This section describes the instruments, technologies, and equipment used to develop and maintain process
control loops. In addition, this section describes how process control equipment is represented in technical
drawings of control loops.
PRIMARY ELEMENTS/SENSORS
In all cases, some kind of instrument is measuring changes in the
process and reporting a process variable measurement. Some of the
greatest ingenuity in the process control field is apparent in sensing
devices. Because sensing devices are the first element in the control
loop to measure the process variable, they are also called primary
elements. Examples of primary elements include:
Primary elements are devices that cause some change in their
property with changes in process fluid conditions that can then be
measured. For example, when a conductive fluid passes through the
magnetic field in a magnetic flow tube, the fluid generates a voltage
that is directly proportional to the velocity of the process fluid. The
primary element (magnetic flow tube) outputs a voltage that can be
measured and used to calculate the fluid’s flow rate. With an RTD, as
the temperature of a process fluid surrounding the RTD rises or falls,
the electrical resistance of the RTD increases or decreases a
proportional amount. The resistance is measured, and from this
measurement, temperature is determined.
TRANSDUCERS AND CONVERTERS
Activities
A transducer is a device that translates a mechanical signal into an
electrical signal. For example, inside a capacitance pressure device, a
transducer converts changes in pressure into a proportional change in
capacitance.
A converter is a device that converts one type of signal into another
type of signal. For example, a converter may convert current into
voltage or an analog signal into a digital signal. In process control, a
converter used to convert a 4–20 mA current signal into a 3–15 psig
pneumatic signal (commonly used by valve actuators) is called a
current-to-pressure converter.
TRANSMITTERS
A transmitter is a device that converts a reading from a sensor
or transducer into a standard signal and transmits that signal
to a monitor or controller. Transmitter types include:
transmitters
control loops. In addition, this section describes how process control equipment is represented in technical
drawings of control loops.
PRIMARY ELEMENTS/SENSORS
In all cases, some kind of instrument is measuring changes in the
process and reporting a process variable measurement. Some of the
greatest ingenuity in the process control field is apparent in sensing
devices. Because sensing devices are the first element in the control
loop to measure the process variable, they are also called primary
elements. Examples of primary elements include:
- Pressure sensing diaphragms, strain gauges, capacitance cells
- Resistance temperature detectors (RTDs)
- Thermocouples
- Orifice plates
- Pitot tubes
- Venturi tubes
- Magnetic flow tubes
- Coriolis flow tubes
- Radar emitters and receivers
- Ultrasonic emitters and receivers
- Annubar flow elements
- Vortex sheddar
Primary elements are devices that cause some change in their
property with changes in process fluid conditions that can then be
measured. For example, when a conductive fluid passes through the
magnetic field in a magnetic flow tube, the fluid generates a voltage
that is directly proportional to the velocity of the process fluid. The
primary element (magnetic flow tube) outputs a voltage that can be
measured and used to calculate the fluid’s flow rate. With an RTD, as
the temperature of a process fluid surrounding the RTD rises or falls,
the electrical resistance of the RTD increases or decreases a
proportional amount. The resistance is measured, and from this
measurement, temperature is determined.
TRANSDUCERS AND CONVERTERS
Activities
A transducer is a device that translates a mechanical signal into an
electrical signal. For example, inside a capacitance pressure device, a
transducer converts changes in pressure into a proportional change in
capacitance.
A converter is a device that converts one type of signal into another
type of signal. For example, a converter may convert current into
voltage or an analog signal into a digital signal. In process control, a
converter used to convert a 4–20 mA current signal into a 3–15 psig
pneumatic signal (commonly used by valve actuators) is called a
current-to-pressure converter.
TRANSMITTERS
A transmitter is a device that converts a reading from a sensor
or transducer into a standard signal and transmits that signal
to a monitor or controller. Transmitter types include:
- Pressure transmitters
- Flow transmitters
- Temperature transmitters
- Level transmitters
- Analytic (O2 [oxygen], CO [carbon monoxide], and pH)
transmitters
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