Measuring Principles/Basic Tech Flashcards
Pressure:
The force exerted over a unit area, p=F/A
Pressure is force divided by the area over which that force is applied.
Pressure:
The force exerted over a unit area, p=F/A
Pressure is force divided by the area over which that force is applied.
Atmospheric pressure (PISA)
Pressure at the earths surface caused by the weight of the air above it. (14.7 PSIA or 101.4 kPa)
Gauge pressure (PSIG):
Uses atmospheric pressure (14.7PSIA) as a zero point.
Absolute pressure (Pabs)
gauge pressure + atmospheric pressure
Differential pressure:
The difference in pressure between two measurement points in a process.
Pressure Units: “H2O, “WC, “Hg, mm Hg, or torrs (1 mm Hg absolute).
Head
Height of a column of liquid
Expressed in units “H2O, “Hg
4 Common Pressure Scales
(D)ifferential Pressure
(A)bsolute
(V)acuum
(G)auge
DAVG
3 types of mechanical pressure elements
(B)ourdon Tubes
(B)ellows
(D)iaphragm
2 Types of Analog Signals
-Pneumatic 3-15 psi
-Electric 4-20 mA
1-5 volts
10-50 mA
Manometers
-Measures positive or negative pressure & differential pressure
-Range depends on it’s vertical height and the specific gravity of it’s fluid
3 Types of Manometers:
Utube:
Well:
Incline
Inclined-tube manometer range:
min: 0-4 “H2O
optimal: 0-6 “H2O
max: 0-8 “H2O
Barometer
Used to measure atmospheric
pressure.
Diaphragm Range
0-0.05 “H2O to 3000 psi
Identification of Instruments: Symbols and Tags
1st Letter: Required. (v)ariable being measured
2nd Letter: Optional. (M)odifier to first letter
3rd Letter: Optional. (o)utput functionality.
Lower Line: Required. (t)ag number
The 3 types of range:
Suppressed: Higher than 0 LRV
Elevated: Lower than 0 LRV
Zero: has a 0 LRV
Relay
Boost output and valve volume
Automatic Feedback Control
-(TT) “eyes” Temperature Transmitter, attached to a temperature indicator.
-(TIC) “brain” Temperature Indicating Controller, attached to the TT that has a set point=desired temperature
-(TCV) “hands” Temperature Control Valve, attached to TIC used to control desired result
The purpose of a pressure element
To create mechanical motion
from an input pressure.
Bourdon tubes
-Flexible flat cross sectional area
-Brass or stainless steal
Types of Bourdon tubes
C-shaped
Helical
Spiral
5 Types of Callibration Errors:
Zero
Span
Linear
Hysteresis
Repeadibility
Bourdon Tubes:
Flat cross section
Used for pressures + 40 psi
Insufficient spring movement to accurately detect pressures below 30 psi
Bourdon Tube range:
Minimum: 40
Maximum: 100 thousand
Span
URV-LRV
C Shaped Bourdon Tube Range
Range: 0-10 psi to 20 000 psi
Accuracy: +/- 1% span
Spiral Bourdon Tube Range
Range: 0-10 psi to 100 000 psi
Accuracy: +/- 0.5% of span
Helical Bourdon Tube Range
0-30 psi to 0-80 000 psi
Accuracy: +/- 1 % of span
Bellows Element Range
Range: 0-2.5 “H2O to 0-2000 psi
Accuracy: +/- 2 % span
Diaphragm Element Range
Range: 0-0.5 “H2O to 3000 psi
Accuracy: +/- 1%
Pressure Gauges Range:
Range: 0 - 10 000 psi
When Ordering a gauge
(P)ressure range scale
(F)ace size
(A)ccurary
(C)onnection location
(T)ype of mounting
PFACT
3 Gauge Accessories
-Oil filed
-Snubbers
-Chemical Seal
Incline- Tube Manometer Advantages
-Measures extremely low pressure, even in a vacuum (negative pressure)
-Extremely accurate
-Unlike mechanical or electronic manometers, inclined manometers don’t have any parts that could wear out or deteriorate.
-Sensitivity of inclined tube manometers is high
Gauge Accessory: Oil Filled
(R)educes plant vibration
(I)ncreases gauge life
(C)an be filled with glycerin, etc..
Gauge Accessory: Snubbers
(R)educes oscillation caused by fluctuations and process pressure
(U)ses porous filter disks, pistons, variable orifices or oil filled rubber bulbs
(S)low response time so pressure element can respond to pressure changes
Gauge Accessory: Chemical Seal
-Protects gauge from corrosive or hot services
-Isolates the internal parts from the process material steam, acid, slurries
-Filled with non compressible fluid
Diaphragm Element
- Stainless steel and filled with liquid
-Used in both pneumatic and electric transmitters - Movement dependent on metal thickness, diameter, and # of corrugations
Bellows Element
- Used in pneumatic transmitters, recorders, controllers
- brass or stainless steel
- more sensitive than bourdon tubes so used for low pressures
-Sensitivity increases with diameter
Helical Bourdon Tube
- Used for continuously fluctuating services
- Higher pressure span = more coils
Spiral Bourdon Tube
- More motion than C type
- Causes flapper to move closer to nozzle
- Used in pneumatic motion balance transmitters
C Bourdon Tube
-Used in direct indicating gauges
-Forces a bar in flapper assembly
-Linear indication with gears, cams, pinions
Flapper Nozzle Assembly
Converts Mechanical Movement into a pressure signal
Typical ranges for Manometers
Utube: 0-200 “H2O
Well:0-20 “H2O
Incline: 0-8 “H2O
Range
Region between limits within a measured quantity expressed by stating the lower and upper range values
Span with examples
Difference between upper and lower range values
(URV) - (LRV) = span
(15) - (3) = span
When calibration of an instrument is correct :
-Zero input matches the zero output
-Span (maximum input) value matches the max output value
- The midpoint input value matches the midpoint output value
Calibration Procedure
- Obtain Instruction Manual
- Check for damaged or loose parts making sure everythings tight
- Make a callibration hook up diagram and hook up your equipment accordingly
- Apply 0% input and adjust the zero for the proper output
- Apply 100% input and adjust the span for the proper output
- Repeat steps 4 & 5 until the zero and span values are correct
7.Check midpoint value for linearity error. If provided, adjust it to correct value, then repeat steps 4 to 7 until correct. If no linearity adjustment is provided, record the linearity error and proceed to step eight. - Check for repeadibility and hysteresis errors by applying input values of 0, 25, 50, 75 & 100 in both directions, compare use a table.
Basic Elements of an Automatic Control Loop
Transmitter
Controller
Valve
Direct Acting
As input goes up, output goes up
when speaking relays inputs the back pressure output is the 3-15 psi
Reverse Acting
input up, output down
when speaking relays inputs the back pressure and output is the 3-15 psi
Purpose of a flapper nozzle
Ends the elements mechanical motion and creates our standard output signal
What is the purpose of a restriction before the nozzle
To reduce the volume of the flow
What would happen on a flapper nozzle if blocked the nozzle
The back pressure would rise up to the supply pressure (20psi)
Why is it necessary to have an exhaust valve and port on a relay
output air will escape to atm when backpressure decreases
Fixed Orifice Restriction
provide free flow in one direction and Restricted (metered) flow in the reverse direction.
Nozzle
a restriction in series, the constant supply of pressurized air is applied to the nozzle through the orifice restriction. The pressurized air comes out of the nozzle through the gap between the nozzle and flapper.
Baffle or Flapper
The flapper is positioned in front of the nozzle in such a way to cover or uncover the nozzle and vary the gap between them when moved.
When the flapper is moved towards the nozzle, the gap between the nozzle and flapper gets reduced. This increases the restriction to the outflow of air through the nozzle and also increases the nozzle back pressure.
