UT 2. Symbols and Instrumentation
What variable is measured by an instrument identified with the first letter T?
Temperature
A transmitter has a measurement range from −25 °C to 100 °C. What is the span?
125 °C
What is the main function of a controller in a feedback control loop?
To compare the measured variable with the setpoint and minimize the error
What are the two main components of a typical control valve?
Body and actuator
What type of control structure uses measured disturbances to take corrective action before affecting the process?
Feedforward control
What does the second letter I represent in an instrument tag?
Indicator
An instrument has a rangeability of 5:1. If the minimum measurable flow is 2 m³/h, what is the maximum measurable flow?
10 m³/h
Which type of controller provides only two possible output states?
On/Off controller
What type of actuator is most commonly used in industrial control valves due to its simplicity and fast response?
Pneumatic actuator
In ratio control, how many manipulated variables are typically adjusted to maintain the desired proportion?
One
State the full meaning of the instrument tag LIT-23B.
Level indicating transmitter, loop 23, element B
A pressure transmitter is configured from 20 psi to 100 psi with a 4–20 mA output. What pressure corresponds to 12 mA?
60 psi
Which control action eliminates steady-state error in a control system?
Integral action
If the pressure ratio (r) is much less than 1 (r << 1), what type of installed characteristic does a linear valve tend to exhibit?
Quick opening characteristic
In cascade control, what is the role of the primary (master) controller?
To provide the setpoint for the secondary controller
What type of signal is represented by a line with short diagonal dashes according to ISA standards?
Electrical signal
What is the term for the maximum difference in readings for the same input value when measured in both increasing and decreasing directions?
Hysteresis
Which control action anticipates future error based on the rate of change of the measured variable?
Answer: Derivative action
A control valve is open when no air signal is applied and closes when air is supplied. What is the valve action?
Direct action
What type of selector is typically used in override control to ensure safe operation?
High or low selector switch
Given the instrument tag PDAL-201, identify the condition that triggers the alarm.
Low differential pressure
A transmitter has a span of 200 °C and a drift of 0.5% over time. What is the maximum expected output variation?
1 °C
What is the effect of increasing proportional gain (Kp) excessively in a control system?
It can cause instability and oscillations
Which type of inherent valve characteristic is most suitable when high rangeability is required and system dynamics are not well known?
Equal percentage (isopercentual)
What phenomenon may occur in non-selected controllers in override control due to integral action?
Reset windup