Questions and Answers
What is the correct interpretation of 'fail closed'?
Which part of the controller serves as the 'target' for desired values?
Which component of the PID algorithm measures the time the process variable has remained away from the setpoint?
Which aspect of the PID algorithm focuses on adjusting based on the rate of change of the process variable?
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In a controller's 'Auto' mode, what does the operator adjust?
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What effect do higher gains have on a controller's performance?
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Which strategy would reduce the tendency of a controller to overshoot the setpoint?
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Which part of the controller is responsible for the actual output adjustment?
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Which side of the transmitter is connected to the bottom part of the tank when using a DP cell to measure level?
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What measurement in the pressure is produced by higher flow when using a DP cell with an orifice?
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What will the pressure transmitter read if a plug occurs in the orifice hole while measuring flow?
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If a restriction occurs downstream of the orifice, what will the flow transmitter read?
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What value will the temperature transmitter indicate if a thermocouple breaks at the junction?
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What value will the temperature transmitter indicate if a thermocouple shorts out?
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What will the temperature transmitter indicate if a thermocouple or RTD falls out of the thermowell?
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What is the most common use of a transducer in a control loop?
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Study Notes
Differential Pressure Measurements
- For a DP cell measuring level, connect the low side of the transmitter to the bottom of the tank, as this area experiences higher pressure from the liquid height.
- In flow measurement using a DP cell and an orifice, increased flow results in higher differential pressure readings under normal conditions.
Effects of Blockages in Flow Measurement
- If a plug forms in the orifice, the pressure transmitter will read lower flow than actual due to flow restriction.
- A restriction downstream of the orifice will lead to the flowmeter reading lower than the actual flow because the pressure differential is affected after the orifice.
Temperature Transmitter Behavior
- A broken thermocouple at the junction will indicate ambient temperature.
- If a thermocouple shorts, the transmitter will show full scale high.
- If a thermocouple or RTD falls out of the thermowell, the transmitter will read ambient temperature.
Transducers in Control Loops
- Common transducer function includes converting 3-15 psig to 4-20 mA, which facilitates signal compatibility in control systems.
Control Terminology
- "Fail closed" can also be described as "air to close," indicating that, without air, the valve closes.
- The "setpoint" in a controller defines the target or desired value for the process.
Measurement and Output in Controllers
- The "process variable" represents the measurement within the control system.
- The "manipulated variable" denotes the output of the controller.
PID Control Algorithm
- The proportional part measures "how far away" the process variable is from the setpoint.
- The integral part focuses on "how much time" the process variable has stayed away from the setpoint.
- The derivative component addresses "how fast the process variable is changing."
Controller Tuning and Stability
- Higher controller gains increase aggressiveness towards reaching the setpoint, potentially resulting in overshoot.
- Reducing gain can enhance stability and diminish the tendency to overshoot.
Controller Modes
- In "Auto" mode, operators change the setpoint.
- In "Manual" mode, the manipulated variable (output) is modified by the operator.
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Description
Test your understanding of differential pressure (DP) cells and their application in measuring liquid levels and flow. This quiz covers key concepts like pressure relationships in tanks and effects of flow on pressure readings.
