Measuring Principles Unit 7 & 8 Flashcards
7 Types of Electrical Pressure Elements
Strain Gauge
Capacitance
Resonance
Potentiometric
Piozelectric
Magnetic
Optical
Define A Strain Gauge
Resistance of wire changes when stretched. The change is proportional to pressure applied.
Stress is measured in all directions due to applied force
When a strain gauge is distorted:
Length of wire is increased and cross sectional area decreases both increasing resistance
Strain Gauges detect what? And good at what?
Very small movements
good for very small pressures
Bonded strain gauges:
have wires that are bonded to an insulating carrier material like epoxy that can be shaped to match the shape of the element
A strain gage is usually measured with a?
wheat stone bridge
Resonance Principle Operation
Causes wire to vibrate at it’s resonant or natural frequency
Sensor converts changes in pressure into changes in frequency
Resonance is dependent on what 3 things?
(M)ass of wire
(S)quare Root of the tension
(L)ength
In resonance a wire under tension is:
Located in the field of a permanent magnet assembly
In resonance a ____ circuit:
Oscillating circuit causes the wire to vibrate continuously at it’s resonant frequency
In resonance as pressure is applied:
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
In resonance mass and length :
Are constant
Any change in frequency will be proportional to the tension in the wire which in turn is PROPORTIONAL to the applied pressure
Capacitance Pressure element works on the principle:
of variable capacitance
- Most common principle of operation
- 2 conductive metal plates separated by insulator (diaelectric)
-Small durable, accurate
Value of a capacitor can be changed by altering:
1) Size of plates
2) Distance between plates
3) Dielectric material
Value of dielectric remains constant, the size of plates do not change
In a capacitance pressure element as:
pressure is increased to the sensor, the plates get pushed together, therefor changing capacitance thusly changing the output
Potentiometric provides a
simple method for obtaining an electrical output from a mechanical pressure element
A potentiometric sensor has whipper arms that are:
often linked to bourdon or bellows elements
In a potentiometric sensor a change in
process pressure moves the connecting arm (whipper) over a precision potentiometer. Which converts pressure into electrical resistance
Potentiometric span, advantages and disadvantages
Span: 5-10 PSI
Advantages: small and low cost
Disadvantages: Prone to wear with short lifespan because of mechanical movement
Piezoelectric principle operation
Crystal compressed under a pressure will produce a small voltage or a change in resistance that is proportional to applied pressure
Piezoelectric’s effect is:
reversible
Tension and compression create opposite voltage polarities
Piezoelectric is typically only used in:
Processes where pressure changes rapidly
When piezoelectric is in static
i.e. not moving pressure conditions the electrical potential drops off producing error
3 types of Magnetic electrical pressure elements
1) Inductive Type
2) Reluctive Type
3) LVDT Type
LVDT stands for:
Linear Variable Differential Transformer
Magnetic: Inductive Type
Process pressure moves magnetic iron core inside a coil which alters the self inductance of the coil
In a Magnetic Inductive type the core is pushed
inside the coil, the inductance is increased in proportion to the amount of metal inside the coil
Magnetic: LVDT Type
Has 3 coils wounded in a insulating tube consisting of an iron core and an alternating current is applied to primary, center coil
If in a LVDT’s center iron core
equals voltages are induced and as the core moves right to left, a voltage is induced in the other 2 coils
LVDT span and limitations
0-30 PSI up to 10k PSI
Limitations mechanical wear, also vibration sensitive
Magnetic: Reluctive Type
Uses a magnetic vein attached to a bourdon tube which changes
In a reluctive type, the bourdon tube changes
the gap of the magnetic flux path of 2 iron cores and the magnetic reluctance varies with the size of the gaps
Magnetic: Optical Type
Movement of pressure element is used to operate the optical sensor
In optical type, when the diaphragm moves up:
a vane is lifted in front of a infrared light beam, the vane reduces the amount of light sensed by measuring diodes
Differential Pressure Transmitters (PDT) are used to measure:
- Low pressure values
- Flow (differential pressure)
- Level
- Negative pressure
- Pressure drop
- Density
Differential Pressure Transmitter (PDT) ranges:
Min: -3 to 3 “H2O
Max: -2000 to 2000 PSIG
PDT operation: pressure is applied to:
high and low sides, force bar movement is proportional to the difference between the high and low side pressure
PDT operation: Movement of the:
force bar causes a flapper nozzle back pressure change proportional to the sensor differential pressure position
D/P Transmitter Leg Elevations: Liquid Process That Does Not Require a Seal Liquid
The leg elevations (location of the transmitter) is such that the impulse lines, remain filled with the process liquid
D/P Transmitter Leg Elevations: Liquid Process That Does Not Require a Seal Liquid: tubing size:
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
D/P Transmitter Leg Elevations:
Steam or gas flow that requires a seal liquid requires:
that the transmitter must be mounted bellow differential pressure element (ex. steam flow)
D/P Transmitter Leg Elevations:
Steam or gas flow that requires a seal liquid
it is critical that:
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
When measuring a gas that does require a seal liquid:
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)
Manifold consists of:
3 Valves
1) High side
2) Low side
3) Equalizer
A Manifolds main purpose is to:
remove or install a dp transmitter from service
Manifolds: Steps to isolate or take out of service a transmitter on a 3 way manifold:
1) Close the high side
2) Open equalizer
3) Close the Low Side
Manifolds: Steps to put the transmitter back into service on a 3 way manifold:
1) Open Equalizer
2) Open Low Side
3) Close Equalizer valve
4) Open High side
Manifolds, in general, never:
have all 3 valves open at the same time
In a manifold, an equalizer should:
never be open with both H and L sides open or else process will flow through the equalizer valve and evacuate the seal liquid
A 5 Valve manifold serves:
the same purpose as a 3 way manifold except vents are added
To isolate of Take Transmitter outta service with a 5 valve manifold:
1) Close High side
2) Open the high equalizer
3) Open the Low equalizer
4) Close the low side
5) Open the vent
