Variable Capacitance Ceramic Pressure Sensors

These are a type of sensor that uses a ceramic diaphragm combined with a variable capacitance sensing device to measure pressure.

The ceramic diaphragm is a thin, flexible material that is sensitive to changes in pressure. Variable capacitance technology relies on a flexible diaphragm and a fixed plate forming a capacitor. Pressure applied to the diaphragm changes the distance between the plates, altering the capacitance. This change is measurable electronically and converted into a pressure reading.

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Find out more about Variable Capacitance Ceramic Pressure Sensors to determine which product options and capabilities will best meet your application requirements.

How It Works

The primary sensing element of a capacitive pressure sensor is a deformable diaphragm and a fixed electrode, which together form a capacitor. When pressure is applied to the diaphragm, it deflects, causing a change in the distance between the diaphragm and the fixed electrode. This change in distance directly affects the capacitance of the capacitor.

Variable capacitance pressure sensor assembly

To measure this capacitance change, an excitation voltage, typically a high-frequency AC signal, is applied to the capacitor. The resulting current or voltage across the capacitor is then measured. Since the capacitance is proportional to the applied pressure, the amplitude of the resulting AC signal also varies with pressure.

The raw output from this sensing element is an AC signal with an amplitude that fluctuates in response to the applied pressure. The frequency of this signal is the same as the excitation frequency, but its amplitude carries the information about the pressure being measured. This AC signal is then fed into the signal conditioning electronics, where it is rectified, filtered, and amplified to produce a stable DC voltage or digital output that is proportional to the applied pressure.

Properties

High precision

Pressure sensors which are based on the variable capacitance technology can detect minute pressure changes, making them suitable for precise measurements.

Capacitive pressure sensors achieve high precision due to several factors:

Sensitivity to small changes: The capacitance changes proportionally with the distance between the plates. Even minute pressure variations result in detectable capacitance shifts, enabling high-resolution measurements.
Linear response: The relationship between pressure and capacitance is often linear over a wide range, simplifying calibration and ensuring accurate readings throughout the measurement range.
Low noise: Capacitance measurement is inherently low noise, minimizing the impact of electrical disturbances on the signal.
Digital signal processing: Modern sensors often incorporate digital signal processing techniques to further enhance precision by filtering out noise, compensating for non-linearities, and providing temperature compensation.

These combined factors contribute to the high precision of capacitive pressure sensors, making them suitable for demanding applications where accuracy is critical.

Good long-term stability

Capacitance doesn’t drift much over time, ensuring consistent accuracy.

Variable capacitance pressure sensors exhibit good stability primarily due to the inherent properties of capacitance measurement and the materials used in their construction. Capacitance is a fundamental electrical property determined by the geometry and materials of the capacitor, which are less prone to drift or change over time compared to other sensing mechanisms.

Additionally, the materials used in the diaphragm and plates are carefully chosen for their stability and resistance to environmental factors like temperature and humidity. This minimizes the potential for long-term changes in capacitance that could affect the sensor’s accuracy and reliability.

Furthermore, some advanced designs incorporate reference capacitors that are not sensitive to pressure changes. This allows for compensation of any minor drifts in the main sensing capacitor, further enhancing the overall stability of the sensor.

Low power consumption

To operate, variable capacitance sensor typically require less power than other types of sensor, making them an ideal choice for battery-powered devices.

Capacitive pressure sensors have lower power consumption primarily due to the absence of continuous current flow. Unlike some other pressure sensor types that require constant excitation or heating, capacitance measurement involves applying a brief voltage pulse to charge the capacitor and then measuring the resulting voltage change. This pulsed operation significantly reduces the average power consumption, making them ideal for battery-powered and energy-efficient applications.

Temperature insensitivity

The performance of a variable capacitance sensor isn’t significantly affected by temperature fluctuations.

Variable capacitance pressure sensors are less sensitive to temperature fluctuations mainly because the capacitance change is primarily dependent on the distance between the plates, not the properties of the materials themselves. While some materials used in the sensor construction might have slight temperature dependencies, the overall effect on capacitance is minimal compared to other pressure sensor technologies where material properties play a more significant role in the measurement principle.

Ceramic diaphragm

Ceramic is an excellent electrical insulator which provides a base substrate for printing conductive materials to provide electrical function such as the electrodes which are incorporated into the inside surface of the the ceramic diaphragm to provide the moving part of the variable capacitance sensing device.

Ceramic is a very hard material, with an extremely high resistance to abrasion. It’s made from tightly packed and bonded crystalline material, and provides superb protection from media which contain abrasive particles such as slurries and liquids which contain sediment.

Applications

Chemical process monitoring

In chemical process monitoring, ceramic capacitive pressure sensors with low pressure gauge references play a crucial role in various applications that require precise measurement of pressure differentials relative to atmospheric pressure. Their corrosion resistance, high accuracy, and stability make them well-suited for harsh chemical environments.

Pharmaceutical Manufacturing

In pharmaceutical manufacturing, many processes require precise monitoring of low pressures, typically ranging from a few millibars to several bars. These processes often involve sensitive substances that are susceptible to degradation or contamination if exposed to incorrect pressures.

Environmental Monitoring

Measuring the pressure of contaminated groundwater or industrial wastewater often involves low pressures and corrosive substances. A chemically resistant variable capacitance sensor would enable accurate monitoring of these environments.

Research and Development

In scientific research, precise pressure measurements are often required in controlled environments involving corrosive substances or extreme conditions. A versatile variable capacitance sensor is a valuable tool for use on lab equipment.

Advantages

High Accuracy and Sensitivity: Can detect minute pressure changes, ideal for precise measurements and control.
Excellent Long-Term Stability: Capacitance remains stable over time, ensuring consistent accuracy and reliability.
Corrosion Resistance: Ceramic diaphragms are resistant to various corrosive media, suitable for harsh environments.
Wide Temperature Range: Can operate reliably across a broad temperature range, making them suitable for demanding applications.
Good Overload Protection: Can withstand high overpressure without damage, ensuring durability and longevity.
Low Power Consumption: Typically require less power than other types of pressure sensors, ideal for battery-powered devices.
Versatility: Can be used for both absolute and gauge pressure measurements, offering flexibility in various applications.

Disadvantages

Cost: Can be more expensive than other pressure sensor technologies due to the complex manufacturing process and materials used.
Fragility: Ceramic diaphragms can be susceptible to damage from mechanical shock or vibration, requiring careful handling and installation.
Limited Pressure Range: While they can handle high overpressure, the measurable pressure range might be limited compared to other technologies.
Complexity: The measurement principle and signal conditioning circuitry can be more complex than simpler sensor types.

FAQ

Questions and Answers about variable capacitance ceramic diaphragm pressure sensor technology.

Sensing Diaphragm Material & Performance

How well does the sensing diaphragm material perform for elasticity, hysteresis, repeatability and robustness?

Despite its high hardness, ceramic exhibits a restricted elastic range, rendering it more prone to fracture under stress than alternative materials.

Media Compatibility

How protected is the pressure sensor module assembly against different types of aggressive media, and how customisable is the construction to enhance protection to a particular media type?

The chemically inert ceramic diaphragm, renowned for its exceptional hardness, provides robust corrosion resistance against a wide spectrum of aggressive media, notably acids and bases. It is typically encapsulated within a metallic or polymeric housing, which also integrates the process connector. An elastomeric seal, acting as an interface between these two components, facilitates a hermetic enclosure. This modular design approach offers unparalleled adaptability to diverse media types by enabling the selection of chemically compatible materials for both the elastomer seal and process connector.

Oil Leakage

Is there a risk of oil leakage from this sensor, which could impact performance or safety?

Ceramic sensing modules offer broad media compatibility, eliminating the need for protective oil-filled isolation diaphragms. Their inherent rigidity and sufficient size enable direct mounting within a process connector. A pressure seal is achieved by compressing an elastomeric O-ring against the module’s outer diameter. All electrical connections are made to PCB solder pads on the reverse side.

Pressure Containment

How well are the internal components joined together to provide a seal to withstand pressurisation that exceeds normal operating conditions, without degradation due to pressure and temperature cycling over the lifetime of the sensing device?

Elastomeric O-rings, functioning as the primary sealing interface between the ceramic diaphragm and process connector, represent a potential point of failure that could compromise pressure integrity. Although they offer adequate sealing capabilities for low-to-medium pressure applications and mitigate the effects of differential thermal expansion on the ceramic, exposure to specific chemical media or extreme thermal conditions can adversely impact their sealing efficacy. Such exposure can induce physical alterations in the elastomer, manifesting as either softening or hardening, thereby diminishing its sealing performance.

Small Size

How small a pressure sensor is it possible to make with this sensing technology?

The requirement for a substantial diaphragm surface area to achieve adequate output sensitivity through capacitance variation renders variable capacitance pressure sensors among the largest in diameter.

Corrosion Resistance

How resistant are the media exposed materials to corrosive substances?

The exceptional chemical inertness of ceramic, rendering it highly resistant to acids and bases, significantly enhances the service life of pressure sensors deployed in demanding environments like chemical processing, waste management, and marine applications.

Abrasion Resistance

How resistant are the media exposed materials to abrasive substances?

Ceramic’s exceptional hardness confers superior resistance to abrasion and pitting, even when exposed to high-velocity particulate matter in gaseous or liquid media.

High Pressure Spikes

What are the chances of a noticeable effect on the sensors operational performance, or damage to the sensors internal components, from exposure to very dynamic or quickly changing pressures, such as hydraulic pressure spikes or water hammer?

The ceramic diaphragm’s inherent rigidity and constrained elastic range render it susceptible to catastrophic failure when subjected to rapid, high-amplitude pressure transients, such as hydraulic pressure spikes or water hammer events. These transients can induce localized stress concentrations exceeding the diaphragm’s fracture threshold, resulting in irreversible damage like cracks or complete rupture.

Furthermore, low-pressure ceramic diaphragms, characterized by their thinner construction and larger diameters, exhibit heightened vulnerability to such dynamic pressure fluctuations. Their reduced thickness and increased surface area amplify the impact of stress concentrations, further elevating the risk of mechanical failure.

Overpressure Handling

How affected is the accuracy performance of the sensor by an overpressure condition, where the pressure exceeds 100% of range?

The inherent sensitivity of variable capacitance technology permits the utilization of thicker diaphragms, conferring an overpressure rating on the order of 10 times the full-scale range, with even higher ratings achievable at lower pressure ranges. This significantly enhances the sensor’s tolerance to overpressure conditions exceeding 100% of the specified range.

Very Low Pressure Ranges (<100 mbar)

How well does it perform measuring very low pressures with a pressure range lower than 100 mbar?

The inherent high sensitivity, low hysteresis, and minimal thermal sensitivity of variable capacitance technology render it well-suited for measuring very low pressures. However, this necessitates a larger diaphragm compared to exposed silicon chip strain gauge technology.

Low Pressure Ranges (100-1000 mbar)

How well does it perform measuring low pressures with a pressure range between 100 and 1000 mbar?

The inherent high sensitivity, low hysteresis, and low thermal sensitivity of variable capacitance technology make it well-suited for measuring low pressures within the 100 to 1000 mbar range, although this necessitates a larger diaphragm diameter compared to silicon chip strain gauge technology.

Medium Pressure Ranges (1-100 bar)

How well does it perform measuring medium pressures with a pressure range between 1 and 100 bar?

Within the lower end of the medium pressure range (1-100 bar), the technology exhibits commendable performance, particularly due to its high overpressure rating, surpassing that of thick-film strain gauge sensors. However, its larger diameter and associated higher manufacturing costs render it less practical compared to silicon chip strain gauge and thin-film technologies in this pressure regime.

High Pressure Ranges (100-1000 bar)

How well does it perform measuring high pressures with a pressure range between 100 and 1000 bar?

In high-pressure applications (100-1000 bar), the limited strength of ceramic diaphragms becomes a critical constraint. The necessity for larger diaphragm diameters in variable capacitance sensing further exacerbates the challenges, leading to increased manufacturing complexity and costs compared to alternatives like thin-film, silicon chip, and bonded foil strain gauge sensors. Consequently, variable capacitance ceramic diaphragm technology proves less viable in this pressure regime.

Very High Pressure Ranges (>1000 bar)

How well does it perform measuring very high pressures with a pressure range greater than 1000 bar?

In the realm of very high-pressure applications exceeding 1000 bar, the inherent strength limitations of ceramic diaphragms, coupled with the necessity for larger diameters in variable capacitance sensing, lead to heightened manufacturing complexities and costs compared to bonded foil strain gauge technology.

Furthermore, the constraints of very high-pressure applications necessitate the use of much smaller process connections, thicker materials, and welded seals, which are incompatible with the physical characteristics and assembly requirements of ceramic variable capacitance sensors. Consequently, this technology proves prohibitive for such extreme pressure regimes.

Absolute Pressure Ranges

Can this sensor technology be easily adapted to measure absolute pressures, or pressures referenced to a hard vacuum?

Absolute pressure ranges are achieved in ceramic variable capacitance sensors by sealing the gauge reference hole in the centre of the PCB side under full vacuum. This adaptation is a relatively straightforward manufacturing modification. In contrast, silicon chip technology necessitates a fundamental redesign from the chip level up to incorporate an absolute reference. Adapting thin-film and bonded foil strain gauge technology on a metal diaphragm for absolute pressure measurement presents significant challenges.

Accuracy

What level of accuracy performance is achievable from the combined effects of linearity, hysteresis, short term repeatability, temperature errors, and long term stability?

The reduced number of components in the primary sensing element minimizes sources of instability and temperature-induced errors. This results in highly linear and repeatable measurements. However, imperfections and variations in ceramic quality can affect elasticity, and the larger diaphragm diameter makes the sensor more susceptible to environmental influences. These factors can impact overall accuracy performance.

Output Sensitivity

What is the level and quality of the output produced from the transducer technology, before any signal conditioning electronics is added?

The inherent sensitivity of variable capacitance sensing, amplified by the relatively large diaphragm surface area, results in substantial capacitance changes even for minor diaphragm deflections. This translates to a significantly higher unconditioned output signal level compared to strain gauge-based technologies like bonded foil and thick-film.

Frequency Response

How quickly does the sensor’s output respond to changes in pressure?

The response time of variable capacitance ceramic diaphragm technology is relatively slow compared to thin-film or silicon chip strain gauge technologies. This is primarily attributed to the larger and heavier diaphragm, which exhibits higher inertia, impeding its rapid movement in response to pressure changes. Additionally, the signal conditioning process, which involves rectifying and filtering an AC signal to generate a DC output, introduces further delays due to the inherent time constants of these filtering circuits.

Compare with other pressure sensor technologies

How do pressure sensors that use a variable capacitance ceramic diaphragm compare to other technologies?

Sensing technology	Si-Chip	Si-Isolated	Thick-Film	Var-Cap	Thin-Film	BFSG
Sensing diaphragm material	Excellent	Excellent	Fair	Fair	Good	Good
Media compatibility	Bad	Good	Excellent	Excellent	Good	Good
Oil fill leak risk	Excellent	Bad	Excellent	Excellent	Excellent	Excellent
Pressure containment	Poor	Good	Fair	Fair	Excellent	Excellent
Small size	Excellent	Good	Fair	Poor	Good	Fair
Corrosion resistance	Bad	Good	Excellent	Excellent	Fair	Fair
Abrasion resistance	Bad	Bad	Excellent	Excellent	Fair	Fair
High pressure spikes	Bad	Poor	Fair	Poor	Good	Good
Overpressure rating	Good	Excellent	Fair	Good	Poor	Poor
Very low pressure < 100mb	Good	Poor	Bad	Good	Bad	Bad
Low pressure 100…1000mb	Good	Good	Poor	Good	Bad	Bad
Medium pressure 1…100bar	Fair	Excellent	Excellent	Fair	Good	Fair
High pressure 100…1000bar	Bad	Good	Fair	Bad	Excellent	Good
Very high pressure > 1000bar	Bad	Bad	Bad	Bad	Good	Excellent
Absolute pressure ranges	Fair	Excellent	Good	Good	Bad	Bad
Accuracy	Excellent	Good	Poor	Fair	Good	Fair
Output Sensitivity	Excellent	Excellent	Fair	Excellent	Good	Fair
Frequency response	Excellent	Good	Fair	Poor	Good	Fair
Comparison rating (90 max)	56	63	59	61	62	57

Which technology to use for your application?

Find out whether variable capacitance ceramic pressure sensors will be the most suitable choice for your application compared to other technologies.

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Product Requirements Checklist

This comprehensive checklist serves as a guide for selecting and specifying the ideal ceramic variable capacitance pressure sensor for your specific application. It covers essential parameters such as pressure range, accuracy, overpressure protection, and output signal type. It also delves into specific details related to ceramic diaphragms, including material selection, temperature range, and compatibility with process media. Electrical requirements, environmental considerations, and additional factors like response time and long-term stability are also addressed. By carefully reviewing and completing this checklist, you can ensure that the chosen sensor aligns with your needs and performs optimally in your intended application.

General Requirements:

Pressure Range:
- Minimum pressure (e.g., 0.1 psi, 10 mbar)
- Maximum pressure (e.g., 100 psi, 7 bar)
Pressure Type:
- Gauge (relative to atmospheric pressure)
- Absolute (relative to vacuum)
Differential (difference between two pressures)
Accuracy:
- Percentage of full scale (%FS)
- Percentage of reading (%RD)
Overpressure Protection:
- Maximum allowable overpressure without damage
Output Signal:
- Analog (e.g., voltage, current)
- Digital (e.g., I2C, SPI)

Specific to Ceramic Diaphragm:

Diaphragm Material:
- Specific ceramic type (e.g., alumina, zirconia)
- Thickness
- Coating (if any)
Wetted Parts Materials:
- Compatibility with process media (e.g., chemical resistance)
Temperature Range:
- Operating temperature range (e.g., -40°C to 125°C)
- Storage temperature range

Electrical Requirements:

Supply Voltage:
- Nominal voltage
- Allowable range
Current Consumption:
- Maximum current draw
Output Impedance:
- For analog output
Electrical Connections:
- Connector type (e.g., cable gland, terminal block)
- Wiring diagram

Environmental Requirements:

Vibration and Shock Resistance:
- Specify levels and durations
Electromagnetic Compatibility (EMC):
- Compliance with relevant standards (e.g., CE, FCC)

Additional Considerations:

Response Time:
- Time to reach 90% of final value
Long-Term Stability:
- Drift over time (e.g., %FS/year)
Hysteresis:
- Maximum difference between increasing and decreasing pressure readings
Certifications:
- Industry-specific certifications (e.g., ATEX for hazardous areas)
Package/Housing:
- Material (e.g., stainless steel)
- Ingress protection (IP) rating
Mounting:
- Thread type, flange type, etc.

By carefully specifying these parameters, you can ensure that the selected ceramic variable capacitance pressure sensor meets the requirements of your specific application.