Chapter_9_Control Loops

Discrete Sensing Element: A sensing element that operates independently and is connected to the transmitter via sensor wires.
Impulse Tubing: A stainless steel tube used for transmitting process variables to the sensor in the transmitter.
Instrument Scale: A range of marks indicating numerical values for measuring a process variable.
Integrally Mounted Sensing Element: A sensing element that is physically part of the transmitter.
Linear Scaling: The relationship between two scales (input vs. output) that is linear.
Lower Range Value (LRV): The minimum value on the scale of a measuring instrument.
Operating Range: The difference between the upper (URV) and lower range values (LRV) on the scale.
Scaling: The process of equating numerical values of one scale to another.
Sensor: Detects the process variable and can be part of a transmitter.
Signal Converter Transducer: Converts the process variable into a standard instrument signal.
Span: The difference between URV and LRV of a scale, expressed as a single number.
Standard Signal: A common communication language used by instruments (e.g., 4-20 mA).
Upper Range Value (URV): The maximum value on the scale of a measuring instrument.

Focus on primary sensors, transmitters, and transducers within a control loop.
Transmitters combine sensing and measuring functions to produce a signal for other loop components.
Control loop components include the controller, recorder, indicator, PLC, and DCS.
Figure 9-1 illustrates a simple liquid-level measuring control loop.

Control loop functions include separate sensing and measuring roles.
Transmitter: Produces an output signal representing measurement information.
Both sensors and transducers may perform the sensing and measuring functions.
In sensor-driven loops, sensors serve directly as transducers to supply input to the controller.
In transmitter-driven loops, transducers convert the process variable into a standard instrument signal.

Sensors can be mechanical or electronic.
Examples include thermocouples, which connect directly to a controller.
The primary role of sensors is to detect process variables.
Discrete sensing elements are distinct devices connected to transmitters, while integrally mounted sensors are part of the transmitter.
Figure 9-3 shows examples of discrete temperature sensors such as thermocouples and RTDs.
Differential pressure (d/p) transmitters may have integrally mounted sensors requiring impulse tubing for measurement.

Transmitters: The primary method for providing measured inputs to control loops.
Transmitters can house primary sensors or be connected externally.
After sensing, transmitters convert measurements into standard instrument signals (e.g., 4-20 mA).
D/P transmitters measure the difference between two pressures, producing standardized outputs.
The d/p cell inside the transmitter is crucial for converting measurements into standard signals.
Various applications include level measurement in tanks through appropriate tubing connections.

Transmitters typically convert outputs to standard signals for compatibility in control loops.
Linear Scaling: Discussed primarily, as it simplifies input/output relationships.
Common Signals: 4-20 mA and 3-15 psig used for electronic and pneumatic signals, respectively.
Live zero value describes a signal's zero output as greater than absolute zero, facilitating easier calibration.

Calibration adjusts input values against output signals to correlate them accurately.
Various scaling terms explained include URV, LRV, and span.
Table 9-1 summarizes point calibration for temperature transmitters.

Transducers: Devices converting one form of energy into another (e.g., pressure to electrical signals).
Common transducer types include I/P transducers that convert electronic signals to pneumatic outputs.
This conversion is essential in systems using pneumatic-actuated control valves.

Pneumatic (3-15 psig) and electronic (4-20 mA) signals are widely used in process industries.
Smart transmitters produce both types of output, improving communication between devices.
A live zero makes adjustments during calibration simpler.

Observing the control loop usually starts at the measurement point.
Each instrument processes incoming signals to produce measurements and outputs standard signals.

The chapter explores primary components of control loops, focusing on sensors, transmitters, and transducers.
Sensors detect variables like flow, level, or pressure and can be standalone or part of transmitters.
Transmitters convert sensed measurements into standard instrument signals for the next loop component.
Understanding input/output relations and scaling is crucial for effective control loop operation.