Pressure Transducer Selection Criteria

In the world of pressure measurement, the selection of the right pressure transducer is critical for obtaining reliable and accurate data. When faced with a plethora of options, making the right choice can often seem daunting. It is essential to examine various parameters, such as pressure range, accuracy, compatibility with media, and the conditions of the environment. This article will delve into each of these parameters to assist users in making educated decisions when selecting a transducer. For further insights into how these devices function, Click here to get more.

Selection criteria

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Pressure range

Determining the range of a pressure transducer is paramount. This range indicates the maximum and minimum pressures the device can accurately sense. Two primary aspects should be evaluated during this selection:

• Accuracy: A narrower range generally leads to better accuracy, which will be discussed in detail later; the normal operational pressure should lie at the center of this range.
• Overpressure protection: Understanding the potential risks associated with exceeding normal operating pressures due to malfunctions, errors, or experimental mishaps is vital. Specifying the necessary overpressure protection guarantees the transducer's resilience to pressures that surpass normal levels.

Example

Imagine selecting a transducer for a hydraulic system operating at psi (137.8 bar).

• Accuracy: Opt for a pressure transducer designed with a range that centers on this psi (137.8 bar) for peak accuracy. For example, a transducer ranging between psi (103.4 bar) and psi (172.3 bar) would be an ideal selection, ensuring accurate readings since the average operating pressure occupies the midpoint of the sensor's range.
• Overpressure protection: It's critical to consider the implications of a malfunction, such as a stuck valve causing the pressure to soar to psi (206.8 bar). In this case, the transducer should exhibit robust overpressure protection capabilities, ideally rated for pressures up to psi.

In this context, the most suitable pressure transducer may range from psi to psi for optimum accuracy, while safeguarding against overpressure up to psi.

Connection size and voltage

The connection size indicates how seamlessly the transducer connects within the system. Ensuring compatibility with existing components is crucial to establishing a secure and leak-free bond. Moreover, select a transducer with a corresponding voltage range that guarantees efficient operation and integration with the power supply system. It's vital that the transducer’s thread size aligns with the equipment or fittings to prevent potential leaks and ensure secure connections.

Linearity

Linearity pertains to the transducer's capability to generate an output that corresponds directly to the input pressure. In layman's terms, if the pressure doubles, the output should ideally double as well, signifying perfect linearity. However, in real-world applications, minor deviations in linearity may occur, with non-linear readings ranging from 0.2 to 0.5 percent, which indicates potential discrepancies in measured values that could deviate by as much as 0.5 percent of the overall measurement range.

Process media

The portions of a pressure transducer in direct contact with the fluid are referred to as 'wetted parts.' The materials used in these parts must be compatible with the media being measured. Key process media considerations include:

• Compatibility: Choose materials that will not corrode or become contaminated by the specific process fluid.
• High nickel alloys: For corrosive fluids such as seawater, high nickel content alloys are robust choices due to their excellent resistance to corrosion.
• Stainless steel: Stainless steel 304, 316, and 17-4 are prevalent materials for pressure gauges in diverse applications due to their durability and resistance to corrosion.
• Sanitary fittings: Industries that prioritize hygiene (such as food, beverage, and pharmaceuticals) prefer pressure transducers with sanitary fittings to promote cleanliness and avert bacterial growth. These fittings facilitate quick and efficient cleaning, whether by hand or through automated systems.

Figure 2: Stainless steel pressure transducer

Output type

Pressure transducers can feature various outputs, each with unique attributes and benefits:

• Ratiometric output: The output signal adjusts proportionally to the power supply, beneficial in situations where the power supply's stability is questionable, thus maintaining output accuracy.
• mV/V output: This represents the simplest and least expensive output type, providing an unamplified millivolt per volt output directly from the sensing element. It is ideal for short-distance transmissions typically found in controlled environments.
• Amplified voltage output: Certain transducers come equipped with built-in amplifiers that enhance the output signal, rendering it less susceptible to electrical noise and suitable for transmission over extended distances.
• mA output: The 4-20 mA current output is standard when signals must traverse long distances since current does not degrade over distance as voltage may. This output type is commonly employed in industrial settings.
• Digital outputs: Some pressure transducers offer digital outputs, allowing for direct interfacing with computers or digital systems.

Application and measurement type

Choosing the appropriate pressure measurement type for a pressure transducer primarily hinges on its intended application and the specific requirements of the associated system. Common pressure measurement types encompass:

• Absolute: This type determines pressure in reference to a perfect vacuum, measuring the total pressure exerted by a fluid, including atmospheric pressure.
• Gauge: Gauge pressure is comparative to regular atmospheric pressure, measuring pressures above or below local atmospheric levels.
• Differential: This type measures the pressure differential between two points within a system, proving invaluable in scenarios where the pressure difference is critical.
• Vacuum: Vacuum pressure quantifies pressure readings below atmospheric levels, typically used in suction devices or situations demanding lower pressure.
• Bidirectional: Bidirectional transducers are capable of measuring both positive and negative pressures (above and below atmospheric levels), necessary when pressure fluctuations can occur in both directions relative to a reference point.

Refer to our article on atmospheric, absolute, gauge, and differential pressure for additional details about different pressure types.

Temperature range

It is essential to ascertain that the pressure transducer can operate within the specified temperature range. Transducers utilizing thin film technology exhibit superior performance at elevated temperatures due to the resilience of their thin layers of material (such as metals or semiconductors), which are less prone to expansion, contraction or degradation under extreme heating conditions.

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Extreme temperatures may distort the output of a transducer, often expressed as a percentage full scale over 1°C (%FS/°C). For instance, if a pressure transducer has an error rate of 0.2% full scale per degree Celsius (%FS/°C) and experiences a temperature increase of 10°C, the output could deviate by 2% of its full scale, thereby affecting the accuracy of pressure readings.

Accuracy

The accuracy of pressure measurements provided by a pressure transducer indicates how closely its readings align with the actual pressure being measured. For example, with a full-scale range of 0 to 6.9 bar (0 to 100 psi) and an accuracy of 0.5% of full scale, the device's readings might fluctuate by as much as 0.03 bar (0.5 psi) from the actual pressure at any given point within that range.

In typical applications, a pressure gauge with 0.5% accuracy suffices for reliable measurements; however, higher precision is required in critical situations such as aerospace, pharmaceuticals, or semiconductor manufacturing, where precise pressure readings are indispensable.

Resolution

The resolution of a pressure transducer refers to the smallest pressure variation it can detect. For instance, if a pressure transducer possesses a full scale of 100 psi with a 0.1% resolution, it can identify changes as minuscule as 0.1 psi. Resolution is a crucial factor as it significantly influences measurement accuracy. Applications requiring precise readings necessitate high-resolution transducers, while general detection tasks may only require lower resolution units. The corresponding costs tend to increase with higher-resolution transducers.

Response time

The response time denotes how swiftly the transducer reacts and produces a signal following a pressure change. This metric is important as it influences both the accuracy and speed of pressure detection. For applications demanding real-time monitoring (such as in medical ventilators), selecting transducers with rapid response times is essential, while slower ones may suffice for gradual pressure changes and present a more budget-friendly alternative.

Installation environment

Factors like corrosion and exposure to explosive vapors significantly impact the performance and safety of transducers.

• Corrosion: Hazards arise when corrosive liquids or gases splash onto the housing surface.
• Explosive vapors: For installation areas characterized by explosive vapors, the transducer and its power supply must be appropriately rated for such environments. Typically, this involves utilization of explosion-proof housing or intrinsically safe designs, which cap the power available to electrical equipment to levels below those capable of igniting gases.

Design features

Some pressure transducers are available in specialized versions with flush designs tailored for applications dealing with viscous or particulate-laden media. These flush designs incorporate a small orifice that facilitates fluid movement across the sensing element, effectively cleaning it and preventing debris accumulation. This feature proves particularly valuable in food processing, pharmaceuticals, and chemical manufacturing industries. Transducers featuring food-grade liquid fills are intentionally engineered for food and beverage applications. These devices utilize an inert fill fluid that is entirely compatible with food products, ensuring that the integrity and safety of processed food remains intact.

Maintenance

Opting for pressure transducers equipped with self-calibration features or ones that can be easily calibrated helps to lessen maintenance needs and minimize downtime. Selecting durable and consistent transducers can also reduce the frequency of necessary maintenance tasks, thus ensuring dependable performance over extended periods.

FAQs

How does the accuracy of a pressure transducer affect its selection?

Accuracy is crucial since it determines the closeness of a measured value to the actual value. Certain critical applications, such as aerospace and pharmaceuticals, necessitate higher accuracy.

Why is the pressure range important in choosing a pressure transducer?

Establishing a suitable pressure range ensures that the transducer operates within its designed limits, preventing damage and inaccuracies in readings.

Explore our online assortment of pressure transducers!

• Pressure Transducers