. 15 EXPLAIN how a mechanical pressure bellows device detects and measures

pressure.

KEO 2. 16 EXPLAIN how a mechanical pressure double-ended piston device detects and

measures pressure

KEO 2. 17 EXPLAIN how an electrical transducer works

KEO 2. 18 EXPLAIN how a resistance pressure strain gauge transducer works.

KEO 2. 19 EXPLAIN how a capacitance pressure transducer works.

KEO 2. 20 EXPLAIN how a reluctance pressure transducer works.

KEO 2. 21 EXPLAIN how a piezoelectric pressure transducer works.

KEO 2. 22 EXPLAIN how a differential pressure (d/p) cell transducer works.

KEO 2. 23 EXPLAIN how to correctly use manometers to measure pressure dealing with:

a. Moisture Condensation

b. Measuring Liquids

c. Using Valve Manifolds


Pressure Module

KEO 2. 24 EXPLAIN methods used to protect pressure gauges and sensors from:

a. Over Pressure

b. Over Temperature

c. Corrosion or Contamination

KEO 2. 25 DESCRIBE what a Deadweight Tester is and how it is used to calibrate pressure

sensors.

KEO 2. 26 EXPLAIN how manometers are used to calibrate pressure sensors and the

limitations associated with using manometers.

KEO 2. 27 EXPLAIN how to connect a FLUKE model 744 Electronic Calibrator to calibrate

a 4-20 mA pressure transmitter.


a Current to Pneumatic (I/P) transducer.


Pressure Module

PRESSURE MEASURMENT

KEO 2. 1 Define Pressure

The measurement of Pressure is one of the major process measurements used for process

control. The pressure of almost any liquid or gas that is stored or moved must be known to

ensure safe and reliable operations. Pressure is defined as force divided by the area over which

that force is applied. Force is anything that changes or tends to change the state of rest or motion

of a body. Area is the number of unit squares equal to the surface of an object.

Figure 3-1 (top half) page 89

Formulas: Pressure = Force ÷ Area (P=F÷A)

Force = Pressure × Area (F=P×A)

Area = Force ÷ Pressure (A=F÷P)

As a comparison, the formula for Ohms law is:

Voltage = Current × Resistance (V=I×R)

Resistance = Voltage ÷ Current (R=V÷I)

Current = Voltage ÷ Resistance (I=V÷R)


Pressure Module

KEO 2. 2 DEFINE Fluid/Liquid Pressure

Fluid/Liquid Pressure is any material that flows and takes the shape of its container. Gasses

and liquids are both fluids. Fluid pressure may be due to the weight of a fluid column, or due to

applied mechanical energy. Mechanical energy is provided by such devices as a pump or blower

and stored in the form of a fluid under pressure, at an elevated height, or both.

For comparison, remember that a gas will completely fill a container and a liquid (solid) will

retain its shape regardless of the container. Liquids are fairly dense materials and the effect of

gravity on liquids is substantial.

KEO 2. 3 DEFINE Atmospheric Pressure

Atmospheric Pressure is the pressure due to the weight of the atmosphere above the point

where it is measured. Atmospheric pressure is depicted below:

Figure 3-2 page 90

Atmospheric pressure changes at different elevations because at higher elevations there is less

weight of air above that elevation than at lower elevations. Atmospheric pressure also changes

with from day to day with changes in the weather.


Pressure Module

KEO 2. 4 DEFINE Head Pressure

Head Pressure is the actual height of a column of liquid. A container or vessel can be any

shape; but the head is only determined by the height of the liquid. For example, the head of

water in water towers of a different shape depends only on the height of the water as depicted

below:

Figure 3-3 page 90

Head is expressed in units of length such as inches or feet, and includes a statement of which

liquid is being measured. Head may be expressed as inches or feet of water or inches of

mercury.


Pressure Module

KEO 2. 5 DEFINE Hydrostatic Pressure

Hydrostatic Pressure is the pressure due to the head of a liquid column and is frequently

referred to as head pressure. The difference is that not only is pressure dependent height, but

also on the properties of the liquid. For example mercury is heavier than water with different

densities. Where mercury is much denser than water, a shorter column of mercury produces a

hydrostatic pressure equivalent to a much taller column of water. The formula for determining

pressure is: Pressure = Density times the Height (P = D×H) as depicted below:

Figure 3-4 page 91

KEO 2. 6 DEFINE Mechanical Pressure

Mechanical Pressure may also be mechanical energy in the form of a fluid under pressure such

as pneumatic or hydraulic pressure. Pneumatic pressure is air or another gas that is compressed

and hydraulic pressure is pressure in a confined hydraulic liquid that has been subjected to the

action of a pump. Pneumatic pressure is used to send a signal in a pneumatic control system and

Hydraulic pressure is used to move objects or do other work.


Pressure Module

KEO 2. 7 DEFINE Pascal’s Law

Pascal’s Law states that pressure applied to a confined static fluid is transmitted with equal

intensity throughout the fluid. The Hydraulic Press Operation below depicts how a force is

amplified through the application of Pascal’s Law:

Figure 3-5 page 92

The hydraulic press operations depicted above is the principle used for dead weight testers used

to calibrate pressure sensing devices.


Pressure Module

Depicted below is a Dead Weight Tester:

Figure 3-37 page 121

KEO 2. 8 DESCRIBE four common pressure scales:

a. Absolute

b. Gauge

c. Vacuum

d. Differential

There are many ways to report pressure, depending on the application. Pressure is reported in

many units as well as on different scales. The four common pressure scales are absolute, gauge,

vacuum and differential pressure. Common units of pressure are atmospheres, psi, and inches of

water.


Pressure Module

KEO 2.8.a Absolute Pressure is pressure measured with a perfect vacuum as the zero point

of the scale. When measuring absolute pressure, the units increase as the pressure increases.

Absolute pressure cannot be less than zero and is unaffected by changes in atmospheric pressure.

Absolute Zero Pressure is a perfect vacuum as depicted below:

Figure 3-6 page 93

When measuring pressure using a gauge to show anything above absolute pressure, the gauge to

indicate both pressure greater than atmospheric pressure and the actual atmospheric pressure is

called a PSIA gauge. The PSIA gauge is a pound-per square-inch gauge that will also measure

the pressure less than the atmospheric pressure.


Pressure Module

A 30 PSIA and a 30 PSI gauge are depicted below (notice the PSIA gauge on the top is not

connected to a pressure source and that it is indicating the atmospheric pressure of 13.7, where

the PSI gauge on the bottom is not connected to a source and it reads zero PSI):

PSIA Gauge Reading Atmospheric Pressure

PSI Gauge Reading Zero

Note: A common mistake is to see a PSIA guage reading atmospheric pressure and someone has

reset the pointer to zero because it is not connected to a pressure source and they believe it to a

reading error.


Pressure Module

KEO 2.8.b Gauge Pressure is pressure measued with atmospheric pressure as the zero point

on the scale. When measuring gauge pressure, the units increase as the pressure increases.

Negative Gauge Pressure is gauge pressure less than zero. Negative gauge pressure indicates the

presence of a partial vacuum. The only difference between absolute pressure and gauge pressure

is the zero point on the scale. A gauge that indicates the difference between absolute pressure

and gauge pressure is depicted below:

Pressure Vacuum Gauge

Note: If a vessel is kept at a constant absolute pressure, the gauge pressue can vary when the

atmospheric pressure varies. This may be significant if very accruate pressure measurements are

needed of the measurements are made at different locations or elevations. For example, if a

process requies a particular absolute pressure, the gauge pressure reading will b different if the

process is in Denver that if the process is at sea level.

KEO 2.8.c Vacuum gauge pressure is pressure measured with atmospheric pressue as the

zero point on the scale as indicated on the Pressure Vacuum Gauge above. When measuring

vacuum, the units will decrease below the zero indicaton of the gauge into the vacuum range


Pressure Module

reading. Vacuum pressrue measurement is used when a prcess measruement is used when a

process is maintained at less than atmosperic pressure. For example, a vacuum pressure gauge

may be installed on the suction side of a pump to check for a clogged suction line, a dirty

strainer, or a closed suction valve.

KEO 2.8.d Differential Pressure is the difference between two measuement points in a

process. Differential pressue is an important process variable measurement in that is can be used

to do more than just measure pressure. For example, it can be used to measure positive and

negative pressure, flow, liquid level, and liquid density. These other process measurements will

be discussed in their respective process variable modules.

The actual pressure at the different points may not be known and there is no reference pressure

used when measuring differential pressure. Pressure Drop is a pressure decrease that occurs due

to friction or obstructions as an enclosed fluid flows from one point in a process to another . A

pressure drop measurement can significantly improve the measurement resolutoion when

compared to using two gauges or absolute pressure measurement. For example, when air is

filtered in an HVAC system, the air pressure before a filter is higher than the air pressure after a

filter as depicted below:

Figure 3-7 page 95

The pressure drop is very small compared to the absolute pressue, so the pressure drop is

monitored to determine when a filter needs to be cleaned or replaced.

An other application for differential pressure is for a facility needing to maintain the air pressure

to either a positive or negative pressure to control, or to prevent a release of contaminates from

or into the facility. Differential pressure is used to set off an alarm or to close doors to control

the desired necessary facility pressure as required.

KEO 2. 9 CONVERT Pressure Equivalents


Pressure Module

Being able to Convert Pressure from one equivalent to another is required of an Instrumentation

and Control Technician. There are some conversions that should be memorized and others need

only be referenced and know where to be found. Today with the ease of access to the internet,

conversion tables and calculators are readily available. Below is a conversion table with the

basic conversions dealing with pressure equivalents:

Figure 3-8 page 96

Based on the Pressure Equivalents Table, there are three pressure equivalents that need to be

understood as common units and conversions and are used often. They are as follows:

1. 1 PSI = 27.7 Inches of Water

2. 1PSI = 2.036 Inches of Mercury

3. 1 Inch of Mercury = 13.61 Inches of Water

Understanding and knowing these three pressure conversions are essential to many process

control measurements used in industry. Memorizing these units will ensure you have the

necessary knowledge to make correct choices in selecting test equipment to perform critical

calibration tasks associated with measuring and controlling pressure process variables.

KEO 2. 10 EXPLAIN how manometers are used to measure pressure


Pressure Module

Manometers are used to provide a visible pressure measurement and to perform accurate

pressure measurement of pressure sensing devices. Manometers are used as an indicating device

connected to pressure sensing devices with actual pressure being applied to both the pressure

sensing device and the manometer to verify accurate sensing capabilities to the device being

calibrated.

An example of how a typical connection may look is when a manometer is used to check the

calibration of a pressure device is depicted below:


Pressure Module

Manometers are used to measure positive or negative pressure, and differential pressures. They

are used in a process control environment to provide a variety of process measurement functions

such as, pressure, level, and density. Operations personnel use them as a visual indication for

recording and documenting specific process conditions as depicted below (this example indicates

an applied pressure to the solution in a glass tube providing a level reading of the solution being

monitored):

Manometers are indicating devices and cannot be remotely transmitted; however the pressure

being applied to the solution can be detected and transmitted. This concept will be addressed

later on when pressure transmitting devices are discussed.

Manometer Note: When reading manometers, if the solution is mercury the accurate

reading is taken from the top of the mercury (meniscus) as pressure pushes mercury up to

form an upward bubble. When reading other solutions like water the pressure pushes up

the sides of the solution leaving a depression and to get an accurate reading, it needs to be

read from the bottom of that depression (meniscus) on the manometer scale.


Pressure Module

KEO 2. 11 DESCRIBE four types of manometers

a. U-Tube

b. Inclined

c. Well

d. Barometer

KEO 2.11.a U-Tube Manometer is a glass tube bent into the shape of elongated letter U.

Liquid, usually water, alcohol, or mercury is poured into the tube until the level in both columns

is at mid scale, or zero. The scale is adjustable to accommodate an accurate zero reading with no

signal applied to the manometer tube.

In operation, a pressure is applied to one of the columns and the other side is left open to

atmospheric pressure. The level in the higher-pressure side decreases and the level in the lower-

pressure side increases. The difference in height of the two liquid columns is represents the

applied pressure (for example, a 2 inch reading would represent a pressure of 4 inches).

Manometer manufactures offer manometer fluids with a choice of densities. Densities are

expressed as specific gravity and are the ratio of the density of a fluid to the density of a

reference fluid. Water is the usual reference fluid for manometers; however other fluids that may

be used in manometers to measure pressures other than inches of water include mercury or

organic chemicals immiscible with water. The Specific gravity fluids available include: 0.826,

1.000, 1.750, 2.950, and mercury at 13.6. Some manometers use water with a dye instead of

using the special manometer fluid having a specific gravity of 1.000.

A picture of a U-Tube manometer used in a calibration of a pressure sensor is depicted below:


Pressure Module

The picture below depicts how U-Tube manometers are connected and read:

Figure 3-9 page 97

KEO 2.11.b Inclined Manometer is a manometer with a reservoir serving as one end and the

measuring column at an angle to the horizontal to reduce the vertical height. The fill liquid is

usually water and may have a die to improve readability. Like the U-Tube manometer, the

reservoir may also be filled with manufactured liquids having a specific gravity of: 0.826, 1.000,

1.750, 2.950, and mercury at 13.6.

Inclined manometers need to me mounted level to the ground and use a bubble level to level

the manometer. The purpose of the angled tube is to lengthen the scale for easier reading. This

type of manometer is used for low-pressure applications because it is difficult to accurately read

low pressures in a vertical tube. For example, an HVAC system may only have a static pressure


Pressure Module

drop of 0.1 inches of water to 0.2 inches of water. Under these circumstances, it is easier to get

an accurate reading with an inclined-tube manometer over an U-Tube manometer.

Below is a picture of an Inclined Tube Manometer depicting where pressure is applied and a

leveling device to ensure accurate indications of pressure. Notice the scale of the inclined

manometer is a negative .10 inches of water to a positive 1.0 inches of water reading.

Figure 3-10 page 98

The following picture depicts a calibration of a pressure sensor using an incline-tube manometer:


Pressure Module

One problem with inclined-tube manometers is even the smallest collection of condensed water

in an inclined-tube manometer can generate very significant measurement errors. If this

happens, it can be corrected by either changing the zero point, or by applying an additional head

to the reservoir and changing the differential pressure measurement.

When dealing with manometers at this small of a scale, it is important to verify the zero setting

prior to its use to ensure the manometer is level and with no signal applied, the scale is adjusted

to read zero.

KEO 2.11.c A Well-Type Manometer is a manometer with a vertical glass tube connected to

a metal well, with the measuring liquid in the well at the same level as the zero point on the tube

scale. The well-type manometer is the most common type of manometer used. With three of

them mounted on a cart on wheels, they can be set up with different solutions like a an organic

solution of 0.826 specific gravity, a solution of 1.000 specific gravity, and mercury with a 13.6

specific gravity. A typical Well-Type manometer is depicted below:

Figure 3-11 page 98


Pressure Module

The picture below depicts a well-type manometer with a fluid specific gravity of 1.000 (colored

blue water) used in the calibration of a pressure sensor.

Using Mercury in any manometer increases the range of measurement over using just water.

Mercury is 13.6 times heaver than water and a manometer 60 inches tall with water can only

measure 60 inches of water (2 PSI). Whereas manometers using mercury that is 60 inches tall,

can measure up to 816 inches of water or a pressure of up to 30 PSI.

The disadvantage with using mercury is the environmental hazard of mercury vapor if a mercury

spill were to occur.

KEO 2.11.d Barometer is a manometer used to measure atmospheric pressure. Barometric

Pressure is a pressure reading made with a barometer. The earliest barometer was a long vertical

glass tube that had been sealed at the bottom and filled with mercury. The open end was then

turned upside down into a container of mercury without allowing any air into the tube. The

mercury in the tube falls to a level where the head of the mercury is equal to the atmospheric

pressure.


Pressure Module

As atmospheric pressure changes, the level of the mercury changes as well. The following

picture depicts the earliest barometer and a pressure equivalent table:

Figure 3-12 page 99

Currently mechanical instruments that sense atmospheric pressure with electronic circuitry that

can produce digital readouts for remote readings have replaced the earlier mercury barometer

and are called Aneroid Barometers.


Pressure Module

KEO 2. 12 EXPLAIN how a mechanical pressure diaphragm device detects and measures

pressure.

Mechanical Pressure sensors use diaphragms to detect and measure pressure. The diaphragm

flexes in response to an applied pressure. This flexing motion moves a pointer on a scale. The

following picture depicts a typical diaphragm device with a cut away view of its internal

components:



Pressure Module

A more common diaphragm mechanical device is a Standard Magnehelic Gauge as depicted

below:

A Magnehelic gauge consists of two pressure tight compartments separated by a molded flexible

diaphragm. The interior of the case serves as the “High” pressure and a sealed chamber behind

the diaphragm serves and the “Low” pressure compartment. Differences in pressure cause the

diaphragm to assume a balanced position between the two pressures. The front of the interior

diaphragm is linked to a leaf spring to detect motion. The motion is detected through an

exclusive magnetic linkage to the indicator pointer. Mechanical pressure devices can activate

alarms and provide signals that can be transmitted for remote operation and control of process

pressures.

KEO 2. 13 EXPLAIN how a mechanical pressure capsule device detects and measures

pressure.

A Mechanical Pressure Capsule device is a mechanical pressure sensor consisting of two

convoluted metal diaphragms with their outer edges welded, brazed, or soldered to provide an

empty chamber. One of the diaphragms is connected at its center to metal tubing to admit fluid to

the chamber. The other diaphragm is fitted with a mechanical connection to the indicator or

fitted with a transducer to transmit the pressure signal.


Pressure Module

A pressure capsule device is depicted below:


KEO 2. 14 EXPLAIN how a mechanical pressure spring device detects and measures

pressure.

A Mechanical Pressure Spring device is hollow tube formed in to a helical, spiral, or C shape.

The bourdon tube is the original pressure C shaped spring that is flattened into an elliptical cross

section. All of the pressure spring devices move with pressure applied and this movement is

captured by a pointing device, switch, or transducer providing a local or remote indication.


Pressure Module

Below are pictures of typical pressure spring sensing elements that indicate or transmit a pressure

reading:

Sprial-Shape C-Shape Bourden Tube

Helical-Shape


Pressure Module

Below are two more examples of bourdon tube devices:

Figure 3-15 page 102 Figure 3-16 page 102

KEO 2. 15 EXPLAIN how a mechanical pressure bellows device detects and measures

pressure.

Bellows-Pressure sensing devices are elastic deformation elements, that flex (twist or expand)

with changes in pressure. The movement is transferred via linkage to indicate or to transmit a

pressure signal remotely. Below are three pictures of typical bellows pressure sensors:



Pressure Module


KEO 2. 16 EXPLAIN how a mechanical pressure double-ended piston device detects and

measures pressure.

A Double Ended Piston is a mechanical pressure sensor consisting of a differential pressure

sensor gauge with a piston that admits pressurized fluid at each end. The piston motion that

results from the in-equality of the pressures is opposed by an internal spring that establishes the

range of the meter. The piston is magnetically coupled to a pointer assembly.


Pressure Module

A double-ended piston is used to measure pressure by balancing the force from the pressure on

the piston with the force needed to compress a spring. A double-ended piston is depicted below:


KEO 2. 17 EXPLAIN how an electrical transducer works

Electrical Transducers are devices that convert an input electrical (40-20 mA-DC) energy into

a different mechanical energy such as pneumatic. An example would be a current to pneumatic

(I/P) transducer. A 4-20 mA electrical input allows a 3-15 PSI pneumatic output to leave the

transducer for indication or control of pneumatic devices.


Pressure Module

Below is a typical I/P transducer:

Pneumatic-Side View

Electronic-Side View


Pressure Module

Cautionary Note: Transducers that are current to pneumatic receive a 4-20 mA-AC signal input

to convert to a 3-15 PSI output. A common mistake is to apply a 24VAC power supply to the

input instead of a 4-20 mA-AC signal. When this is done, damage occurs to the transducer.

Be sure not to apply a power supply to the transducer. Some test equipment can supply

either a mA signal or a voltage source and caution needs to be taken to prevent this damage via

test equipment.

A pressure transmitter is also called a pressure transducer that receives a physical pressure input

and provides an electronic output, such as a 4-20 mA output signal. Pressure transmitters

generally require a 24 VAC power source. Below is a Rosemont pressure transmitter:


Pressure Module

KEO 2. 18 EXPLAIN how a resistance pressure strain gauge transducer works.

A Resistance Pressure Transducer is a diaphragm sensor with a strain gauge as the electrical

output element. Resistance pressure transducers are the most widely used electrical pressure

transducers. A strain gauge is an electrical transducer that measures the deformation, or strain,

of a rigid body as a result of the force applied to the body. The picture below depicts a typical

Strain Gauge:


A strain gauge uses a bridge circuit with a variable resistor that measures the deformation or

strain on the sensor providing a pressure measurement to be transmitted.

KEO 2. 19 EXPLAIN how a capacitance pressure transducer works.

A Capacitance Pressure Transducer/transmitter is a diaphragm sensor with a capacitor as the

electrical element. When pressure distorts the diaphragm it alters the distance between the

plates, the capacitance of the sensor changes.


Pressure Module

Below is a picture depicting the functionality of a capacitance pressure transducer:


A common capacitive pressure transducer/transmitter is the Rosemount Differential Pressure

Transmitter.


Pressure Module

The following picture is of a typical Rosemount DP Transmitter being calibrated and powered

by an external AC power supply with the DVM placed in series with the power supply to

measure the 4-20 mA current output:

Below are pictures of the capacitance pressure diaphragm used with the Rosemount DP

Transmitter:


Pressure Module


KEO 2. 20 EXPLAIN how a reluctance pressure transducer works


Pressure Module

A Reluctance Pressure Transducer is a diaphragm pressure sensor with a metal diaphragm

mounted between two stainless steel blocks. Reluctance is the property of an electric circuit that

opposes a magnetic flux. Embedded in each block is a magnetic core and coil assembly with a

gap between the diaphragm and the core.

The blocks have pressure ports and passages for the fluid media to exert pressure against the

diaphragm. The movement of the diaphragm increases the gap on one side of the diaphragm and

decreases the gap on the other side to vary the magnetic reluctance. This variation is

proportional to the change in applied pressure and produces a signal used in a bridge circuit.

The following picture is of a Reluctance Pressure Transducer showing its mechanical structure

and bridge circuit:



Pressure Module

Reluctance transducers have a relatively high output for a small change in pressure. The input

voltage is 5 VAC with a frequency of approximately 5 kHz and a change in range can be

accomplished by changing the diaphragm.

KEO 2. 21 EXPLAIN how a piezoelectric pressure transducer works

A Piezoelectric Pressure Transducer is a diaphragm sensor combined with a crystalline

material that is sensitive to mechanical stress in the form of pressure. This type of transducer

produces an electrical output proportional to the pressure on the diaphragm. This transducer

does not need an external power source. As the crystal is compressed, a small electric

potential is developed across the crystal.

This potential produced by the crustal is then amplified and conditioned to be proportional to the

applied pressure. Temperature compensation is often included as part of the circuitry. The

following picture shows the mechanical structure of the Piezoelectric Pressure Transducer:


Piezoelectric Pressure Transducers are not appropriate for measuring static pressures because the

signal decays rapidly. They are used to measure rapidly changing pressure that results from

explosions, pressure pulsations, or others sources of shock, vibration, or sudden pressure change.


Pressure Module

KEO 2. 22 EXPLAIN how a differential pressure (d/p) cell transmitter/transducer works.

Differential Pressure (d/p) Cell Transmitter/Transducers convert a differential pressure to an

output signal. It is a device that sends the output to another location where the signal is used for

recording, indicating, or control. Two types of d/p Cell Transducers are Pneumatic and

Electronic. Pneumatic operate with a compressed plant supply and provides an output signal of

3-15 PSI. Electronic d/p Cell Transducers operate with an input power supply of 24 VAC and

provide an output signal of 4-20 mA or 10-50 mV. The earlier versions of these types of devices

used a force balance bar attached to a metallic diaphragm to generate the output signal. Below

are examples of both a pneumatic and an electronic force balance d/p cell transducers:

Pneumatic Transmitter Electronic Transmitter

These transmitters are still in service in process plants world-wide. With the advancement of

technology, transmitters are now smaller and the only moving device is the diaphragm, which

changes electronic properties to supply the output of a 4-20 mA signal when pressure is applied

to one side or the other.


Pressure Module

Examples of older pneumatic d/p cell devices are shown below:


Pressure Module

Example of a more modern electronic d/p cell transmitter is shown below:

The Rosemont transmitter uses a capacitance cell to generate its 4-20 mA output signal:


Pressure Module

Below is an expanded view of the Rosemont d/p Cell Transmitter:

Like any of today’s D/P Cell Transmitters, they are all capable of measuring differential pressure

or both positive and negative pressures the same as the earlier version of the force-balanced

pneumatic and electronic devices. The value and benefit to today’s transmitters are the stability

and freedom of moving parts and is the reason the pneumatic transmitters are being replaced

with the more modern and stable transmitters. There are, however many facilities still using older

devices that are still functional.


Pressure Module

Differential pressure devices have two ports: 1) Low Pressure and, 2) High Pressure. The

following examples show how Differential Pressure transmitters are connected to measure

positive and negative pressures (Pressure and Vacuum):


Pressure Module

KEO 2. 23 EXPLAIN how to correctly use manometers to measure pressure dealing with:

a. Moisture Condensation

b. Measuring Liquids

c. Using Valve Manifolds

KEO 2.23.a Moisture Condensation can result in a collection of water in the manometer fluid

and increase the volume of fluid. If the fluid in the manometer is water, this increase in fluid

will change the manometer zero setting and will have to be reset or fluid will have to be removed

from the manometer to maintain an accurate reading.

If the fluid is a fluid heavier than water, the condensed on top of the manometer fluid causing an

error in the reading. A correct reading can be obtained by measuring differential of the

manometer fluid only, converting this to inches of water, and then subtracting this from the

reading.

The below picture shows how you can deal with correction of condensation in a U-Tube

Manometer:


If there is a possibility of condensed water collecting in a manometer, the connecting piping or

tubing should include a condensate collection pot to intercept the condensed water before it

reaches the manometer.


Pressure Module

KEO 2.23.b Measuring Liquids requires the process side of the manometer and the

connecting piping or tubing to be filled with the process fluid to provide a consistent liquid head

pressure at all times.

The following picture depicts how to compensate for a wet leg of water to the manometer when

using mercury as the manometer fluid show that using water in one leg must have a pressure

adjustment made:


KEO 2.23.c Using Valve Manifolds is critical in maintaining the manometer fluid. A

procedure of cutting in the manometer is essential and if not done properly, the manometer fluid

can be removed from the manometer and forced into the process being measured.


Pressure Module

The following picture depicts how a manometer can be cut in using a valve manifold system:


Cutting out a manometer is the exactly the opposite of cutting in by reversing the steps above.

KEO 2. 24 EXPLAIN methods used to protect pressure gauges and sensors from:

a. Over Pressure

b. Over Temperature

c. Corrosion or Contamination


Pressure Module

KEO 2.24.a Over-Pressure Protection is required to protect sensing equipment from the

starting and stopping of pumps, opening and closing of valves, vibrations in piping, and

unexpected increase or decrease in pressures. Pressure limiting valves are available to protect

equipment. Pulsation dampers (snubbers) are also available to install in inlet lines to protect

against pulsation damage. Examples of devices to protect against over pressure are shown

below:



Pressure Module

KEO 2.24.b Over-Temperature can damage pressure sensors. Adding enough inlet tubing

allowing the process fluid to cool before entering the sensor is standard installation practice. A

siphon system is also used to protect equipment. The below picture shows how siphons can be

used to protect sensors from over temperature:



Pressure Module

KEO 2.24.c Corrosion or Contamination protection is essential as dealing with processes

that are corrosive or contaminated is more of a reality than a norm. There are sealing systems

that keep the process isolated from the sensor. The photo below shows a typical sealing system

that is used:



Pressure Module

Air and water are more often used to isolate the sensor from the corrosive or contaminated

processes. The photos below depict how this is done:




Pressure Module

KEO 2. 25 DESCRIBE what a Deadweight Tester is and how it is used to calibrate pressure

sensors

Dead Weight Testers are devices using hydraulic fluid to develop a pressure to a set of

calibrated weights. When the weights lift and rotate freely, that pressure is equal to the weights

and the pressure is applied to a pressure sensor to verify calibration. Below are pictures of both a

high pressure (HP) and low pressure (LP) dead weight pressure testers set up calibrating pressure

gauges:

HP Tester

LP Tester


Pressure Module

KEO 2. 26 EXPLAIN how manometers are used to calibrate pressure sensors and the

limitations associated with using manometers.

Manometers are used as an indicating device in the calibration process. A pressure source is

connected to both the manometer and the pressure sensor. Pressure is then applied to both the

manometer and the device to complete the calibration process. Limitations to using manometers

are availability, maintaining fluid levels, cutting them in and out and dealing with mercury as a

hazard. The pictures below show calibration tasks being performed with manometers:

Inclined-Tube Manometer used to calibrate a Magnehelic Pressure Gauge. A hand

held pump or a pressure regulated air supply is used at a “Tee” fitting to supply the

pressure source to the Magnehelic gauge and the inclined manometer.


Pressure Module

U-Tube Manometer used to calibrate a Magnehelic Pressure Gauge. A hand held

pump or a pressure regulated air supply is used at a “Tee” fitting to supply the

pressure source.


Pressure Module

Below is an example of a more modern method using a pneumatic calibration box called a

Wallace and Tiernan Calibration Box (Notice this calibration box is equipped with a valve

manifold to cut in or out pressure):


Pressure Module


a 4-20 mA pressure transmitter

The FLUKE 744 calibrator is a high end device that can provide both a pressure input signal and

a mA output signal. This is accomplished via a source pressure display function that requires the

use of an external pressure hand pump and an external Pressure Module. What the FLUKE

calibrator and similar calibrators can do is to provide the option of seeing both the source signal

and the signal from the output of the pressure transmitter/transducer. Connecting the Pressure

Module and setting up the FLUKE 744 calibrator is depicted below:


Pressure Module

The steps for setting up the FLUKE 744 calibrator to use the Sourcing Pressure functionality are

as follows:


Pressure Module

Below are pictures of a calibration set up to show dual output signal and input pressure source

applied to transmitter and the FLUKE 744 Calibrator;

FLUKE 744 Calibration With Pressure Module


Pressure Module

FLUKE 744 Calibration With Pressure Module (Close-Up)


Pressure Module


a Current to Pneumatic (I/P) transducer.

I/P Transducers can be damaged if connected improperly. Applying a 24 VDC power source to

an I/P transducer will damage it. When connecting an I/P Transducer for calibration, be sure to

connect it to a device that will supply a 4-20 mADC signal and place this source in series with

the I/P Transducer. The below picture shows a typical calibration of and I/P Transducer using a

FLULE 744 calibrator as its 4-20 mADC source (Notice the 24 VDC power source in the back

ground above the transducer is not connected to the instrument being calibrated):


Pressure Module

STEP TWO

Pressure Measurement Course

Skill/Performance Objectives

Skill Knowledge Introduction:

Below are the skill knowledge objectives. How these objectives are performed depend on

equipment and laboratory resources available. With each skill objective it is assumed that a set

of standard test equipment and tools be provided.

For example, to be able to perform pressure calibration tasks, the following tools and equipment

will be required:

1. A pressure source such as a regulator, pneumatic calibration box, hand pump, etc.

2. A calibration standard to measure the applied pressure like a manometer, gauge or meter

3. Equipment capable of measuring pressure such as a gauge, transducer, transmitter,

switch, etc.

4. A measuring device capable of measuring / indicating the output signal such as pressure,

voltage, and current

5. An appropriate power supply to power the equipment being calibrated

Skill Terminal Objective (STO)

STO 2. Given a Pressure Measurement Task Checklist, under the direction of an instructor,

complete a series of tasks using calibration equipment, pressure indicating devices,

and pressure transmitting devices to demonstrate mastery of both knowledge and skill

objectives associated with the measurement of pressure.


Pressure Module

Skill Enabling Objectives (SEO)

SEO 2. 1. Calibrate a pressure sensor using a Pneumatic Pressure Calibrator (Wallace &

Tiernan Box)

SEO 2. 2. Calibrate a pressure sensor using a Low Pressure Dead Weight Tester


Pressure Module

SEO 2. 3. Calibrate a pressure sensor using a High Pressure Dead Weight Tester

SEO 2. 4. Calibrate a pressure sensor using a Hand Pump Pressure Source


Pressure Module

SEO 2. 5. Calibrate a pressure sensor using a Ralston or Rosemount 0-200 PSI Hand Pressure

Source

SEO 2. 6. Calibrate a pressure sensor using a Calibration Gauge with regulator and building air

supply


Pressure Module

SEO 2. 7. Calibrate a pressure sensor using a U-Tube Manometer

SEO 2. 8. Calibrate a pressure sensor using an Incline Manometer


Pressure Module

SEO 2. 9. Calibrate a pressure sensor using a Well-Type Manometer

SEO 2. 10. Calibrate a pressure sensor using a Fluke Pressure PV350 Calibrator


Pressure Module

SEO 2. 11. Calibrate a pressure sensor using a Transformation & Manometer


Pressure Module

SEO 2. 12. Calibrate a pressure sensor using a Crystal Calibrator

SEO 2. 13. Calibrate a pressure sensor using a Marsh Calibrator


Pressure Module

SEO 2. 14. Calibrate Three different Pressure Switches

SEO 2. 15. Calibrate Three Pressure Gauges


Pressure Module


Pressure Module

SEO 2. 16. Calibrate a Foxboro 13 or 15 Pneumatic Transmitter

SEO 2. 17. Calibrate a Rosemont 1151 Transmitter

SEO 2. 18. Calibrate a Capacity Tank Circuit


Pressure Module

SEO 2. 19. Disassemble and Reassemble a pressure regulator

SEO 2. 20. Calibrate a pressure sensor using a Foxboro Current Source (Black or Green Box)


Pressure Module


Pressure Module

SEO 2. 21. Calibrate a pressure sensor using a Fluke Multi Processor Function Calibrator

SEO 2. 22. Calibrate a pressure sensor using a Thermo Electric Current Source


Pressure Module

SEO 2. 23. Calibrate a Rosemount I/P Transducer

SEO 2. 24. Calibrate a Moore I/P Transducer


Pressure Module

SEO 2. 25. Calibrate a Fisher I/P Transducer


Pressure Module

module 8 pressure measurement

Documents

measuring pressure

negative pressure measurement

fluidliquid pressure

atmospheric pressure

head pressure keo

hydrostatic pressure

mechanical pressure

common pressure scales