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Pressure transducers are essential in both industrial and commercial settings because accurate pressure monitoring and control are vital for all fluid power systems, whether gas-based or hydraulic.

Establishing Definitions of Pressure Transducers

Pressure transducers are often not as clearly defined as other industrial products. To clarify their role, it is useful to compare them with similar but distinct devices. The terms “pressure sensor” and “pressure transducer” are frequently used interchangeably, though they have important differences. A sensor is a device that detects and responds to physical phenomena or changes, whereas a transducer not only senses these phenomena but also converts the information into a different format that is easier to interpret. Although sensors can also convert the data they detect, they do not always do so. Pressure gauges are a good example of devices that act as sensors without necessarily functioning as transducers. Analog pressure gauges, in particular, detect and respond to mechanical force without converting the input into non-mechanical formats. By definition, transducers always perform a conversion in addition to sensing. In practice, the terms are often used interchangeably because most pressure sensors also function as pressure transducers, but it is helpful to be aware of the technical distinction.

SITRANS P Pressure Measurement Components – Siemens Process Instrumentation

Additionally, it is important to differentiate between pressure transducers and load cells. The distinction between these devices is often blurred. Load cells are generally considered a type of pressure transducer that converts mechanical force into an electrical output. Since most pressure transducers produce electrical output, many of them can also be classified as load cells. However, not all pressure transducers are load cells. Some pressure transducers, for example, produce optical output instead of electrical output. Moreover, pressure transducers typically measure pressure, defined as force distributed over an area (measured in Pascals, bars, psi, etc.), while load cells directly measure force (measured in Newtons).

Pressure Transducer Production

Modern high-accuracy pressure transducers are constructed using a diverse array of materials. The primary components are typically made from stainless steel, copper, ceramic, titanium, carbon, and other materials. The wetted materials, which are the parts of the transducer that come into contact with pressurized media or fluid, may include not only these metals (such as stainless steel) but also bronze, plastics, glass, and silicon. Pressure transducers can greatly vary in size and material composition. On average, a pressure sensor measures approximately one cubic inch, though it can be manufactured to be as small as 1/100th of a cubic inch.

How Pressure Transducers Work

Various types of pressure transducers exist, but they all function based on similar fundamental principles. They convert mechanical pressure into electrical signals by detecting the deflection of a sensing element inside them. This element can be a Bourdon tube, bellows, or diaphragm. (For more detailed explanations of these elements, refer to the article on Pressure Gauges.) The deflection and resulting strains in these elements cause changes in electrical properties, such as resistivity, which can be measured and interpreted to determine mechanical pressure and its variations.

Strain gauge pressure transducers are the most prevalent type. These transducers use sensing elements, often made of materials like silicon, polysilicon film, or metal foil, that change their electrical resistivity when deformed by mechanical pressure. Typically, the strain gauge deforms in a secondary step after a primary sensing element, such as a diaphragm, deforms from pressure. Strain gauges are usually wired in a Wheatstone bridge circuit, which amplifies and optimizes the transducer’s output, making them ideal for high-pressure applications and typically having a cylindrical shape.

Piezoelectric pressure transducers operate similarly to strain gauges. They measure pressure by directly detecting electric charges that accumulate on sensing elements in response to applied force. Some piezoelectric transducers incorporate strain gauges, creating a hybrid type.

Other transducers, called capacitive pressure transducers, detect pressure changes through decreases in electric capacitance when a diaphragm deforms. These are generally used for low-pressure applications, such as around 40 bar.

There are also pressure transducers that do not rely on sensor deflection. Resonant pressure transducers measure pressure changes by observing shifts in the resonance frequency of a sensing element. Sometimes, the resonating element is exposed to the media being measured, making its frequency dependent on media density. Another example is thermal pressure transducers , like Pirani gauges, which calculate pressure changes based on variations in a gas’s thermal conductivity.

Types of Pressure Transducers

Pressure sensors can be classified in several ways, such as by the type of pressure they measure, the pressure range they cover, and their operating temperature range. Below is an overview of different categorization approaches.

Categorizing by Types of Output

Pressure transducers typically need an electric power source for excitation to operate, unlike analog pressure gauges. They are primarily characterized by their capability to produce electrical output signals based on pressure detection. These electrical outputs can take the form of voltage (e.g., mV or V), current (e.g., mA), or frequency.

Millivolt-generating transducers usually produce outputs that do not exceed 30 mV and are very sensitive to changes in excitation. In contrast, transducers that generate voltage can produce higher electrical outputs due to built-in signal conditioning, and their output is not directly proportional to changes in excitation. Transducers that generate current are also known as pressure transmitters. Although all pressure transducers are sometimes called pressure transmitters, this is technically incorrect. The term “pressure transmitters” specifically applies to transducers that generate current, typically known as 4-20 mA output pressure transducers due to their standard output range.

Some pressure transducers do not produce electrical output signals at all. A common alternative is optical signal output, where many pressure transducers rely on physical changes in an optical fiber to detect mechanical strain and corresponding pressure changes. Other transducers use layered elastic films to deduce pressure changes through variations in reflected optical wavelengths.

Categorizing by Different Measuring Standards

Another way to categorize pressure transducers is based on the standard they use to operate. From this perspective, pressure sensors fall into five main categories: sealed pressure sensors, gauge pressure sensors, vacuum pressure sensors, absolute pressure sensors, and differential pressure sensors.

• Gauge pressure sensors measure pressure relative to atmospheric (or barometric) pressure, which is created by the weight of the air in Earth’s atmosphere.
• Sealed pressure sensors measure pressure against a predetermined fixed pressure, which may or may not correspond to the surrounding atmospheric pressure.
• Vacuum pressure sensors measure pressures lower than atmospheric pressure, indicating the difference between the low pressure and atmospheric pressure.
• Absolute pressure sensors calculate pressure relative to a perfect vacuum, including atmospheric pressure in their total pressure calculations, unlike gauge pressure sensors.
• Differential pressure sensors sensors measure the difference between two or more pressures at different locations on the sensor. They are used to assess flow rates and pressure drops within pressurized or enclosed vessels.

Categorizing by Measured Phenomena

Pressure transducers are sometimes categorized by the specific phenomena they measure, such as a blood pressure transducer. A significant category is air pressure sensors, which are commonly used with pneumatic tools or air compressors to measure airflow pressure and provide readable data to operators. A crucial type of air pressure sensor is the atmospheric (or barometric) pressure sensor, used for meteorological purposes to measure atmospheric pressure. Typically, air pressure sensors are either absolute pressure sensors or differential pressure sensors.

Application-Specific Categorization

Pressure transducers are often categorized by the types of applications they serve. Examples include PC board mountable transducers, heavy-duty/industrial pressure transducers, and high-temperature pressure transducers. Pressure transducers can be adapted or customized for a wide range of applications, including altitude sensing (useful in aerospace and navigation), flow sensing, leak testing, and pressure sensing (vital for weather instruments, automotive functioning, aircraft operation, and chemical processing).

Categorizing by Size

A category of pressure transducers defined primarily by size is the miniature pressure transducer. These small sensors are designed for critical applications, such as biological and medical procedures, where instruments must be as non-intrusive as possible. Most miniature sensors have an error margin of less than one percent, maintained through proper calibration and backup systems.

Pressure Transducer Accessories

To refine outcomes, operators often use additional devices and mechanisms alongside pressure transducers. These can include pressure regulators, pressure calibrators, level transmitters, torque transducers, temperature transducers, and integrated circuits.

Pressure regulators enhance the ability to observe and control the pressure within a system. They are typically programmable to alert operators if the pressure surpasses a safety threshold. This feature is particularly beneficial when used with millivolt transducers.

Pressure regulators help maintain the desired system pressure level or reduce it. The most critical aspect of designing pressure regulators is adjusting the range, which defines the upper limit for control. Various types of pressure regulators include back pressure, vacuum pressure, differential pressure, and pressure reducers, tailored for specific fluids like oil and gasoline. These regulators are compatible with media types such as air, fuel and oil, hydraulic fluids, liquids, steam, and others.

A back pressure regulator (BPR) is a device that maintains a set pressure upstream of its inlet. It opens to release excess pressure when fluid pressure at the BPR’s input exceeds the setpoint. The BPR continuously adjusts to keep the inlet pressure at the desired setpoint, functioning similarly to relief valves but focusing on steady-state pressure management rather than on/off activation. BPRs are used to regulate supercritical, mixed-phase, liquid, gas, and liquid phases.

Construction of Pressure Regulators

Pressure regulators have three fundamental components. The loading mechanism affects the regulator’s setting and delivery pressure. The sensing component responds to both the loading mechanism’s force and the pressure differential between the inlet and outlet. A valve acts as the control element, reducing inlet pressure to outlet pressure by incorporating feedback from other system components.

Pressure regulator bodies can be constructed from materials such as acetal, aluminum, brass, bronze, cast iron, steel, stainless steel, and zinc. Common connection sizes range from 1/8″ NPT to 2″ and include metric and British standard pipe threads. Mounting styles encompass subplate or manifold mount, stack or line mount, pipe or line mount, and cartridge mount.

Types of Pressure Regulators

• Back Pressure Regulators:
  These regulators maintain a specified pressure at their intake. Their applications include:

1. 1. Managing upstream pressure in analytical or processing systems.
   2. Preventing overpressure damage to sensitive equipment.
   3. Reducing pressure differences over components that cannot withstand significant variations.
   4. Being used in vent or flare lines or gas sales lines.
   5. Being used in manufacturing vessels such as separators, heater treaters, or free water knockouts.
   6. Being used in hyperbaric rooms.
   7. Use in dive helmets.

• Dome-Loaded Pressure Regulators:
  These regulators are controlled by gas pressure.
• Spring-Loaded Pressure Regulato:
  These regulators control the pressure of a gas or liquid in a container, whether the flow is steady or erratic.

Pressure Reducers or Lowering Regulators:
These regulators maintain the appropriate downstream pressure by reducing a higher supply pressure to a controlled lower pressure at the outlet. In contrast, reducing back pressure works oppositely, controlling pressure at the inlet by adjusting the opening to maintain the desired upstream pressure. Their applications include:

• Widespread use in the aerospace, aviation, and cooking industries.
• Reducing water pressure effectively.
• Use in cutting and welding operations.
• Controlling propane/LP gas.
• Use in recreational and gas-powered vehicles.
• Providing breathing gas.
• Use in the mining and natural gas industries.

Pressure calibrators:
These devices calculate and report the pressure, flow, and level of certain system instruments to ensure safe and efficient operations. By connecting to an existing system, they receive input to compare with the system’s gauge, allowing operators to quickly determine the gauge’s accuracy.

Level transmitters:
Level transmitters measure variables such as solids, slurries, or liquids within a space. They typically sound an alarm or trigger a shut-off switch if the buildup becomes too great for safe operations.

Torque and Temperature Transducers:
These devices extend the functionality of pressure transducers by measuring rotational movement and heat content, respectively.Torque transducers, also known as torque sensors or torque transmitters, measure both static and dynamic twist in a rotating system. Temperature transducers can measure either remotely by assessing thermal radiation or directly by being immersed in a substance.

Integrated circuit (ICs):
Also known as silicon chips or microchips, integrated circuits are miniature electronic circuits. Pressure transducers increasingly use integrated chips to communicate with other equipment and maintain accuracy levels.

Considerations for Pressure Transducers

When choosing a pressure transducer, it’s essential to prioritize the specific requirements of your application and select a device accordingly. For instance, millivolt transducers are widely used because they are cost-effective. However, they are susceptible to electrical noise and interference. In industrial environments, opting for a transducer with a higher voltage output might be a better choice. If the pressure transducer needs to function over long distances, using a voltage-based transducer may not be ideal. In such cases, a pressure transmitter, which is more resistant to electrical noise and can accommodate lead wires over a thousand feet, might be the best option.

A crucial aspect of selecting a pressure transducer is considering the physical threats it may face. Like most industrial equipment, pressure transducers are complex but delicate devices that should be handled with care to ensure longevity. Although they are designed to operate under various conditions, using them in environments for which they are not intended can quickly lead to malfunction.

Over-pressurization is a significant risk for pressure transducers. A common sign of over-pressurization is an upward shift from a zero reading. To prevent this, a proactive approach during the selection process is advisable. Many manufacturers and suppliers recommend choosing transducers with maximum pressure ranges significantly higher than the anticipated operating pressures to provide a safety margin against unexpected spikes. Ideally, the expected operating pressures should be 50-60% of the transducer’s maximum capacity. For example, using a 5000 psi transducer in a hydraulic system with expected pressures of 2500-3000 psi follows this strategy. Additional safety measures, such as installing snubbers—small orifices in the piping to protect the transducer from pressure spikes—can further extend the transducer’s lifespan and effectiveness.

Extreme temperature variations also pose a significant threat to pressure transducers. Despite being designed to function independently of non-pressure variables, transducers are still affected by temperature changes due to their composition of various materials. A preemptive strategy in transducer selection can help mitigate this risk. If you anticipate using a transducer in high-temperature conditions, opt for high-temperature pressure transducers to ensure optimal performance.

Considerations for Pressure Transducer Supplier Selection

When choosing a pressure transducer supplier, there are some key considerations to keep in mind. Look for a supplier with a broad range of experience and a diverse product selection. In the case of transducers, partnering with a supplier who has extensive experience in your specific application or niche can be particularly beneficial, as it may reduce the need for extensive product testing on your part.

Additionally, it’s important to familiarize yourself with industry-specific terminology to ensure you ask the right questions when evaluating potential suppliers. The terminology and publishing standards in the transducer field are not always clearly defined. For instance, standards for resolution (the smallest pressure change a transducer can detect) can vary significantly among manufacturers. Some may publish maximum resolution values, while others provide average resolution values. By asking focused questions about these topics, you can assess a supplier’s expertise and their commitment to customer service, helping you make an informed decision.