Instrument Consultant Program: Pressure Measurement

Pressure Measurement Units

• Pascal (Pa) is the SI unit of pressure, defined as one Newton per square meter (N/m²) and is not referenced to a column of fluid.
• Conversion factor from bar to psi: 1 bar = 14.5038 psi
• Conversion of 40 bar to psi: 40 bar * 14.5038 psi/bar = 580.152 psi

Density and Temperature

• Temperature affects liquid density: A liquid's density changes with temperature, increasing as temperature decreases and vice versa.

Pressure Measurement Units and Applications

• Safe for liquid applications: Units like psi, bar, and kPa are commonly used for measuring liquid pressures.
• Primary reason for industrial pressure measurement: Process control, monitoring, and safety are key reasons for measuring pressure in industrial settings.
• Differential pressure measurement: Used to gauge the pressure difference between two points, often for flow measurement or level indication.

Mechanical Pressure Gauges

• Function of Bourdon tube gauges: These gauges translate pressure into physical movement of a pointer across a calibrated scale.
• Gas pressure and container: Gas pressure is directly related to the volume of the container it occupies; an increase in volume leads to a decrease in pressure, and vice versa.

Types of Electronic Pressure Transmitters

• Strain gauge: One of the common types of electronic pressure transmitters.

Pressure Measurement Terms and Concepts

• Head pressure: Refers to the pressure exerted by a column of liquid due to its height and density.
• Benefit of accurate pressure measurement: Optimal process efficiency, reliable system performance, and improved safety.
• Distinction between absolute and gauge pressure: Absolute pressure measures pressure relative to a perfect vacuum, while gauge pressure measures pressure relative to atmospheric pressure.
• Effect of increased temperature on gas pressure: An increase in temperature leads to an increase in gas pressure within a tank.

Importance of Pressure Measurement in Process Industries

• Reasons for pressure measurement: Safety, process control, monitoring, equipment protection, and efficiency analysis.
• Preventing equipment failure: Accurate pressure measurements help prevent system overpressure or underpressure conditions that can lead to equipment damage.
• Inferred measurement variable associated with pressure: Flow, level, and density are inferred variables often associated with pressure measurement.

Cost savings and Risk Reduction through Pressure Measurement

• Cost saving: Efficient process control leads to reduced waste, improved efficiency, and lower energy consumption.
• Risk reduction: Accurate pressure measurement mitigates risks associated with equipment failure, process instability, and safety hazards.
• Process efficiency and pressure control: Precise pressure control directly contributes to optimizing process efficiency.

Pressure Measurement Accuracy and its Significance

• True statement about pressure measurement: Maintaining accurate pressure measurement ensures reliable process control and minimizes safety risks.
• Pressure at the bottom of a liquid column: When density remains constant, the pressure increases linearly with the height of the liquid column.

Density, Specific Gravity, and Pressure

• Effect of temperature on liquid density: Generally, a liquid's density decreases with increasing temperature.
• Specific gravity of a liquid: The specific gravity is 0.8 for a liquid with a density of 800 kg/m³ (water density at 4°C is 1000 kg/m³).
• Top pressure in a closed tank: Refers to the pressure at the surface of the liquid in a closed tank.
• Gasoline density: Gasoline with a specific gravity of 0.66 has a density of 660 kg/m³ at 50 °F (10°C).
• Pressure in a vacuum above a liquid: The pressure at the surface of the liquid in a vacuum is near absolute zero pressure.

Hydrostatic Pressure and its Factors

• Factor not affecting hydrostatic pressure: The shape of the container holding the liquid does not influence hydrostatic pressure.
• Equation for mass, density, and volume: Mass = Density * Volume

Types of Pressure Transmitters and Their Characteristics

• Reference for gauge pressure transmitters: Gauge pressure is relative to atmospheric pressure.
• Output of a gauge pressure sensor: The output of a gauge pressure sensor reflects the difference between the measured pressure and atmospheric pressure.
• Advantage of differential pressure measurements: They are less susceptible to fluctuations in atmospheric pressure.
• Connections in differential pressure instruments: Differential pressure instruments typically have two connections, one for high pressure and one for low pressure.
• Significance of 'g' in pressure units: 'g' denotes gauge pressure, which is referenced to atmospheric pressure.

Pressure Measurement Applications and Considerations

• Reason behind lower gauge pressure than atmospheric pressure: A potential vacuum condition exists within a closed system.
• Approximate atmospheric pressure in psi: Atmospheric pressure is around 14.7 psi at sea level.
• Specific gravity calculated in pressure measurement: Measured density divided by a reference liquid density (often water).
• Standard liquid column height for density calculations: 1 meter (3.28 feet)

Interface Measurement and Applications

• Interface measurement identification: Determines the boundary line between two immiscible liquids based on their densities.
• Atmospheric pressure measurement instrument: A barometer is used to measure atmospheric pressure.
• Role of pressure measurement in fermentation: Monitoring pressure helps control the fermentation process, as it can indicate the production of gases or pressure buildup within the fermentation vessel.
• Oil and water relationship in interface measurement: Oil typically floats on top of water due to its lower density, providing a clear interface.

Electronic Pressure Transmitters and Other Applications

• Capacitance-based pressure transmitter: A capacitance-based electronic pressure transmitter operates based on changes in capacitance caused by pressure variations.
• Determining when to stop pouring off top fluid: You would use a level sensor or interface sensor to determine when to stop pouring off the top fluid in a tank.