Overview of Pressure Transmitters

The Smart Measurement ALDPT series of pressure transmitters are used for measuring process pressure, hydrostatic level, and liquid or gas volumetric or mass flow rates in all kinds of process applications. Within the ALDPT Series there are five different types of instruments to accommodate the wide variety of applications that exist in industry. SmartMeasurement’s staff of experienced application engineers is available to help users select among these choices and specify the instrument that provides the best solution for any pressure measurement application.

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Why Choose a Pressure Transmitter

Pressure measurement is the most widely measured variable in industrial process control applications.  Pressure measurement is commonly performed in applications involving all media phases including liquid, gas, and steam. For example, in the oil & gas industry, wellhead pressures are measured in subsea and topside locations. In water towers, pressure is measured to monitor and control water levels.  Pressure sensors are used to monitor the water line pressure in automatic sprinkler systems. Barometric pressure is used to predict the weather.

Special Cases

The pressure transmitter plays an integral role in applications where flow, level, and temperature measurements are performed. A differential pressure (DP) transmitter can be used to measure both flow and differential pressure when used in conjunction with a primary flow element. They may also infer liquid level as a function of pressure differential in sealed, pressurized tanks. Some pressure transmitters also utilize temperature sensors to measure process temperature. This type of pressure transmitter may be used to calculate mass flows of compressible media such as steam and natural gas when used in conjunction with a primary flow element.

Benefits of Pressure Transmitters 

Pressure transmitters offer numerous advantages across various industrial applications, making them indispensable in modern process control and monitoring systems. Here are some key benefits:

Accuracy and Reliability

Pressure transmitters provide highly accurate measurements, often within ±0.075% of the calibrated range. This precision is crucial for maintaining optimal process efficiency and product quality.

Versatility

From gauge pressure transmitters to differential pressure transmitters, these devices can measure various pressure types, adapting to diverse industrial needs.

Remote Monitoring

Pressure transmitters can send data over long distances, enabling centralized control and monitoring of multiple pressure points across a facility.

Durability

Modern pressure sensors are designed to withstand harsh industrial environments, including extreme temperatures, vibrations, and corrosive substances.

Cost-Effective 

While initial investment may be higher than simpler pressure transducers, the long-term benefits of pressure transmitters often result in significant cost savings through improved process efficiency and reduced maintenance.

Safety Enhancement

By continuously monitoring pressure levels, these devices help maintain normal operating pressure, preventing potentially dangerous situations in high-pressure systems.

Digital Integration 

Many pressure transmitters offer digital output options, facilitating easy integration with modern control and data acquisition systems.

Diagnostics 

Advanced pressure transmitters often include self-diagnostic features, alerting operators to potential issues before they become critical.

By leveraging these benefits, industries can significantly improve their processes, enhance safety, and optimize resource utilization. Whether it’s in oil refineries, chemical plants, or water treatment facilities, pressure transmitters play a crucial role in ensuring efficient and safe operations.

Key Applications 

Process Control

Pressure transmitters are crucial in process control, providing real-time data for maintaining optimal operating pressure and flow conditions using a primary element flow body. They help ensure product quality and process efficiency across various industries. 

Liquid Level Measurement

Pressure transmitters, including differential pressure transmitters, accurately determine liquid levels by measuring hydrostatic pressure. This method is particularly useful for large storage tanks and challenging environments. 

Primary Flow Elements

Differential Pressure transmitters used with a primary flow element like orifice plates, venturis, Vcone etc. and a flow computer can measure difficult flows such as multi-phase, extremely high temperatures and/or pressures.  Oftentimes traditional flowmeters would fail in these difficult applications.  

Pump & Compressor Control

Pressure transmitters monitor and control pump and compressor operations, preventing cavitation and ensuring efficient performance. They help optimize energy consumption and extend equipment life by maintaining normal operating pressure. 

Hydrostatic Level Measurement

For deep tanks or those with difficult-to-access tops, hydrostatic level measurement using pressure transmitters is ideal. This method provides accurate level data without the need for top-mounted sensors. 

Oil & Gas

In the oil and gas industry, pressure transmitters and transducers are essential for wellhead monitoring, pipeline management, and custody transfer. They withstand harsh conditions while providing critical pressure data. 

Chemical & Petrochemical

Pressure transmitters in chemical plants ensure safe operations and product quality. They monitor reactor pressures, control distillation processes, and assist in leak detection. 

Liquid Storage Tanks

For inventory management and overfill prevention in liquid storage tanks, gauge pressure transmitters offer reliable level measurement. They can be easily installed and maintained, even on large tanks. 

Food & Beverage

In food and beverage processing, sanitary pressure transmitters monitor critical pressures in tanks, pipes, and packaging equipment. They help maintain product quality and ensure compliance with hygiene standards. 

Pharmaceutical

Pressure transmitters in pharmaceutical manufacturing ensure precise control in critical processes like fermentation and filtration. They help maintain the sterility and quality of pharmaceutical products. 

Marine & Shipping

On ships and offshore platforms, pressure transmitters monitor hull stress, tank levels, and engine performance. They provide crucial data for safe and efficient maritime operations.

Selecting a Pressure Transmitter 

Choosing the ideal pressure transmitter for your application demands careful consideration of various factors. This critical decision can significantly impact process efficiency, safety, and overall system performance. Here’s a comprehensive guide to help you navigate the selection process:

  1. Measurement Type
    Determine the specific pressure measurement required:
    Absolute pressure transmitters: Measure pressure relative to a perfect vacuum
    Gauge pressure transmitters: Measure pressure relative to atmospheric pressure
    Differential pressure transmitters: Measure the difference between two pressure points.

  2. Pressure Range and Turndown Ratio
    Select a transmitter that covers your normal operating pressure and potential extremes. Consider the turndown ratio – the ratio between the maximum and minimum measurable pressures. A higher turndown ratio offers greater flexibility but may come at a premium.

  3. Accuracy and Precision
    Evaluate the required level of accuracy for your application. High-precision pressure transmitters might be essential for custody transfer or critical process control, while standard accuracy may suffice for general monitoring. Remember, accuracy often degrades at the extremes of a transmitter’s range.

  4. Environmental Considerations
    Assess the operating environment, including:
    • Temperature extremes and fluctuations
    • Humidity and potential for condensation
    • Exposure to corrosive substances or abrasive materials
    • Vibration and shock levels
    Choose a transmitter with appropriate materials, seals, and ratings to withstand these conditions without compromising performance.

  5. Output Signal and Communication Protocols
    Determine the required output signal based on your control system’s compatibility:
    Analog: 4-20 mA is common for its noise immunity
    Digital: HART, Fieldbus, or Profibus for advanced diagnostics and multi-variable readings
    Wireless: For hard-to-reach locations or rotating equipment

  6. Safety and Compliance
    Ensure the pressure transmitter meets necessary certifications:
    • Intrinsic safety or explosion-proof ratings for hazardous areas
    • Industry-specific certifications (e.g., 3-A for sanitary applications)
    • Regulatory compliance (e.g., FDA, ASME, PED)

  7. Advanced Features
    Consider additional capabilities that might benefit your application:
    • Multivariable measurements (e.g., simultaneous pressure and temperature readings)
    • Built-in diagnostics for predictive maintenance
    • Local displays for easy reading in the field
    • Customizable damping to smooth out rapid pressure fluctuations

  8. Total Cost of Ownership
    Look beyond the initial purchase price and consider:
    • Installation costs and ease of integration
    • Long-term stability and recalibration requirements
    • Power consumption, especially for remote or battery-powered installations
    • Availability of spare parts and technical support

  9. Compatibility with Process Media
    Ensure the transmitter’s wetted parts are compatible with your process fluid to prevent corrosion or contamination. This is particularly crucial in industries like food and beverage or pharmaceuticals where product purity is paramount.

  10. Future-Proofing
    Consider potential changes in your process or industry regulations:
    • Scalability for potential range adjustments
    • Firmware upgradability for new features or improved performance
    • Compatibility with emerging IIoT (Industrial Internet of Things) platforms

By meticulously evaluating these factors, you can select a pressure transmitter that not only meets your current needs but also provides long-term value and reliability. Whether you’re monitoring compressed air systems, measuring hydrostatic tank levels, or controlling critical processes in chemical manufacturing, the right pressure transmitter will serve as a cornerstone of your instrumentation strategy, ensuring optimal performance, efficiency, and safety in your operations.

Troubleshooting Pressure Transmitters 

Even the most reliable pressure transmitters can occasionally encounter issues. Proper troubleshooting techniques can help identify and resolve problems quickly, minimizing downtime and ensuring accurate measurements. Here are some common issues and their potential solutions:

  1. Zero Offset or Drift
    Symptom: Transmitter output doesn’t return to zero when pressure is removed
    • Possible Causes: Temperature fluctuations, mechanical stress, or sensor degradation
    Solution: Perform a zero calibration or adjust the zero point. If persistent, consider recalibrating or replacing the transmitter

  2. Erratic or Noisy Output
    Symptom: Unstable or fluctuating pressure readings
    Possible Causes: Electrical interference, loose connections, or process turbulence
    Solution: Check wiring and grounding, ensure proper shielding, and verify installation in a low-turbulence area of the process

  3. Non-Linear Response
    Symptom: Transmitter output doesn’t correspond linearly to pressure changes
    Possible Causes: Improper calibration, damaged sensor, or operation outside the specified range
    Solution: Verify operating conditions, recalibrate if necessary, or consider replacing the transmitter

  4. No Output Signal
    • Symptom: Transmitter produces no output despite applied pressure
    • Possible Causes: Power supply failure, damaged electronics, or severed signal wire
    • Solution: Check power supply and wiring, verify transmitter electronics are functional

  5. Slow Response Time
    Symptom: Transmitter output lags behind pressure changes
    Possible Causes: Clogged impulse lines, incorrect damping settings, or damaged sensor
    Solution: Clean impulse lines, adjust damping settings, or inspect the sensor for damage

  6. Overpressure Damage
    Symptom: Transmitter output becomes erratic or fixed after a pressure spike
    Possible Causes: Exposure to pressure beyond the transmitter’s rated capacity
    Solution: Verify if the transmitter has overpressure protection. If damaged, replacement may be necessary

  7. Environmental Factors
    Symptom: Transmitter performance degrades in certain conditions
    Possible Causes: Temperature extremes, humidity, or chemical exposure
    Solution: Ensure the transmitter is rated for the operating environment. Consider adding protective measures or choosing a more suitable model

  8. Communication Errors (for smart transmitters)
    Symptom: Unable to communicate with the transmitter using HART or other protocols
    Possible Causes: Incorrect configuration, damaged communication circuit, or interference
    Solution: Verify communication settings, check for physical damage, and ensure proper wiring and termination

When troubleshooting pressure transmitters, always consult the manufacturer’s documentation for specific guidance. For complex issues or when dealing with critical applications, consider seeking assistance from the manufacturer or a qualified instrumentation specialist. Regular maintenance and calibration can prevent many common issues, ensuring your pressure transmitters provide reliable, accurate measurements across their entire operating range.

Theories of Operation

Pressure is defined as the exertion of force per unit area.  There are many different instruments used for pressure measurement, including pressure gauges, pressure sensors, pressure transducers, as well as pressure transmitters. There are also specialized devices for measuring pressure extremes, such as vacuum pressure.

Pressure Sensors

Theory Of Operation Of Diagram Seal Pressure sensors operate by sensing pressure and converting it into an electrical quantity. Piezoresistive and capacitive are the two most commonly seen types in industrial applications with piezoresistive being the most commonly used.

The electrical resistance of piezoresistive materials, measured in Ohms (Ω), will change when they are strained or compressed. A piezoresistive pressure sensor consists of a micro-machined silicon diaphragm that has a piezoresistive strain gauge diffused into it. The diaphragm is fused to a silicon or glass backplate. The sensor contains resistors that are typically arranged in the form of a Wheatstone Bridge Circuit. As pressure increases on the piezoresistive material, it is more resistant to the electrical current passing through it. This results in the output of the Wheatstone Bridge, measured in millivolts, being directly proportional to pressure.

Capacitive pressure sensors use a thin diaphragm that serves as one capacitor plate. This diaphragm is usually a metal or metal-coated quartz component. The diaphragm is exposed to a reference pressure on one side and to the process pressure on the other. Changes in pressure will cause a slight deformation of the pressure sensing plate, which will in turn cause changes in the electrical capacitance. These changes in capacitance are directly proportional to the pressure applied to the plate that is exposed to the process. 

Pressure sensors detect variations in pressure by converting changes in resistance or capacitance into electrical values. These electrical signals are relatively weak and require amplification to carry these values over a distance. Pressure transmitters are typically made up of a pressure sensor, an amplifier, and signal conditioning circuitry which converts the raw signal into an industry standard output such as 4-20 mA or a frequency/pulse signal which may be transmitted over a long distance. If the raw signal is converted to a 0-5 or 0-10 VDC signal which may only be transmitted over an intermediate distance, the device may be referred to as a pressure transducer. The output signal from the pressure transmitter or pressure transducer relays the pressure reading to a flow computer, controller, or distributed control system (DCS).  Pressure transmitters also typically contain an LED or LCD display element that allows the user to view the measured pressure value directly at the point of measurement.