The way you do the things you do
I like the way you do the things you do
The way you do the things you do
I like the way you do the things you do

("The Way You Do the Things You Do," by Smokey Robinson and Bobby Rogers but made famous by The Temptations, 1964)

 

TYPES OF PRESSURE TRANSDUCERS

Form and Function of Strain Gauge Pressure Transducers

How many PTOA Readers and Students often feel strained? Fred sometimes feels like he is being twisted longward ... making him feel all tensed-up ... just to contract back to where he was, and then the cycle repeats over and over again.

The wires in a Strain Gauge are likewise repeatedly stretched and contracted.

A Strain Gauge is a very fine wire that will reversibly change its Electrical Resistance when stretched and then relaxed. Nowadays the wire is made out of a semiconducting material (semiconductors were explored in PTOA Segment #116 and should be reviewed at this juncture.)

Imagine that the single Strain Gauge shown in the nearby graphic is fabricated from semiconductor material and physically attached to the surface of a very flexible Diaphragm. In this case, the Diaphragm IS NOT made of metal. In order to increase Responsivity of the Pressure Transducer, the Diaphragm will be made of a flexible material, like Silicon.

When the Force Factor of the PV Pressure being sensed impacts the Diaphragm, the Strain Gauge is stretched (middle picture), and its Electrical Resistance increases. Alternatively, when the Force Factor of the PV Pressure decreases, the Strain Gauge is compressed (right side picture) and its Electrical Resistance decreases.

The photo below shows an actual Strain Gauge.

 

The cutaway graphic below shows where the Strain Gauge would be located within the Strain Gauge Pressure Transducer.

The Electrical Resistance output from the Strain Gauge would not be useful when the goal is to monitor and control the PV Pressure real-time in the process industries.

The actual manufacturing process of a Strain Gauge Pressure Transducers includes what is referred to as a "sputtering process" which integrates a Wheatstone Bridge Circuit into the flexible Diaphragm's surface. An excitation voltage is applied. When the Force Factor of the PV Pressure impacts the flexible Diaphragm, the directly proportional unbalanced strains on the Wheatstone Bridge Circuit are converted into a more robust voltage output.

PTOA Readers and Students already know how a Wheatstone Bridge Circuit works and how the technology is used to convert an Electrical Resistance into a more robust voltage measurement. This topic was featured in PTOA Segment #114 and PTOA Segment #115. PTOA Readers and Student must stop and reread these PTOA Segments at this juncture because the content is directly applicable to the form and function of the Strain Gauge Pressure Transducer.

Recall that the RTD used for accurate PV Temperature measurement utilized a 4-leg Wheatstone Bridge Circuit which compensated for ambient Temperature changes. The Wheatstone Bridge Circuit used in Pressure Transducers/Transmitters is likewise a 4-leg architecture.

The nearby graphic features a Futek Full Bridge Strain Gauge Circuit. PTOA Readers and Students can observe how the voltage output varies from the Wheatstone Bridge Circuit that has been incorporated into the Futek Pressure Transducer. Access the Futek Strain Gauge Bridge Circuit animation HERE.

When the PV Pressure is at the desired Set Point, the circuit is balanced around 350 ohms and the net voltage read from the voltmeter is 0. Otherwise, the voltage shown on the voltmeter increases and decreases, indicating an increase or decrease in the PV Pressure sensed by the flexible Diaphragm which has stretched or compressed the Strain Gauge, respectively.

The PTOA does not advocate for any particular brand of instrument. That being understood, the Rosemont Pressure Transducer/Transmitter is by far the most popular in USA process industry. The Rosemont 3051 can be modified for many applications in Process Industry. Rosemont has since been acquired by Emerson.

If the Low-Pressure port of the Pressure Transducer remains open to atmosphere, the device measures/transduces/transmits a Gauge Pressure (psig). If the Low-Pressure Port is sensing a lower process PV Pressure, the Pressure Transducer measures/transduces/transmits a Differential Pressure (psi).  If the Low-Pressure port is exposed to a hermetically sealed vacuum Pressure, the Pressure Transducer measures/transduces/transmits an Absolute Pressure (psia).

The popularity of Strain Gauge Pressure Transducers is explained by their wide range of PV Pressure detection and measurement ... from 3 inches of water Vacuum Pressure to the maximum PV Pressure that will be encountered in an industrial processing facility. Strain Gauge Pressure Transducers work best in Differential Pressure applications and where the PV Pressure to be measured has a narrow span. Strain Gauges fabricated from bonded foil are a better choice when the process service has a probability of over-pressuring.

The Strain Gauge Pressure Transducer is extremely accurate. Some Rosemont models claim to have 0% error. Of course, all brilliant PTOA Readers and Students completely understand that such a claim of accuracy is inferring that a robust linear relationship exists between the PV Pressure sensed by the Diaphragm and the transduced Electrical Resistance output AND that a second robust linear relationship exists between the transduced Electrical Resistance and the voltage output from the Strain Gauge Wheatstone Bridge Circuit.

Strain Gauge Pressure Transducers are also "resistant" (pardon the pun ... ha-ha-ha!) to shock, vibration and the constant, dynamic pressure changes that characterized industrial processes.

 

 

Form and Function of Capacitance Pressure Transducers

Capacitance Pressure Transducers were the original, go-to Pressure Transducer when Vacuum Pressures needed to be measured.

Modern Capacitance Pressure Transducers can measure high vacuum pressures in the micron range all the way to 10,000 psig (70 MPa). Their sweet spot application is in low-differential and low-absolute PV Pressure measurement service.

From now on, when PTOA Readers and Students hear the words "Capacitor" or "Capacitive" they need to visualize two metal plates with air or some other insulator between them.

This two-metal plates-separated-by-an-insulator describes a Capacitor. Capacitors have the ability to store electrical charge. The magnitude of the stored electrical charge is called "Capacitance."

One of the plates is stationary (the Stator) and one of the plates can move closer to or further from the Stator.

The Capacitance of the Capacitor varies. When the plates are close together, the maximum Capacitance (aka, the ability to hold a charge) is observed. Capacitance decreases as the flexible plate moves further from the stationary plate.

The Capacitor is connected to an ac power source which gives one of the plates a positive charge and one of the plates a negative charge as is illustrated in the nearby graphic.

The changing Capacitance is converted into a quality voltage output.

In a Capacitance Pressure Transducer, the flexible plate is a stainless steel or high nickel-steel alloy Diaphragm. The Stator plate is in an unmovable, fixed position.

When the sensed PV Pressure increases, the flexible Stainless-Steel Diaphragm moves closer to the Stator (labelled "Rigid plate" in one of the nearby diagrams).

The Capacitance of the Capacitor changes proportionally with the varying gap between the Diaphragm and the Stator.

The quality voltage output signal from a Capacitive Pressure Transducer is not bothered by low or moderate EMI or Radio Frequency Interference (FRI). Another benefit of converting to a high-quality voltage signal is that, unlike the Strain Gauge Pressure Transducer, no signal amplification is necessary

The Responsivity of the Capacitance Pressure Transducer is moderate compared to the Strain Gauge but much better than the Responsivity of a Potentiometric Pressure Transducer. The Diaphragm just has to move a few microns to generate a distinct electrical output that can be easily converted to volts.

Capacitance Pressure Transducers are a better choice when over-pressuring is anticipated. They also work well when static (aka non-dynamic, aka not changing) PV Pressures need to be measured.  They work excellent in static, low PV Pressure-measuring applications ... like 1 psi.   They also have great hysteresis characteristic (see PTOA Segment PTOA #228 to refresh what "hysteresis" means).

 

Form and Function of Potentiometric Pressure Transducers 

PTOA Readers and Students: Don't let the ten-dollar word "Potentiometric" bother you!

That word just means that the Bourdon Tube, Bellows or Diaphragm Pressure Sensor is connected via mechanical linkages to a Wiper arm. The movement of the linkages amplifies the deflection movement of the Pressure Sensor.  The Wiper slides across a resistance-creating element that has been given the name "the Potentiometer." 

The Wiper has multiple electrical contacts on a "sliding track." Note that the sliding contact track is connected by a wire to a third terminal, hence by construction grounded.

In the nearby schematic, the Pressure Sensor is a C-Shaped Bourdon Tube. The Potentiometer is an (unlabeled) vertical-standing rectangle made from non-conducting ceramic material which is wrapped with wire.  Both ends of the wire are connected to terminals.

As the Pressure is sensed and measured, the Bourdon Tube in the nearby photo is deflected, expanding outward. The linkages are pulled upward which likewise slides the Wiper upward on the Potentiometer.  In other words, the magnitude of the Bourdon-Tube's deflection determines the position of the Wiper on the Potentiometer.

As the Wiper moves up the Potentiometer, the contacts slide over the Potentiometer and an Electrical Resistance output is transduced from the deflection of the Bourdon Tube.

The amount of Electrical Resistance transduced depends upon the position of the Wiper on the Potentiometer because the position of the Wiper determines how much wire ... and thus how much Electrical Resistance ... is between the end of the Wiper and the Potentiometer.

The Wiper will have a secondary set of sliding contacts that convert the Electrical Resistance into a voltage output. This voltage signal is made available for transmission at the output terminal of the Potentiometer.

Another way to describe a Potentiometer Pressure Transducer is a "variable resistor" in which the Electrical Resistance is varied to control the flow of electric current.

NOTE! The Potentiometer Pressure Transducer described above is limited to a discrete output. The Wiper contacts the wire only at certain positions, not continuously.  Ergo, the Electrical Resistance output jumps from one value to the next; the PV Pressure reading cannot indicate smaller values of the PV Pressure between each node on the Potentiometer. 

A more precise Potentiometer will be fabricated from a mixture of ceramics and metal or conductive plastic.

The above description should alert all PTOA Readers and Students that there are many mechanical parts and linkages in a Potentiometric Pressure Transducer ... and all of them can wear out. As the components wear out, the amount of electrical interference increases. The resulting electrical "noise" will confuse the PV Pressure measurement observed.

And, as was mentioned above, the Responsivity of a Potentiometric Pressure Transducer is inferior compared to the Capacitive Pressure Transducer and the Strain Gauge Pressure Transducer.

On the other hand, the benefits of the Potentiometric Pressure Transducer include being cheap and small ... small enough to fit into a 4.5-inch dial PV Pressure gauge. Additionally, their robust electrical output does not require signal amplification. Therefore, Potentiometric Pressure Transducers can be used in low-power applications.

In the Process Industries, Potentiometric Pressure Transducers are used as automatic control valve "positioners." PTOA Readers and Students will learn about Automatic Control Valves in future PTOA Segments.  A "Positioner" on a Flow Control Valve will yield a more accurate measurement of the PV Flowrate.The Potentiometer Pressure Transducer can transmit positive feedback regarding the valve position or can be an input signal for position control of an actuator.

Potentiometer Pressure Transducers have many automotive service applications including pressure gauges on a dashboard, brake positioning, clutch positioning, throttle positioning. They are also found in ground support equipment at airports and positioning the fork in forklifts.

 

HUMONGOUS HIGH FIVE TO PTOA READERS AND STUDENTS!

The PTOA Process Variable Pressure Focus Study was just completed!

 

 

 

TAKE HOME MESSAGES: 

The Strain Gauge Pressure Transducer/Transmitter

• Electrical Resistance is transduced when the sensed PV Pressure expands and contracts a Strain Gauge which is adhered to the Pressure Sensor.
• An embedded Wheatstone Bridge Circuit converts the Electrical Resistance into a voltage. 
• The smooth analog signal output is repeatable with high accuracy and the fastest Responsivity over a large range of Pressure measurement.

 

The Capacitance Pressure Transducer/Transmitter

Capacitance is the ability for a device called a Capacitor to hold an electrical charge.

The architecture of the Capacitor within the Capacitance Pressure Transducer includes a flexible Stainless-Steel Diaphragm that serves as a flexible metal plate and a rigid metal plate. An insulating material lies between the two plates. An ac power sources gives one plate a positive charge and the other a negative charge.

The gap between the two plates determines the Capacitance of the Capacitor. The movement of the flexible Stainless-Steel Diaphragm determines if the Capacitance is increasing (Pressure increase) or decreasing (Pressure decrease). Without amplification, the Capacitance can be transduced into a quality voltage signal.

Capacitive Pressure Transducers accurately measure Vacuum Pressures and static (non-dynamic) Pressures.

 

The Potentiometric Pressure Transducer

The movement of a Bourdon Tube Pressure Sensor is amplified by linkages that are attached to a Wiper which contains contacts that slide over a Potentiometer (a resistance generating device).

The Potentiometric Pressure Transducer transduces the movement of the Pressure Sensor into an Electrical Resistance.

The circuitry of a Potentiometer is the same as would be used for a Variable Resister.  Variable Resisters are also referred to as "voltage dividers."

Potentiometric Pressure Transducers are used to give feedback on the position of hardware. In Process Industry, Potentiometric Pressure Transducers are used to more accurately position the movement of a Flowrate Control Valve. A more accurate PV Flowrate results. 

 

©2022 PTOA Segment 0231

PTOA PV PRESSURE STUDY AREA
PV PRESSURE INSTRUMENTATION

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