Understanding Pressure: Types and Measurement

• Pressure is the force exerted per unit area.
• There are different types of pressure measurements including gauge pressure, absolute pressure, and differential pressure.

Vacuum

• A vacuum refers to a space where the pressure is lower than atmospheric pressure.
• This is achieved by evacuating air from a sealed space until it contains little to no air.

Absolute Pressure

• Is the pressure measured relative to a perfect vacuum.

Atmospheric Pressure

• Is the average pressure exerted by the Earth's atmosphere.

Overpressure

• Is pressure above atmospheric pressure.

Underpressure

• Is pressure below atmospheric pressure.

Differential Pressure

• Is the difference between two absolute pressures.

Membrane Pressure Meter

• A membrane pressure meter consists of a thin metal plate clamped between two flanges.
• When pressure is applied, the membrane deforms, moving a pointer.
• These meters are robust and can measure small pressures (8-16 mbar) to medium pressures (10-40 bar).

Bourdon Tube Manometer

• The lower the measuring range, the larger the area of the membrane needs to be.
• This type of meter can be used for absolute pressure measurements.
• The meter can be connected via a capillary tube to measure pressure from a distance, often using a Bourdon tube.
• These type of meters are similar to the membrane pressure meters.
• The Bourdon tube is made of metal and must be of a certain length to allow movement.
• These meters are sensitive to vibrations and temperature fluctuations.
• While aggressive substances can damage the meter, Bourdon tube manometers are more accurate (0.5% to 1.6%) and cheaper than membrane pressure meters.

Capacitive Pressure Meter

• When plate B deforms under pressure, the distance changes and thus the capacitance of the capacitor changes.
• This change is converted into a standard current or voltage signal using a transmitter.
• These meters are accurate (0.1%) and can be used in barometric applications.

Strain Gauges

• Strain gauges are made of resistance wire that is flexible.
• They can be bent when pressure is applied, becoming longer and thinner, which changes the resistance.
• The strain gauge is attached to a membrane in a pattern of two or four.

Zero and Span

• Zero is the temperature that corresponds to 4 mA.
• Span is the difference in temperature between the maximum temperature and the zero temperature.

Standard Signals

• The output of a sensor is usually a 4-20 mA signal.
• Other standards exist (0-20 mA, 0-10V, 1-10V)
• However, 4-20 mA is the most commonly used.
• Voltage standards are only used when the sensor and regulator are close together.
• If this is not the case, voltage losses can lead to poor measurement results.
• Current standard is the industry standard
• The 4-20 mA current signal is less susceptible to interference and there are no voltage losses.
• The sensor, power supply, and regulator are placed in series.
• The current in a series circuit is the same everywhere.
• The HART protocol is compatible with the 4-20 mA signal, where a digital HART signal is placed on the analog 4-20mA signal.

Digital Signals

• A 1200 Hz signal represents a logical "1", and a 2200 Hz signal represents a logical "0".
• A special module filters the "1"s and "0"s out of the signal and extracts the information.

Measuring Pressure

• Pressure is the force exerted per unit area.

Types of Forces

• pressure force
• gravity
• frictional force
• centripetal force.
• A fluid exerts pressure, causing deformation.

Equations

• Pressure = Force / Area
• 1 Pascal (Pa) = 1 Newton / meter² (N/m²)

Hydrostatic Pressure

• The pressure exerted by a fluid is determined by hydrostatic pressure
• P = ρ * g * h, where P is pressure in Pascal, ρ is density, g is gravitational acceleration, and h is depth.

Units of Measure

• 1 bar = 100,000 Pa = 10 N/mm²
• 1 atmosphere (ATM) = 101325 Pa = 1013.25 hPa = 1.01325 bar.
• Pounds per square inch (PSI): 1 PSI = 6894.757 N/m².

Transmitters

• Transmitters convert temperature to voltage or current.
• Regulators with direct PT100 usually contain a circuit that converts the resistance change of the PT100 into a voltage change.
• This circuit is called a Wheatstone bridge.

Four-Wire Connection

• Since the PT100 resistance is small, the connection wires can influence the measurement
• Three-wire arrangements, connecting the sensor using three identical wires of the same length, thickness and material can eliminate this error.
• In instruments where the four wires are not identical, a four-wire setup is necessary.

Thermocouples

• A thermocouple consists of two wires made of different metal alloys joined together.
• One junction is the hot junction and the other is the cold junction.
• When the hot junction is heated, a voltage is generated.
• When a metal or alloy is heated, electrons are "pushed" away from the hot end, which causes a voltage difference between the two ends.
• Can be connected the regulator or PLC.
• Connections do not affect the signal

RTD vs Thermocouples

• RTDs have better accuracy
• Thermocouples are faster
• RTDs are smaller
• Thermocouples are cheaper.