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Elex 7355 > Midterm > Flashcards

Midterm Flashcards

1
Q

what is a transducer

A

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 (accelerometer, thermocouple)

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2
Q

What is a transmitter

A

a transmitter is a transducer that responds to a measured variable by mean of a sensing element and coverts it to a standardized transmission signal which is a function only of the measured variable

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3
Q

Parts of a transmitter

A

Transducer, raw output transducer, signal conditioner

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4
Q

What are the standard signal formats?

A

4-20 mA and 1-5V dc

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5
Q

What is the range of an instrument?

A

The range of an instrument is defined as the region between the limits within which a quantity is measured, received, or transmitted, expressed by stating the lower and upper range values

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6
Q

What is static error?

A

The difference between the indicated (measured) value and the actual value

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7
Q

What are the several types of errors and limitations for a transducer system?

A
  • bias error
  • span error
  • linearity error
  • hysteresis error
  • dead band error
  • repeatability
  • Resolution
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8
Q

Definition of absolute pressure

A

The pressure with respect to absolute pressure (a perfect vacuum)

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9
Q

Definition of gauge pressure

A

The pressure with respect to atmospheric or barometric pressure (the reference pressure is not constant)

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10
Q

Types of instruments used to measure pressure

A

manometer, bourdon tube, diaphragm

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11
Q

What is level measurement

A

the continuous or single point measurement of the level of a liquid or solid in a tank or vessel

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12
Q

Direct methods of level measurement

A

dipstick, sight glass, constant displacement float systems

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13
Q

inferential methods of level measurement

A

ultrasonic level measurement, microwave or radar level measurement, capacitance level measurement, nuclear level measurement, variable displacement float system, hydrostatic level measurement

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14
Q

definition of contact thermometry

A

involves placing a temperature sensor is close proximity to the object being measure i.e. touching or immersed (simple glass thermometer, resistance thermometer, thermocouple)

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15
Q

Definition of non-contact thermometry

A

non-contact thermometry is usually based on measurement of the electromagnetic radiation of an object (infra red thermo, spectroscopic, ultrasound thermo)

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16
Q

What is transfer lag (temperature)

A

The lagging response of indicated temperature’s exponential rise from original temperature, dependent on the thermal conductance and heat capacitance of the measurement system

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17
Q

what does a safety blow out do for a gauge?

A

It allows for for the gauge (if affected by line pressure) to essentially blow off and not injure personal working nearby

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18
Q

what does an oil filled case do for a gauge?

A

it dampens mechanical vibrations, making the gauge easier to read and extend its service life

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19
Q

what does a snubber do for a gauge?

A

A needle valve or porous metal disk is used to “dampen” or restrict pressure fluctuations or pulsations which could quickly wear out the gauge and make it difficult to read

20
Q

What does a chemical seal/diaphragm seal/remote seal do for a gauge?

A

A diaphragm or fluid capsule is placed between the gauge and process fluid. This extends gauge life when the process fluid is corrosive or capable of plugging the sensing element

21
Q

What is transfer lag

A

a resistance/capacitance or time constant lag

22
Q

What is transport lag

A

this is distance/velocity lag found wherever a mass is transported over a distance

23
Q

what are some control system objectives?

A

minimize settling time
minimum peak error
minimum residual error

24
Q

what is positive feedback?

A

a closed loop system arranged such that the action of the controller tends to increase the error or difference

25
Q

what is negative feedback

A

a closed loop system arranged such that the action of the controller tends to minimize the error or difference between the desired value (set point) of the process and the actual value (measurement)

26
Q

controlled variable definition

A

the condition which is being regulated or maintained (the variable being directly or indirectly measured)

27
Q

measured variable definition

A

the condition which yields information about the controlled variable

28
Q

manipulated variable definition

A

the condition being adjusted to maintain or regulate the controlled variable

29
Q

uncontrolled variable definition

A

any condition which can affect the controlled variable but which itself is uncontrolled (change in load, external wind, hills)

30
Q

load variable(s) definition

A

a special type of uncontrolled (disturbance) variable(s) that affect(s) the mass or energy balance of a process and which the manipulated variable is directly countering

31
Q

Set point definition

A

the desired condition (value) of the controlled variable

32
Q

Error definition (into to automatic control)

A

the difference between the desired condition and the actual condition of the measured variable

33
Q

Advantages of platinum RTD

A
  • Good accuracy
  • Repeatable/reproduce easily
  • very linear in typical process range
  • Fast reacting
34
Q

Disadvantages of platinum RTD

A
  • relatively expensive (compared to thermistor) however cheaper than thermocouple
  • unsuitable for extreme temperatures
35
Q

Advantages of thermistor

A
  • small size
  • inexpensive
  • non-linear but still accurate and repeatable
36
Q

Disadvantages of thermistor

A
  • highly non-linear
  • narrow measurement range
37
Q

What is an RTD

A

RTD’s (Resistance temperature detector) are made by wrapping a length of fine metal wore around a supporting ceramic or glass core. This structure may then be enclosed in a stainless steel sheath to protect the RTD from physical damage and hostile environments.

They work by measuring the resistance from the metal due to an increase or decrease in temperature

38
Q

What is a thermistor

A

A thermistor is a semiconductor device made from metal oxides. They usually exhibit a negative resistance - temperature coefficient (NTC). As temp. increases, resistance decreases. However, positive temperature coefficient thermistors (PTC) are also available

39
Q

What is a thermocouple

A

A thermocouple is formed by joining two dissimilar homogeneous metals. They work by producing an emf or voltage when there is a temperature difference between the hot and cold junctions

40
Q

types of inferential methods of flow measurement

A

orifice plate
venturi tube
flow nozzle
annubar and pitot tube

41
Q

types of direct methods of flow measurement

A

variable flowmeter
turbine flowmeter
vortex shedding flowmeter
magnetic flowmeter
Coriolis flowmeter
ultrasonic flowmeters (doppler type, transit type)

42
Q

when a valve actuator is considered direct acting? (valve at pipe leading into beginning of tank)

A

using fail open valve, requires increase in air to close value

43
Q

when is a valve actuator considered reverse acting? (valve at bottom of tank)

A

using fail open valve, increase in air results in reduced output (value is closing)

44
Q

when is a valve actuator considered reverse acting (valve at pipe leading into beginning of tank)

A

using fail closed valve, increase in air results in reduced output (value is closing)

45
Q

when is a valve actuator considered direct acting (valve at bottom of tank)

A

using fail closed valve, increase in air results in increase in output (valve opening)

Elex 7355 (2 decks)

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