Module D: Pressure measurements Flashcards
(32 cards)
Pressure definition
Force applied per unit area [static or dynamic]
Measured wrt absolute vacuum or atmosphere
- Absolute pressure is measured relative to vacuum
- Gauge pressure is measured relative to atmospheric pressure
[Pgauge= Pabs- Patm]
How does height of column and weight of fluid influence fluid pressure?
The taller the column = the lower the fluid pressure
The heavier the fluid = the higher the fluid pressure
What is static fluid pressure and how is it measured?
The mass of the fluid column is related to its volume and density.
P = F/A = mg/A = p(density)ghA/A = pgh
What are the different pressure measurement tools?
Barometers, manometers, diaphragms and bellows, bourdon gauges
What are barometers?
Absolute pressure measurement
Made of tube w/ fluid, along with scale
(can contain water or mercury)
- mercury has shortest column given higher density (P=pgh (static fluid pressure))
What are manometers - single tube?
Gauge pressure measurement
fluid [COLUMN]
What are manometers - u-tube?
Gauge pressure measurement - relative to ATM OR Pressure difference between 2 GASES
Variation in pressure: deltaP = p(density)gdelta(h)
How can you measure pressure differences between liquids in manometers?
2 fluids of different DENSITY are required (1 fluid inside pipe, 1 fluid inside manometer tube)
the difference in height in the u-tube is proportional to the pressure difference AND to the differences in fluid densities
delta (P) = delta(h)g(p1-p2)
How can you increase sensitivity of manometers?
Use different kinds of manometers (large well where fluid enters and small tube when difference in height is determined)
= bigger accuracy
What are pressure sensors with electrical output?
[barometers and manometers rely on inspection to determine change in height]
other methods can be used which convert the pressure measurement to an electrical signal
- bourdon gauges
- diaphragms and bellows
What is the measurement system for pressure sensors with electrical output?
Pressure input —-> [Pressure sensing and transduction] —-> Movement in pressure sensor —-> [Transduction Element] —-> Electrical output —–> [Signal processing element] ——> Cleaned or amplified signal
What are the characteristics of diaphragms and bellows? How do they work? What kind of pressure can they measure?
- Elastic elements made of metal, plastic, ceramic
- Bellows are made of metal and are more convoluted
Diaphragm: Pressure input —> [Diaphragm: sensing, transduction] —-> translational movement—-> [strain gauge or piezoresistive sensor] —–> electrical output
- Diaphragms can measure gauge pressure, absolute pressure, or pressure differential
(absolute: diaphragm in vacuum, gauge: diaphragm in atmosphere, differential: diaphragm in pressure 2)
What are the characteristics of bourdon gauges? How do they work? What kind of pressure can they measure?
- Elastic element sensors made of flexible metal tubes (sensors are dependent on the type of metal used)
- Displacement is recorded as a responses of an applied pressure in the open end of the tube
- Sensitivity depends on the thickness of the gauge, biggest range will be for gauges that provoke less displacement for same pressure input
- Can be connected to alarms
- Measure GAUGE pressure
Choosing a pressure sensor: What would you use if:
You need direct signal transmission?
Diaphragms and bellows or bourdon gauges
Choosing a pressure sensor: What would you use if:
You need to measure absolute, gauge and differential pressures?
Absolute (barometer, diaphragms and bellows
Gauge or differential (manometer, diaphragms and bellows)
Gauge (bourdon gauges)
Choosing a pressure sensor: What would you use if:
You were limited by reading errors?
Diaphragms and bellows or bourdon gauges (produce electrical output so no reading error)
Choosing a pressure sensor: What would you use if:
You needed a minimum accuracy of 1%
Bourdon gauges (1%), diaphragms and bellows (0.5%)
Choosing a pressure sensor: What would you use if:
You wanted to be able to control sensitivity?
Manometers (different configurations), bourdon gauges (by selecting a gauge that is thinner)
Choosing a pressure sensor: What would you use if:
High pressures
Bourdon gauges
Choosing a pressure sensor: What would you use if:
Low pressures?
Diaphragms and bellows
What instrument is used to calibrate pressure sensors? how does it work?
= Deadweight gauges (on top of metrology traceability pyramid)
The fluid pressure is therefore calculated in terms of the weight added to the platform and the known area of the piston.
Measuring vacuum. Why is it different than measuring normal pressure? What instruments must be used?
Vacuum is lower than atmospheric pressure; needed for many instruments
Vacuum instruments require HIGH SENSITIVITY: must be able to detect small changes in pressure
Low vacuum, medium vacuum, high vacuum. What are the instruments?
Low vacuum: capsule gauges
Medium vacuum: McLeod, thermal, capacitative gauges
High vacuum: ion gauges
Capsule Gauges. How do they work? What pressure can they detect?
Similar to diaphragm gauges (think small expanding air pockets) but more SENSITIVE:
Adding more capsules amplifies the signal.
Like diaphragm gauges, they can be used to measure gauge pressure, absolute pressure and differential pressure!
