Process Variables and Instrumentation Quiz

Study Notes

Process Variables and Measurements

Main process variables in industry include temperature, pressure, flow, and level.
In an open tank, fluid volume increases with temperature rise due to thermal expansion.
Pressure in an open tank increases when the temperature rises due to the increase in kinetic energy of fluid molecules.
The density of a liquid generally decreases when its temperature increases, assuming it does not change phase.
Increasing pressure raises the boiling point of a liquid, preventing it from boiling at standard atmospheric conditions.
True or False: In an analog system, measurements change continuously, representing the actual process variable.
True or False: Digital signals transition from one discrete value to another, also reflecting the actual process.

Instrumentation and Control

Pneumatic instruments derive their power from compressed air.
Typical pneumatic signals for control valves operate around 3-15 psi.
Electronic analog devices typically produce a standard output signal of 4-20 mA.
True or False: Digital instruments use microprocessors to manage binary-coded information as ones and zeros.
Differential pressure refers to the difference in pressure between two points in a system, crucial for flow measurement and control.

Head Pressure Calculations

Head pressure exerted by 5 ft of liquid with specific gravity 0.75 equals approximately 1.5 inches of water.
If temperature rise increases liquid volume or height in example 13, head pressure will also generally increase due to greater fluid weight.
According to the Bernoulli principle, as fluid speed increases, the pressure within the fluid decreases.

Control Loops

A control loop comprises sensors, transmitters, controllers, and final control elements.
True or False: An open control loop does not provide feedback.
A transducer converts one form of energy to another and plays a key role in measurement systems.

Signals and Transducers

Live zero on pneumatic signals typically represents 3 psi.
Live zero on the milliamp (mA) scale often represents 4 mA.
In closed control loops, the signal from a transmitter is compared at the controller with the setpoint (sp).
Control valves often serve as the final control element in a closed loop.

Transmitter Output Characteristics

For a 4-20 mA transmitter, the outputs corresponding to PSI inputs can be filled in a table correlating level to milliamp output.
Primary sensor signals in milli-volts pertain primarily to temperature sensors.
Purpose of “live zero” ensures that the system can detect faulty conditions by providing a non-zero baseline signal.

Types and Definitions of Transducers

The most common type of transducer in industry is the pressure transducer due to widespread applications in various processes.
Other types of transducers include temperature sensors and level sensors.
LRV (Lower Range Value) and URV (Upper Range Value) define the minimum and maximum values a transmitter can measure, respectively.
Equating values between different scales mathematically is referred to as scaling or calibration.
A differential pressure transmitter measures the pressure difference across two points, often used in flow metering applications.
True or False: The primary sensor, transmitter, and transducer can sometimes be integrated in a single device.
True or False: The output of a transmitter is designed to represent the full range of the measured process value from 0% to 100%.

Electronic Signal Calculations

For a pressure transmitter reading from 500 to 1000 lbs, a pressure reading of 687 lbs yields a corresponding mA signal of 10.5 mA.
If a signal of 14 mA is sent back into the transmitter, the pressure reading would be approximately 743 lbs.
A temperature measurement of 425°F on a transmitter ranging from 200°F to 800°F would output an mA signal of around 10.625 mA.
For a 14 mA signal, the corresponding temperature reading would be approximately 375°F.
A level transmitter indicating 7 mA equates to a level around 16.67% in a vessel operating on a range from 0 to 100%.

Description

Test your knowledge on the main process variables in industry such as temperature, pressure, flow, and level. This quiz covers concepts related to fluid behavior in open tanks and instrumentation used in control systems. From pneumatic instruments to digital signals, challenge yourself with true or false questions.