Final Flashcards
What are open-loop control systems?
they have no negative feedback (feedback from output to input)
What are closed-loop control systems?
systems that have negative feedback path (from output to input)
What does a typ. closed-loop control system look like?
Input (Setpoint) - Controller - FCE - Process - Output
I I
- Measurement -
What is the difference between a transmitter and transducer?
- a transducer is a device which receives information in the form of one quantity and converts it to information in the form of the same or another quantity
- a transmitter is a transducer that responds to a measured variable by means of a sensing element and converts it to a standardized transmission signal which is a function only of the measured variable
What is the difference between pneumatic and electronic signals?
- pneumatic signals use pressure of a gas (air) to carry signal information
- electronic signals use electrical voltage or current to carry signal information
What are the common pneumatic signals?
20-100 kPag
3-15 Psig
What are the common electronic signals?
1-5 V
4-20 mA
What are the different types of errors in transducers?
1) Bias error
2) Span error
3) Linearity error
4) Repeatability
5) Resolution
Define Control Variable:
the condition which is being regulated or maintained
Define Measured Variable:
the condition that yields information about the controlled variable
Define Uncontrolled Variable:
any condition that can affect the controlled variable but itself is uncontrolled
Define Manipulated Variable:
condition that is being adjusted to regulate or maintain the controlled variable
Define Load Variable
a special type of uncontrolled variable that affects the mass or energy of a process and which the manipulated variable is directly countering
Define Setpoint:
the desired condition of the controlled variable
Define Error:
the difference between the desired condition and the actual condition of the measured variable
What are the control system objectives?
1) minimize settling time
2) minimal peak error
3) minimal residual error
What is transfer lag?
product of resistance and capacitance
- capacitance: the ability any control system component to store mass or energy
- resistance: the opposition to the flow of mass or energy by any control system component
What is transport lag?
the distance/velocity lag found wherever a mass is transported over a distance (the length of time required to travel the distance)
What are the different types of strain?
- axial strain (delta L/L)
- lateral strain (delta d/d)
Define the relationship between stress and strain:
strain is directly proportional to the stress applied
List seven flow measurement systems:
1) Head type flowmeters
2) Variable area flowmeter (rotameter)
3) Turbine flowmeter
4) Vortex shedding flowmeter
5) Magnetic flowmeter
6) Coriolis flowmeter
7) Ultrasonic flowmeter
List some Head Type flowmeters:
1) Orifice Plate
2) Venturi
3) Pitot tube
List some Ultrasonic flowmeters:
1) Transit time
2) Doppler
What is the difference between gauge and absolute pressure and how does it affect a reading?
- gauge pressure is with respect to atmospheric pressure while absolute pressure is with respect to absolute zero pressure
- gauge is affected by elevation while absolute is not
- if elevation is increased atmospheric is decreased and gauge is decreased, but absolute stay the same
List some pressure gauge protection devices:
1) Pigtail/Siphon
2) Safety blowout
3) Oil filled case
4) snubber
5) chemical seal/diaphragm seal
What does a safety blowout do?
it vents process fluid away from the front of the gauge to prevent gauge glass from breaking (away from glass and blowout from back)
What does an oil filled case do?
filling the gauge with oil dampens mechanical vibrations, making the gauge easier to read and extending its service life
What does a snubber do?
used to dampen/restrict pressure fluctuations which could wear out the gauge and make it difficult to read
What is a chemical seal/Diaphragm seal used for?
it is placed between the gauge and the process fluid to help extend the gauge service life in case where the fluid is corrosive or to prevent fluid from plugging the sensing element
What is a pigtail or siphon?
they prevent high temperature steam from entering the bourdon tube by forming a condensate buffer, isolating the high temp. steam from the gauge (to avoid degradation in gauge performance)
List some Direct Level Measurement Methods:
1) Dipstick
2) Sightglass
3) Constant Displacement Float and Tape System
List some Inferential Level Measurement Methods:
1) Ultrasonic Level Measurement
2) Microwave/Radar Level Measurement
3) Capacitance Level Measurement
4) Nuclear Level Measurement
5) Variable Displacement Float System
6) Hydrostatic Level Measurement
7) Open Tank Measurements
List the three common temp. transducers (w/ definitions):
1) RTD (Resistance Temp. Detectors): measures change in resistance - linear
2) Thermistor: measures change in resistance - non-linear
3) Thermocouple: creates a small voltage based on temp. difference
List some applications for pitot tube:
- measure airspeed (planes)
- HVAC systems
- formula one cars
What are some adv. and disadv. for a rotameter?
Adv.:
- direct indication of flow
- low cost
- low pressure drop
- no power required
Disadv.:
- must be mounted vertically
- easily breaks
- fluid must be clean
- sensitive to fluid density
What are some adv. and disadv. for turbine flowmeter?
Adv.:
- high accuracy
- repeatable measurement
Disadv.:
- high cost
- moving parts
- not good for low flow
What is an application example for turbine flowmeters?
oil & has blending
What are some adv. and disadv. for vortex shedding flowmeter?
Adv.:
- no moving parts
- good accuracy
Disadv.:
- not suitable for dirty, abrasive fluids
- not suitable for laminar flow
- midrange, expensive
What are some adv. and disadv. for magnetic flowmeter?
Adv.:
- no restriction of flow
- very accurate
Disadv.:
- only works for conductive fluids
What are some adv. and disadv. for coriolis flowmeter?
Adv.:
- highest accuracy
Disadv.:
- not good for slurries
- expensive
What are some adv. and disadv. for ultrasonic flowmeters?
Adv.:
- simple construction
- easy to install
- can handle corrosive fluid
Disadv.:
- disturbed by electrical noise easily
- build up on pipe wall affects the reading
List some strain gauge applications:
1) Weighing scales
2) Electronic scales
3) Bicycle power meters
4) To measure strain placed on components of buildings
