Midterm

Question 1

what is a transducer

Answer 1

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)

Question 2

What is a transmitter

Answer 2

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

Question 3

Parts of a transmitter

Answer 3

Transducer, raw output transducer, signal conditioner

Question 4

What are the standard signal formats?

Answer 4

4-20 mA and 1-5V dc

Question 5

What is the range of an instrument?

Answer 5

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

Question 6

What is static error?

Answer 6

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

Question 7

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

Answer 7

• bias error
• span error
• linearity error
• hysteresis error
• dead band error
• repeatability
• Resolution

Question 8

Definition of absolute pressure

Answer 8

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

Question 9

Definition of gauge pressure

Answer 9

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

Question 10

Types of instruments used to measure pressure

Answer 10

manometer, bourdon tube, diaphragm

Question 11

What is level measurement

Answer 11

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

Question 12

Direct methods of level measurement

Answer 12

dipstick, sight glass, constant displacement float systems

Question 13

inferential methods of level measurement

Answer 13

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

Question 14

definition of contact thermometry

Answer 14

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

Question 15

Definition of non-contact thermometry

Answer 15

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

Question 16

What is transfer lag (temperature)

Answer 16

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

Question 17

what does a safety blow out do for a gauge?

Answer 17

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

Question 18

what does an oil filled case do for a gauge?

Answer 18

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

Question 19

what does a snubber do for a gauge?

Answer 19

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

Question 20

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

Answer 20

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

Question 21

What is transfer lag

Answer 21

a resistance/capacitance or time constant lag

Question 22

What is transport lag

Answer 22

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

Question 23

what are some control system objectives?

Answer 23

minimize settling time
minimum peak error
minimum residual error

Question 24

what is positive feedback?

Answer 24

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

Question 25

what is negative feedback

Answer 25

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)

Question 26

controlled variable definition

Answer 26

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

Question 27

measured variable definition

Answer 27

the condition which yields information about the controlled variable

Question 28

manipulated variable definition

Answer 28

the condition being adjusted to maintain or regulate the controlled variable

Question 29

uncontrolled variable definition

Answer 29

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

Question 30

load variable(s) definition

Answer 30

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

Question 31

Set point definition

Answer 31

the desired condition (value) of the controlled variable

Question 32

Error definition (into to automatic control)

Answer 32

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

Question 33

Advantages of platinum RTD

Answer 33

- Good accuracy - Repeatable/reproduce easily - very linear in typical process range - Fast reacting

Question 34

Disadvantages of platinum RTD

Answer 34

- relatively expensive (compared to thermistor) however cheaper than thermocouple - unsuitable for extreme temperatures

Question 35

Advantages of thermistor

Answer 35

- small size - inexpensive - non-linear but still accurate and repeatable

Question 36

Disadvantages of thermistor

Answer 36

- highly non-linear - narrow measurement range

Question 37

What is an RTD

Answer 37

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

Question 38

What is a thermistor

Answer 38

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

Question 39

What is a thermocouple

Answer 39

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

Question 40

types of inferential methods of flow measurement

Answer 40

orifice plate venturi tube flow nozzle annubar and pitot tube

Question 41

types of direct methods of flow measurement

Answer 41

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

Question 42

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

Answer 42

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

Question 43

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

Answer 43

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

Question 44

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

Answer 44

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

Question 45

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

Answer 45

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