Test 2 Flashcards
What is a strain gauge
Transducer that measures deformation/strain thanks to force
When does strain gauge wire resistance rise?
When stretched
What is Epoxy?
Specific insulating bendable material
What makes strain gauges useful?
Detects very small movements, good for very small pressures
What is change in resistance usually measured with?
Wheatstone bridge
What principle does a capacitance pressure element work on?
Variable capacitance
What three things alter the value of a capacitor?
Size of plates, distance between plates, dielectric material
What does increasing pressure on a capacitor sensor do?
Pushes plates together -> changes capacitance -> changes output
What is a capacitance pressure transducer?
Diaphragm pressure sensor with a capacitor as the electrical element
What are the five communication protocols?
ARCNET, WORLDFIP, HART, Foundation FieldbusH1, Profinet (Profibus)
How do we differentiate between smart and digital transmitters?
Smart has both digital and analog outputs, digital only has digital output
What are the 4 types of orifice plates?
Concentric, eccentric, segmental, conditioning
What does stamping on one side of an orifice mean?
High/sharp side
What are the 5 types of taps?
Corner, Flange, Vena Contracta, Pipe, and Radius
Where are corner taps placed?
Right up against the orifice plate, where a 90 degree angle would form
Where are flange taps placed?
Holes are simply drilled into the flange
Where are Vena Contracta taps located?
Upstream: 1D
Downstream: Point of minimum pressure
- Point in fluid system where diameter of stream is least -> fluid velocity is at maximum
Where are radius taps placed?
Upstream: 1D
Downstream: D/2
Where are pipe taps placed?
Upstream: 2 1/2 D
Downstream: 8D
What do flow elements do?
- Create a differential pressure
Advantages of dp flow meters:
- Familiar/Versatile
- Lack of moving parts
- Suitable for all flow rates in a wide variety of pipes and tube
Disadvantages of dp flow meters:
- Many elements required for measurement
- Square root relationship limits rangeability
What must be used when measuring flow?
Square root extractor
What are orifice plates usually made of?
High grade stainless steel
What can reduce reduce the required length in a pipe before an orifice plate?
Straightening vane
Advantages of orifice plates:
- Ease of installation
- Easily reproduced
- Cheap
- Used with a wide range of pipe sizes
Disadvantages of orifice plates:
- Erosion/corrosion can change characteristic
- High pressure loss
- Can clog
What is the change in capacitance proportional to?
Change in pressure
What does a potentiometric transducer do?
- Provides a simple method for obtaining an electric output from a mechanical pressure element
- Uses bourdon/bellows elements
- Procese pressure moves the wiper over precision potentiometer
- pressure converts to electrical resistance
Span of potentiometric transducer?
5-10psi
Advantages of a potentiometric transducer
- Small
- Cheap
Disadvantage of potentiometric transducer
Prone to wear/short lifespan
Principle operation of resonance?
Cause a wire to vibrate at its resonant
Resonance: since mass and length of a wire is constant…
Any change in frequency will be proportional to wire tension -> which is proportional to applied pressure
The resonant frequency of a vibrating wire is a function of…
- The length
- The mass
- The square root of the tension
What is the principle of operation for a piezoelectric transducer?
A crystal compressed under pressure produces voltage and resistance
Piezoelectric transducer: tension and compression produce…
opposite voltage polarities
LVDT stands for…
linear variable differential transformee
LVDT principle of operation:
- Three coils around insulating tube with iron core
- Alternating current applied to primary coil in center
- If iron core is centered, equal voltages induced
- If iron core moves left/right, voltage is induced in other two coils
LVDT ranges:
0-30psi, 0-10 000psi
Disadvantage of LVDT:
Mechanical ware/vibration sensitive
Magnetic inductive element principle of operation:
- Process pressure moves iron core inside coil
- Self inductance altered
- As core is pushed inside coil, inductance is increased proportionally to metal in coil
Magnetic reluctive element principle of operation:
- Magnetic vein attached to bourdon tube changes the gap of magnetic flux path of two iron cores
- Reluctance varies with size of gap
Optical element principle of operation:
- Movement of pressure element operates optical sensor
- As diaphragm moves upwards, vein is lifted,
Common ranges of DPT?
-> -3-3”H2O
-> -2000-2000psig
When measuring a liquid that does not require a liquid seal…
leg elevations (location of transmitter) are below
When measuring steam/gas flow that requires a liquid seal…
- the transmitter must be mounted below
- filling t’s must be at same height
When measuring steam/gas flow that does not requires a liquid seal…
- the transmitter must be mounted above
- so condensation will fall into pipe
Three valve manifold: to isolate or take the transmitter out of service…
- close high side
- open equalizer
- close low side
- Bleed caulk to get rid of pressure
Three valve manifold: to put back in service…
- open equalizer
- open low-side
- close equalizer
- open high-side
When should all valves in a three valve manifold be open?
Never
Five valve manifold: to take out of service…
- close high side
- open high equalizer
- open low equalizer
- close low side
- open vent
Five valve manifold: to put into service…
- close vent
- open low side
- close both equalizers
- open high side
What must a controller incorporate?
A to D (analog to digital) converter
What is a smart transmitter?
- All internal components are digital
- Output is analog
- Can achieve 0.1% accuracy
What does a smart transmitter become capable of when part of a dcs (distributed control system)
Being controlled remotely
Smart transmitter: Digital “0” represented by…
2200Hz
Smart transmitter: Digital “1” represented by…
1200Hz
Digital transmitters eliminate the need for…
A/D, D/A converters
Digital transmitters eliminate error produced by…
Not reconverting digital signal back to analog
The most common wireless technologies use…
radio waves
What is a pressure relief valve (PRV)?
- Valve used to control/limit amount of pressure in a system/vessel built up by:
- process upset, instrument/equipment failure
How does a pressure relief valve relieve pressure?
- Allowing pressurized fluid to flow from auxiliary passage
Rupture disk advantages over PRV:
- Cost
- Leak tightness
Rupture disk disadvantages over PRV:
- one time use
- can’t be adjusted
When is a rupture pin used?
- When a non-resetting PRD is required as a substitute for a rupture disk
How does a rupture pin offer savings?
- Eliminates premature failures
- Prevents need to break integrity of system to institute reset
- Can be reset without having to remove PRD from system
- Avoids environmental exposure concerns
- Enables quick/easy reset when pin requires changing
DP flow meter advantages:
- Familiar/versatile
- Lack of moving parts
- Suitable for all flow rates in wide variety of pipes/tubes
DP flow meter disadvantages:
- Large amount of elements
- Square root relationship
1 psi to “H20
27.7
1 atm to bar
1.01325
1 bar to psi
14.5
1 atm to psi
14.7
1 psi to kPa
6.895
1 psi to “Hg
2.04
1 inch to mm
25.4
As strain gauge is distorted…
- The length of of wire is increased and cross sectional area decreases
- both jncreasing resistance
A Wheatstone bridge is used…
In strain gauges
1 kg/cm^2 to psi
14.22
