One thing leads to another, yeah, yeah, yeah
One thing (one thing) leads to another

("One Thing Leads to Another," The Fixx, 1982)

 

THE INTRODUCTION TO PRESSURE TRANSDUCERS IS A TRIP DOWN PTOA MEMORY LANE

Pressure Transducers/Transmitters are here, there, and everywhere in an industrial processing facility. That fact is because the PV Pressure is related to the PV Flowrate and the PV Level ... not to mention that sometimes the plain, vanilla PV Pressure needs to be monitored and controlled.

Understanding how a Pressure Transducer/Transmitter works requires a trip down PTOA memory lane to review Electrical Resistance.

Brilliant PTOA Readers and Students learned about the role that Electrical Resistance plays in Ohm's Law way back in PTOA Segment #106.

PTOA Readers and Students learned in PTOA Segment #113 that "(Electrical) Resistance is NOT Futile" because the Electrical Resistance output in ohms from RTDs is used to monitor and control the PV Temperature.

PTOA Readers and Students should take this opportunity to access the links to those PTOA Segments cited above because the information will help with understanding the function of Pressure Transducers. The generation of Electrical Resistance plays a major role in electronic Pressure Transducers/Pressure Transmitters.

Brilliant PTOA Readers and Students more recently learned in PTOA Segment #227 that the 3 most common PV Pressure sensing elements are the Bellows, the Diaphragm and the Bourdon Tube.

All Pressure Transducers/Pressure Transmitters will incorporate one of these primary Pressure Sensors.

 

UNDERSTANDING THE TRANSDUCTION AND CONVERSION FUNCTIONS
THAT OCCUR WITHIN A PRESSURE TRANSDUCER/TRANSMITTER

Energy "Transduction Happens" when one form of energy is changed into a completely different form of Energy.

Just very recently, brilliant PTOA Readers and Students learned in PTOA Segment #229 that these common PV Pressure Sensors are incorporated into Pressure Switches that can be used for On-Off Control (aka "Discrete Control").

Although it wasn't pointed out at the time, the magnitude of the Pressure Sensor's deflection (aka a form of mechanical energy) was transduced into a proportional magnitude of Electrical Resistance output. The Electrical Resistance output in ohms was hence converted into an electronic signal that could be transmitted through a wire for On-Off Control.

The component(s) within the Pressure Switch that changed the mechanical energy input (the deflection of the Pressure Sensor) into a proportional Electrical Resistance output in ohms is(are) called a Pressure Transducer.

Any device that changes energy from one form into a totally different form of energy is a Transducer.

In Pressure Transducers, the magnitude of the Pressure Sensor's deflection is transduced into a proportional electrical output ...typically in the form of Electrical Resistance (units are ohms).

 

Energy "Conversion Happens" when one form of electrical energy changes into a different form of electrical energy.

Truth be known, ohms aren't so helpful for transmitting a signal. There is a second function of a Pressure Transducer that makes it more useful in process industry.

The Electrical Resistance which is transduced from the deflection of the Pressure Sensor is converted into a different expression of electrical energy ... milliamperes (mAs), volts (V).... their digital equivalents ... and even frequency. These forms of electrical energy can be used to send information over long distances to a variety of automatic control instruments.

A device that changes one form of electrical energy into another form of electrical energy is called a Converter.

Uh-oh! Fred does not understand the difference between a Transducer and a Converter!

Fred ...

Consider the Pressure Switch that was recently focused on; the schematic of the Pressure Switch is nearby.

The PV Pressure was sensed and measured by a Diaphragm.

The deflection movement of the Diaphragm is transduced into an Electrical Resistance.  How the change in PV Pressure is related to the generation of an Electrical Resistance is sort of explained later in this PTOA Segment.

Then the Electrical Resistance was converted by the microswitch into the electrical signal (like mAs) so that the signal could be sent through wires to another device that helps with the On-Off circuit control of the PV Pressure.

In summary, the PTOA Department of Redundancy Department reiterates:

All Pressure Transducers will have components which perform the following functions:

• A component that transduces the movement of the Pressure Sensor into an electrical signal output (aka, typically Electrical Resistance) and ...
• A component that converts the electrical signal output (aka, Electrical Resistance) into the desired electrical output format needed for the desired automatic monitoring or control scheme.

 

Why a Pressure Transducer is often called a Pressure Transmitter

When the electrical output from a Pressure Transducer needs to be sent over a long distance, the Pressure Transducer is typically called a Pressure Transmitter,

 

Why?

 

Because the Electrical Resistance that is transduced from the deflection of the Pressure Sensor will be converted into a 4-20 mA signal, voltage signal (or their digital equivalents) which can be transmitted over long distances and used for monitoring or proportional control of the PV Pressure.  The transduction function of the device is typically overlooked. The fact that the device transmits an analog signal dominates in modern automatic instrumentation lingo, thus the device is commonly called a Pressure Transmitter.

 

When the distance between the Electrical Resistance output of the Pressure Transducer and the device that will utilize the output is short, then the Transducer maintains the descriptor "Transducer."

 

PRESSURE TRANSDUCER/PRESSURE TRANSMITTER SERVICE SELECTION

PTOA Readers and Students will soon learn about the most prevalent Pressure Transducers used in process industry. The service selection of a Pressure Transducer/Pressure Transmitter depends upon many criteria, including:

1. The range and type of PV Pressure that needs to be sensed, measured, monitored, and controlled. For example, is it an Absolute PV Pressure (units=psia)? A Differential Pressure (units=psi)?
2. The Responsivity of the Pressure Transducer must be considered. Responsivity refers to the time interval observed between the deflection of the Pressure Sensor and the completion of the transduction function of the Pressure Transducer.  Strain Gauge Pressure Transducers/Pressure Transmitters are much more responsive than Capacitance Pressure Transducers. Potentiometric Pressure Transducers have the slowest comparative response time.
3. The type of electrical output signal needed for the application (volts, mAs, digital equivalents, etc). Which leads to considering ...
4. The distance between the Pressure Transducer and the device that will eventually receive the electrical signal output that is transduced from the movement of the Pressure Sensor.  As was mentioned above, a Pressure Transducer is the optimal choice for short distances whereas a (smart) Pressure Transducer/Transmitter is optimal when sending a 4-20 mA signal over a longer distance.
5. The anticipated electromagnetic interference (EMI) in the application environment must be evaluated and mitigated. For example, welding or power lines or nearby motors can cause electrical interference (aka "noise"). Shielding wire and proper grounding and attention to installation techniques will help reduce EMI.
6. The overall cost of the system during the design phase. A Pressure Transducer with a millivolt output might appear cost effective on paper; however, the signal is so weak that it is easily interfered with by external EMI.

 

The above paragraphs infer that each Pressure Transducer has an optimal range of PV Pressure that can be measured, transduced, converted (and perhaps transmitted) with accuracy.

All Pressure Transducer models operate best between 50%-60% of their maximum rated PV Pressure.

For example, a Pressure Transducer expected to operate in the typical range of 2000-3000 psi should be rated for 5000 psi (because 3000/0.60 = 5000).

All types of Pressure Transducers are impacted by the ambient Temperature that surrounds them. The majority of Pressure Transducers are manufactured to be functionally accurate within a specified ambient Temperature range.

The Relative Humidity of the ambient environment (featured in PTOA Segment #77) will interfere with PV Pressure measurements if condensation in the instrument occurs. Excessive vibration will also result in component failure.

 

THE OPERATING PRINCIPLE OF PRESSURE TRANSDUCERS IS SORT-OF CLARIFIED

Your Mentor could launch into a boring string of paragraphs to explain the below highlighted statements ...

or ...

PTOA Readers and Students could just except that the Universe has once again whispered into the ear of Humankind and shared the secret of the Changing Pressure/Changing Electrical Resistance relationship:

Changes in Pressure cause a change in Electrical Resistance. The amount of Electrical Resistance observed depends upon the materials from which the "Resistance Element" is fabricated. The Changing Pressure/Changing Electrical Resistance relationship is reversible ... less Pressure sensed, less Electrical Resistance generated. NOTE: The Pressure may be expressed as Strain.

The $100 dollar word for the reversible Changing Pressure/Changing Electrical Resistance relationship/phenomenon is "piezo resistivity." Any PTOA Reader or Student who does not want to accept this latest edict from the Universe at face value is welcome to perform their own deep dive into Wonkyville.

The rest of us are happy to accept that a proportional change in the Electrical Resistance of a substance can be generated by a change in sensed and measure Pressure, including strain-induced Pressure. This law of the Universe provides the basic operating principle of modern, smart Pressure Transducers.

The next PTOA Segment describes the form and function of the types of Pressure Transducers/Pressure Transmitters that are prevalently used in Process Industry:

• The Strain Gauge Pressure Transducer/Transmitter.
• The Capacitance Pressure Transducer/Transmitter.
• The Potentiometric Pressure Transducer

 

TAKE HOME MESSAGES: Pressure Transducers are the most prevalent instrumentation in a processing facility, even more numerous than pumps. That is because the PV Pressure is can also be used to determine the PV Flowrate and the PV Level as well as the PV Pressure.

Transduction means changing from one form of energy to a completely different type of energy. Pressure Transducers transduce mechanical movement/deflection of a Pressure Sensor into Electrical Resistance.

Conversion means changing from one type of energy (typically electric) to another form of the same energy (typically electric).

Every Pressure Transducer incorporates a mechanical energy-to-electrical energy Transduction function and an electrical energy Conversion function.

There is a proportional and reversable relationship between Pressure (often in the form of Strain) and the generation of Electrical Resistance. This Pressure/Electrical Resistance relationship is the underlying operating principle of all types of Pressure Transducers. The Pressure Transducer output of ohms is thence converted to a more useful form of electricity, typically 4-20 mA or digital equivalent, or volts.

The selection of a Pressure Transducer depends on many factors which are listed in this PTOA Segment.

All Pressure Transducers should be operated between 50-60% of their maximum rated Pressure.

All Pressure Transducers are impacted by ambient Temperature and Relative Humidity, Vibration, and EMI in the vicinity of installation. Installation techniques must mitigate all of these environmental concerns. 

The Responsivity of a Pressure Transducer/Transmitter refers to the time interval between the PV Pressure measurement of the Pressure Sensor and completing the transduction function which yields the Electrical Resistance output.   

 

©2022 PTOA Segment 0230

PTOA PV PRESSURE STUDY AREA
PV PRESSURE INSTRUMENTATION

 

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