Measuring Principles Unit 7 & 8

Question 1

7 Types of Electrical Pressure Elements

Answer 1

Strain Gauge
Capacitance
Resonance
Potentiometric
Piozelectric
Magnetic
Optical

Question 2

Define A Strain Gauge

Answer 2

Resistance of wire changes when stretched. The change is proportional to pressure applied.
Stress is measured in all directions due to applied force

Question 3

When a strain gauge is distorted:

Answer 3

Length of wire is increased and cross sectional area decreases both increasing resistance

Question 4

Strain Gauges detect what? And good at what?

Answer 4

Very small movements
good for very small pressures

Question 5

Bonded strain gauges:

Answer 5

have wires that are bonded to an insulating carrier material like epoxy that can be shaped to match the shape of the element

Question 6

A strain gage is usually measured with a?

Answer 6

wheat stone bridge

Question 7

Resonance Principle Operation

Answer 7

Causes wire to vibrate at it’s resonant or natural frequency
Sensor converts changes in pressure into changes in frequency

Question 8

Resonance is dependent on what 3 things?

Answer 8

(M)ass of wire
(S)quare Root of the tension
(L)ength

Question 9

In resonance a wire under tension is:

Answer 9

Located in the field of a permanent magnet assembly

Question 10

In resonance a ____ circuit:

Answer 10

Oscillating circuit causes the wire to vibrate continuously at it’s resonant frequency

Question 11

In resonance as pressure is applied:

Answer 11

To the the high pressure side of the transmitter, fluid in the fluid transfer port is compressed
This fluid presses on the back side of the low pressure diaphragm putting tension on resonant wire

Question 12

In resonance mass and length :

Answer 12

Are constant
Any change in frequency will be proportional to the tension in the wire which in turn is PROPORTIONAL to the applied pressure

Question 13

Capacitance Pressure element works on the principle:

Answer 13

of variable capacitance
- Most common principle of operation
- 2 conductive metal plates separated by insulator (diaelectric)
-Small durable, accurate

Question 14

Value of a capacitor can be changed by altering:

Answer 14

1) Size of plates
2) Distance between plates
3) Dielectric material
Value of dielectric remains constant, the size of plates do not change

Question 15

In a capacitance pressure element as:

Answer 15

pressure is increased to the sensor, the plates get pushed together, therefor changing capacitance thusly changing the output

Question 16

Potentiometric provides a

Answer 16

simple method for obtaining an electrical output from a mechanical pressure element

Question 17

A potentiometric sensor has whipper arms that are:

Answer 17

often linked to bourdon or bellows elements

Question 18

In a potentiometric sensor a change in

Answer 18

process pressure moves the connecting arm (whipper) over a precision potentiometer. Which converts pressure into electrical resistance

Question 19

Potentiometric span, advantages and disadvantages

Answer 19

Span: 5-10 PSI
Advantages: small and low cost
Disadvantages: Prone to wear with short lifespan because of mechanical movement

Question 20

Piezoelectric principle operation

Answer 20

Crystal compressed under a pressure will produce a small voltage or a change in resistance that is proportional to applied pressure

Question 21

Piezoelectric’s effect is:

Answer 21

reversible
Tension and compression create opposite voltage polarities

Question 22

Piezoelectric is typically only used in:

Answer 22

Processes where pressure changes rapidly

Question 23

When piezoelectric is in static

Answer 23

i.e. not moving pressure conditions the electrical potential drops off producing error

Question 24

3 types of Magnetic electrical pressure elements

Answer 24

1) Inductive Type
2) Reluctive Type
3) LVDT Type

Question 25

LVDT stands for:

Answer 25

Linear Variable Differential Transformer

Question 26

Magnetic: Inductive Type

Answer 26

Process pressure moves magnetic iron core inside a coil which alters the self inductance of the coil

Question 27

In a Magnetic Inductive type the core is pushed

Answer 27

inside the coil, the inductance is increased in proportion to the amount of metal inside the coil

Question 28

Magnetic: LVDT Type

Answer 28

Has 3 coils wounded in a insulating tube consisting of an iron core and an alternating current is applied to primary, center coil

Question 29

If in a LVDT's center iron core

Answer 29

equals voltages are induced and as the core moves right to left, a voltage is induced in the other 2 coils

Question 30

LVDT span and limitations

Answer 30

0-30 PSI up to 10k PSI Limitations mechanical wear, also vibration sensitive

Question 31

Magnetic: Reluctive Type

Answer 31

Uses a magnetic vein attached to a bourdon tube which changes

Question 32

In a reluctive type, the bourdon tube changes

Answer 32

the gap of the magnetic flux path of 2 iron cores and the magnetic reluctance varies with the size of the gaps

Question 33

Magnetic: Optical Type

Answer 33

Movement of pressure element is used to operate the optical sensor

Question 34

In optical type, when the diaphragm moves up:

Answer 34

a vane is lifted in front of a infrared light beam, the vane reduces the amount of light sensed by measuring diodes

Question 35

Differential Pressure Transmitters (PDT) are used to measure:

Answer 35

- Low pressure values - Flow (differential pressure) - Level - Negative pressure - Pressure drop - Density

Question 36

Differential Pressure Transmitter (PDT) ranges:

Answer 36

Min: -3 to 3 "H2O Max: -2000 to 2000 PSIG

Question 37

PDT operation: pressure is applied to:

Answer 37

high and low sides, force bar movement is proportional to the difference between the high and low side pressure

Question 38

PDT operation: Movement of the:

Answer 38

force bar causes a flapper nozzle back pressure change proportional to the sensor differential pressure position

Question 39

D/P Transmitter Leg Elevations: Liquid Process That Does Not Require a Seal Liquid

Answer 39

The leg elevations (location of the transmitter) is such that the impulse lines, remain filled with the process liquid

Question 40

D/P Transmitter Leg Elevations: Liquid Process That Does Not Require a Seal Liquid: tubing size:

Answer 40

standard tubing size 3/8" but valves sizes are smaller on this type of application typically 1/4" valves whereas 1/2" valves are generally used for liquid and gas flows with a seal liquid

Question 41

D/P Transmitter Leg Elevations: Steam or gas flow that requires a seal liquid requires:

Answer 41

that the transmitter must be mounted bellow differential pressure element (ex. steam flow)

Question 42

D/P Transmitter Leg Elevations: Steam or gas flow that requires a seal liquid it is critical that:

Answer 42

filling tees to be located at the same level, so that equal liquid heads are on both sides of the transmitter; so that no difference in pressure is caused by the liquid heads as opposed to the process being measured

Question 43

When measuring a gas that does require a seal liquid:

Answer 43

the transmitter body needs to be placed above the porcess being measured so that any condensation will fall into the pipe and away from the transmitter (ex: natural gas)

Question 44

Manifold consists of:

Answer 44

3 Valves 1) High side 2) Low side 3) Equalizer

Question 45

A Manifolds main purpose is to:

Answer 45

remove or install a dp transmitter from service

Question 46

Manifolds: Steps to isolate or take out of service a transmitter on a 3 way manifold:

Answer 46

1) Close the high side 2) Open equalizer 3) Close the Low Side

Question 47

Manifolds: Steps to put the transmitter back into service on a 3 way manifold:

Answer 47

1) Open Equalizer 2) Open Low Side 3) Close Equalizer valve 4) Open High side

Question 48

Manifolds, in general, never:

Answer 48

have all 3 valves open at the same time

Question 49

In a manifold, an equalizer should:

Answer 49

never be open with both H and L sides open or else process will flow through the equalizer valve and evacuate the seal liquid

Question 50

A 5 Valve manifold serves:

Answer 50

the same purpose as a 3 way manifold except vents are added

Question 51

To isolate of Take Transmitter outta service with a 5 valve manifold:

Answer 51

1) Close High side 2) Open the high equalizer 3) Open the Low equalizer 4) Close the low side 5) Open the vent