Module # 8 Pressure Measurement

Energy Systems Engineering Technology

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 Page 1


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



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



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



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.

KEO 2. 28 EXPLAIN how to connect a FLUKE model 744 Electronic Calibrator to calibrate a Current to Pneumatic (I/P) transducer.



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

= Force ÷ Area (P=F÷A)

= Pressure × Area (F=P×A)

= Force ÷ Pressure (A=F÷P)

As a comparison, the formula for Ohms law is:

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

Resistance =

Current =

Voltage ÷ Current (R=V÷I)

Voltage ÷ Resistance (I=V÷R)



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.



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:


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.



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:


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.



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:


The hydraulic press operations depicted above is the principle used for dead weight testers used to calibrate pressure sensing devices.



Depicted below is a Dead Weight Tester:


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.



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:


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.



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.



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


Energy Systems Engineering Technology 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:


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



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:


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



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:



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.



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 lowerpressure 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:



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


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


Energy Systems Engineering Technology 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.


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



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:

Pressure Module


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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.



As atmospheric pressure changes, the level of the mercury changes as well. The following picture depicts the earliest barometer and a pressure equivalent table:


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.



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:

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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.



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.



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

Sprial-Shape C-Shape Bourden Tube

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Helical-Shape

Page 28


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:

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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.



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.



Below is a typical I/P transducer:

Pneumatic-Side View

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Electronic-Side View

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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:



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.



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


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

Transmitter .



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:




KEO 2. 20 EXPLAIN how a reluctance pressure transducer works



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:

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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.



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.



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



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:



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.



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):



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.



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.



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



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:

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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:

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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:

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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:


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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

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LP Tester

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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.



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.



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):



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



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



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)

Page 59


KEO 2. 28 EXPLAIN how to connect a FLUKE model 744 Electronic Calibrator to calibrate 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):



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.



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



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



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



SEO 2. 7.

Calibrate a pressure sensor using a U-Tube Manometer

SEO 2. 8.

Calibrate a pressure sensor using an Incline Manometer



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



SEO 2. 11.

Calibrate a pressure sensor using a Transformation & Manometer



SEO 2. 12.

Calibrate a pressure sensor using a Crystal Calibrator

SEO 2. 13.

Calibrate a pressure sensor using a Marsh Calibrator



SEO 2. 14.

Calibrate Three different Pressure Switches

SEO 2. 15.

Calibrate Three Pressure Gauges





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



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)





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



SEO 2. 23.

Calibrate a Rosemount I/P Transducer

SEO 2. 24.

Calibrate a Moore I/P Transducer



SEO 2. 25.

Calibrate a Fisher I/P Transducer




Module # 8 Pressure Measurement

STEP ONE

Knowledge Terminal Objective (KTO)

Knowledge Enabling Objectives (KEO)

PRESSURE MEASURMENT

STEP TWO

Pressure Measurement Course

Skill/Performance Objectives

Skill Terminal Objective (STO)

Skill Enabling Objectives (SEO)

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Module # 8 Pressure Measurement

STEP ONE

Knowledge Terminal Objective (KTO)

Knowledge Enabling Objectives (KEO)

PRESSURE MEASURMENT

STEP TWO

Pressure Measurement Course

Skill/Performance Objectives

Skill Terminal Objective (STO)

Skill Enabling Objectives (SEO)

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