15.2 Pressure and Temperature Measurements
4 min read•july 19, 2024
Pressure and temperature measurement devices are crucial tools in fluid mechanics. From manometers to pressure gauges, these instruments help engineers quantify fluid behavior. Understanding their principles and applications is essential for accurate data collection and analysis in fluid systems.
Temperature measurement techniques like thermocouples and RTDs provide insights into fluid thermal properties. Selecting the right device based on accuracy, environmental factors, and process conditions is key. Proper calibration and installation ensure reliable measurements, critical for effective fluid mechanics studies and applications.
Pressure Measurement Devices
Principles of pressure measurement devices
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- Manometers balance fluid pressure against a liquid column
- uses a U-shaped tube filled with liquid (mercury or water) to determine pressure difference by measuring height difference between the two legs
- similar to U-tube but with one leg inclined provides higher resolution for small pressure differences
- Pressure gauges measure pressure by deformation of a sensitive element
- contains a curved, hollow tube that straightens when pressurized and translates movement to a pointer on a calibrated scale
- uses a flexible diaphragm that deflects under pressure and measures the deflection to convert it to a pressure reading
- Pressure transducers convert pressure into an electrical signal
- measures strain on a diaphragm or pressure-sensitive element using strain gauges that convert mechanical strain into electrical resistance change
- measures change in capacitance between a diaphragm and fixed plate that varies with pressure-induced deflection of the diaphragm
- uses a piezoelectric material generating an electrical charge when subjected to pressure suitable for measuring dynamic pressures and fluctuations
Temperature Measurement Techniques
Temperature measurement techniques
- Thermocouples consist of two dissimilar metals joined at one end (hot junction)
- Temperature difference between hot junction and reference junction (cold junction) generates a voltage (Seebeck effect)
- Voltage is measured and converted to temperature using standard tables or polynomial equations
- Different types of thermocouples (Type J, K, T) suitable for various temperature ranges and environments
- Resistance Temperature Detectors (RTDs) measure temperature by change in electrical resistance of a metal (usually platinum) with temperature
- Resistance increases with temperature in a predictable manner
- commonly used to measure resistance change accurately
- Offer high accuracy and stability over a wide temperature range
- Infrared thermometers measure temperature by detecting infrared radiation emitted by an object
- Non-contact measurement technique suitable for measuring temperature of moving objects or in hazardous environments
- Emissivity of the object must be considered for accurate measurements
- Emissivity is the ratio of energy radiated by an object to that radiated by a perfect blackbody at the same temperature and varies with material, surface finish, and temperature
Accuracy factors in measurements
- Calibration is essential to maintain accuracy and should be performed regularly against known standards traceable to national or international standards
- Environmental factors can affect measurements
- Temperature variations cause or contraction of pressure-sensing elements but compensation techniques (temperature-compensated pressure transducers) can minimize effects
- Ambient temperature fluctuations influence temperature measurements but proper insulation and shielding of sensors can reduce the impact
- Sensor placement and installation are crucial for representative measurements
- Pressure sensors should avoid areas with flow disturbances (bends or valves) that cause localized pressure variations
- Temperature sensors should be in direct contact with process fluid or installed in a thermowell with adequate immersion depth to minimize stem conduction errors
- Extreme temperatures, high pressures, and corrosive or abrasive media can affect sensor performance and longevity so selecting suitable materials and operating ranges is crucial for reliable measurements
- Proper shielding, grounding, signal conditioning (amplification and filtering), and digital communication protocols (HART or Fieldbus) ensure accurate, stable measurements and improve signal integrity
Selection of measurement devices
- Consider process fluid properties
- Chemical compatibility of sensor materials with process fluid
- Operating temperature and pressure ranges
- Presence of particulates, bubbles, or contaminants that may affect measurement
- Determine required accuracy and precision and select a device with appropriate specifications for the application considering impact of measurement errors on process control and product quality
- Evaluate process conditions
- Ambient temperature and pressure fluctuations
- Vibration, shock, or other mechanical stresses
- Corrosive, abrasive, or hazardous environments may require special sensor materials or protective coatings
- Consider installation requirements
- Available space for sensor installation
- Accessibility for maintenance and calibration
- Need for remote monitoring or wireless communication
- Assess cost and maintenance requirements
- Initial cost of measurement device
- Long-term maintenance and calibration costs
- Expected sensor lifespan under given process conditions
- Consult industry standards and guidelines
- Refer to relevant standards (ASME PTC 19.2 for pressure measurement, ASTM E230 for temperature measurement) for best practices and recommendations
- Consider regulatory requirements for hazardous areas or food and pharmaceutical processing
Key Terms to Review (27)
Absolute Pressure: Absolute pressure is the total pressure exerted on a system, measured relative to a complete vacuum. This measurement is crucial in understanding how fluids behave under different conditions and helps in determining forces acting within fluid systems, making it fundamental in pressure measurement, distribution, and instrumentation.
Bimetallic thermometer: A bimetallic thermometer is a temperature measuring device that utilizes the principle of thermal expansion of two different metals bonded together, which allows for the measurement of temperature changes. As temperature increases, the different rates of expansion cause the bimetallic strip to bend, moving a pointer on a dial to indicate the temperature. This type of thermometer is widely used in various applications due to its durability and accuracy in measuring temperatures across a range of environments.
Bourdon tube gauge: A bourdon tube gauge is a mechanical device used to measure pressure, consisting of a curved tube that straightens when pressure is applied. This change in shape translates into a movement that drives a needle on a dial, indicating the pressure value. The design of the bourdon tube allows for accurate and reliable measurements across a range of pressures, making it widely used in various industries for monitoring and control purposes.
Capacitive pressure transducer: A capacitive pressure transducer is a type of sensor that measures pressure by converting changes in capacitance caused by the movement of a diaphragm in response to pressure variations. This device operates based on the principle that the capacitance between two conductive plates changes as the distance between them varies, allowing it to provide precise pressure measurements.
Celsius: Celsius is a temperature scale used to measure thermal energy, where 0 degrees represents the freezing point of water and 100 degrees represents the boiling point at standard atmospheric pressure. This scale is widely used in scientific contexts, especially in fields like physics and engineering, making it essential for understanding temperature-related phenomena.
Density: Density is the mass per unit volume of a substance, typically expressed in units like kg/m³. It plays a crucial role in determining how fluids behave under various conditions, influencing buoyancy, pressure distribution, and flow characteristics.
Diaphragm gauge: A diaphragm gauge is a type of pressure measurement device that uses a flexible diaphragm to detect changes in pressure. When pressure is applied, the diaphragm deforms, which translates into a mechanical displacement that can be measured and displayed. This device is commonly used to measure both gas and liquid pressures, making it versatile for various applications.
Dynamic measurement: Dynamic measurement refers to the process of capturing and analyzing the time-dependent behavior of physical quantities, such as pressure and temperature, as they change in response to varying conditions. This type of measurement is crucial for understanding how fluid systems behave under dynamic conditions, providing insights into their performance and stability. It involves techniques that can monitor rapid fluctuations and transient states, making it essential for applications in engineering and scientific research.
Gauge pressure: Gauge pressure is the pressure of a fluid relative to atmospheric pressure. It is measured using a gauge that subtracts the atmospheric pressure from the total pressure, providing a reading that reflects only the pressure exerted by the fluid itself. This concept is crucial when understanding how fluids behave in different environments and is foundational for various pressure measurement methods.
Ideal Gas Law: The Ideal Gas Law is a fundamental equation that describes the behavior of an ideal gas, relating its pressure, volume, temperature, and number of moles through the equation $$PV = nRT$$. This law is essential for understanding the thermodynamic properties of fluids, as it provides a framework for analyzing how gases behave under various conditions. It also plays a crucial role in describing isentropic flow and helps in making accurate pressure and temperature measurements in fluid systems.
Inclined manometer: An inclined manometer is a type of pressure measuring device that consists of a U-shaped tube, with one side open to the atmosphere and the other connected to a pressure source. This design allows for more precise measurements of low-pressure differences by amplifying small changes in height due to its angle. The inclined setup increases the sensitivity of the measurement, making it particularly useful for applications where accurate pressure readings are critical.
Infrared thermometer: An infrared thermometer is a non-contact temperature measurement device that detects infrared radiation emitted by an object to determine its temperature. This type of thermometer is particularly useful in applications where direct contact with the surface being measured is impractical, allowing for quick and efficient readings without the need for physical interaction.
Liquid-in-glass thermometer: A liquid-in-glass thermometer is a device used to measure temperature, consisting of a glass tube filled with a liquid, typically mercury or colored alcohol, which expands and contracts in response to temperature changes. This type of thermometer operates on the principle that liquids expand when heated and contract when cooled, allowing for accurate readings of temperature by observing the liquid level against a calibrated scale.
Manometer: A manometer is a device used to measure pressure, often by comparing the pressure of a fluid to a known reference pressure. It typically consists of a U-shaped tube filled with a liquid, which moves in response to pressure changes, allowing for the determination of pressure differences in fluids. Manometers are fundamental in various applications, such as determining fluid flow, monitoring system pressures, and ensuring safety in engineering systems.
Pascal: Pascal is the SI unit of pressure, defined as one newton per square meter. It quantifies the force applied per unit area and is crucial for understanding how fluids behave under various conditions. This measurement is essential in analyzing physical properties of fluids, how pressure is distributed within them, and the losses encountered in pipe systems, as well as in taking accurate pressure and temperature measurements.
Piezoelectric pressure transducer: A piezoelectric pressure transducer is a device that converts mechanical pressure into an electrical signal through the piezoelectric effect. This type of transducer is widely used in various applications due to its ability to provide accurate and rapid pressure measurements, making it essential in fields where precise data on fluid dynamics and system behavior are crucial.
Pressure Gauge: A pressure gauge is a device used to measure the pressure of gases or liquids within a system. It provides critical information that helps in monitoring and controlling processes, ensuring safety and efficiency in various applications, such as HVAC systems, pipelines, and water supply systems. Understanding the readings from a pressure gauge is essential for maintaining optimal operating conditions and preventing equipment failure.
Pressure Transducer: A pressure transducer is a device that converts pressure into an electrical signal, allowing for the measurement and monitoring of fluid pressure in various applications. These devices play a crucial role in ensuring accurate pressure readings in systems, enabling engineers and technicians to assess and control fluid dynamics effectively. Their applications range from industrial processes to scientific research, making them essential for understanding pressure behaviors in different contexts.
Resistance Temperature Detector (RTD): A Resistance Temperature Detector (RTD) is a temperature sensor that operates on the principle that the electrical resistance of certain metals changes with temperature. Typically made from pure platinum, RTDs provide accurate and stable temperature measurements, making them essential in various industrial applications. Their ability to offer precise readings over a wide range of temperatures allows them to be crucial for processes requiring stringent temperature control.
Specific Heat Capacity: Specific heat capacity is the amount of heat energy required to raise the temperature of a unit mass of a substance by one degree Celsius. This property is essential for understanding how materials respond to changes in temperature and is crucial in the context of pressure and temperature measurements, as it influences how fluids behave when subjected to thermal variations.
Static measurement: Static measurement refers to the technique of assessing physical properties such as pressure or temperature when the fluid is at rest or not in motion. This method is essential in understanding fluid behavior under stable conditions, providing a baseline for analyzing dynamic changes when the fluid starts to flow. Accurate static measurements are critical for designing systems, calibrating instruments, and conducting experiments in fluid mechanics.
Strain gauge pressure transducer: A strain gauge pressure transducer is a device that converts pressure into an electrical signal by using the principle of strain gauges, which measure the deformation or strain of a material when subjected to pressure. These transducers are widely used in various applications for accurate and reliable pressure measurements, often found in industrial settings and research laboratories.
Thermal expansion: Thermal expansion refers to the increase in volume of a substance as its temperature rises. This physical phenomenon occurs because the particles within the substance move more rapidly and require more space as they gain heat energy. Understanding thermal expansion is crucial for analyzing how fluids behave under temperature changes, how materials interact in different thermal environments, and how pressure and temperature measurements are impacted in various systems.
Thermocouple: A thermocouple is a temperature measurement device made up of two different metals joined at one end, which generates a voltage proportional to the temperature difference between that junction and the other ends. This makes thermocouples very useful in measuring temperature across a wide range of conditions, providing quick response times and a simple construction. They are commonly used in various applications, including industrial processes, HVAC systems, and scientific research.
U-tube manometer: A u-tube manometer is a device used to measure the pressure difference between two points in a fluid system, utilizing a U-shaped tube filled with liquid, typically mercury or water. The pressure difference causes the liquid to rise or fall in each leg of the tube, allowing for the determination of pressure in various applications, such as in fluid mechanics and engineering fields. Its simplicity and effectiveness make it a fundamental tool in measuring static and dynamic pressures.
Viscosity: Viscosity is a measure of a fluid's resistance to deformation and flow, essentially describing how thick or sticky a fluid is. This property plays a crucial role in understanding fluid behavior under different conditions and directly affects various phenomena, such as flow rates, pressure distribution, and energy loss in fluid systems.
Wheatstone Bridge Circuit: A Wheatstone bridge circuit is an electrical circuit used to measure unknown resistances by balancing two legs of a bridge circuit. It works on the principle of null measurement, where the ratio of the resistances in one leg is equal to the ratio in the other leg, allowing for precise measurements of resistance, which is crucial in applications involving pressure and temperature measurements.