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CHEE 291 TEST 2 > Module D: Pressure measurements > Flashcards

Module D: Pressure measurements Flashcards

1
Q

Pressure definition

A

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]

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2
Q

How does height of column and weight of fluid influence fluid pressure?

A

The taller the column = the lower the fluid pressure

The heavier the fluid = the higher the fluid pressure

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3
Q

What is static fluid pressure and how is it measured?

A

The mass of the fluid column is related to its volume and density.

P = F/A = mg/A = p(density)ghA/A = pgh

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4
Q

What are the different pressure measurement tools?

A

Barometers, manometers, diaphragms and bellows, bourdon gauges

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5
Q

What are barometers?

A

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))

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6
Q

What are manometers - single tube?

A

Gauge pressure measurement

fluid [COLUMN]

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7
Q

What are manometers - u-tube?

A

Gauge pressure measurement - relative to ATM OR Pressure difference between 2 GASES

Variation in pressure: deltaP = p(density)gdelta(h)

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8
Q

How can you measure pressure differences between liquids in manometers?

A

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)

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9
Q

How can you increase sensitivity of manometers?

A

Use different kinds of manometers (large well where fluid enters and small tube when difference in height is determined)

= bigger accuracy

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10
Q

What are pressure sensors with electrical output?

A

[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
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11
Q

What is the measurement system for pressure sensors with electrical output?

A

Pressure input —-> [Pressure sensing and transduction] —-> Movement in pressure sensor —-> [Transduction Element] —-> Electrical output —–> [Signal processing element] ——> Cleaned or amplified signal

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12
Q

What are the characteristics of diaphragms and bellows? How do they work? What kind of pressure can they measure?

A
  • 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)
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13
Q

What are the characteristics of bourdon gauges? How do they work? What kind of pressure can they measure?

A
  • 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
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14
Q

Choosing a pressure sensor: What would you use if:

You need direct signal transmission?

A

Diaphragms and bellows or bourdon gauges

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15
Q

Choosing a pressure sensor: What would you use if:

You need to measure absolute, gauge and differential pressures?

A

Absolute (barometer, diaphragms and bellows
Gauge or differential (manometer, diaphragms and bellows)
Gauge (bourdon gauges)

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16
Q

Choosing a pressure sensor: What would you use if:

You were limited by reading errors?

A

Diaphragms and bellows or bourdon gauges (produce electrical output so no reading error)

17
Q

Choosing a pressure sensor: What would you use if:

You needed a minimum accuracy of 1%

A

Bourdon gauges (1%), diaphragms and bellows (0.5%)

18
Q

Choosing a pressure sensor: What would you use if:

You wanted to be able to control sensitivity?

A

Manometers (different configurations), bourdon gauges (by selecting a gauge that is thinner)

19
Q

Choosing a pressure sensor: What would you use if:

High pressures

A

Bourdon gauges

20
Q

Choosing a pressure sensor: What would you use if:

Low pressures?

A

Diaphragms and bellows

21
Q

What instrument is used to calibrate pressure sensors? how does it work?

A

= 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.

22
Q

Measuring vacuum. Why is it different than measuring normal pressure? What instruments must be used?

A

Vacuum is lower than atmospheric pressure; needed for many instruments

Vacuum instruments require HIGH SENSITIVITY: must be able to detect small changes in pressure

23
Q

Low vacuum, medium vacuum, high vacuum. What are the instruments?

A

Low vacuum: capsule gauges
Medium vacuum: McLeod, thermal, capacitative gauges
High vacuum: ion gauges

24
Q

Capsule Gauges. How do they work? What pressure can they detect?

A

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!

25
Q

Ion gauges. How do they work? Where are they used?

A

Current is measured when it flows from anode to cathode

Electrons encounter gas molecules, ionizing them. Ions are collected on a thin wire. Current measured is proportional to pressure. The greater the pressure, the more ions, the greater the current.

Used in electron microscopes.

26
Q

Pirani gauges. How do they work?

A

Based upon heat dissipation of a filament (metal) wire. Heat dissipation is proportional to pressure. It is based on heat transfer/ thermal conductivity of a gas.

Thermal conductivity in absolute vacuum is 0 (there are no molecules in vacuum to transfer heat to)
Thermal conductivity of air in ambient conditions is 0.025 W/m*K (still quite small)

Thermal conductivity varies for ALL GASES

27
Q

How are temperature and pressure related in Pirani gauges?

A

The transfer of heat from the filament to the surroundings (in the Wheatstone bridge), depend on the pressure of these surroundings.

Low pressure (high vacuum): no molecules, heat has nowhere to go, filaments heats up.

High pressure (low vacuum): many molecules, heat transfers, filament cools down.

28
Q

What is the measurement system for a pirani gauge?

A

Excitation voltage —-> [Wheatstone bridge:sensor, transducer] —> current —-> [Filament: transmitter]—> Heat —-> [Sensor tube] —-> temperature

29
Q

How can we calculate power (heat) dissipated?

A

Consider the wire as a resistance temperature device (RTD), where the temperature of the wire is dependent on the dissipation coefficient of the gas.

We determine Kt (dissipation coefficient), which varies with gas pressure

30
Q

McLeod gauges. How do they work?

A

Use compression of a low pressure gas to determine pressure of unknown gas. Follows Boyle’s law:

PuVu = PcVc = gas with unknown pressure * volume of gas = compressed gas (in gauge) * volume of compressed gas

The volume of the bulb corresponds to the volume of the unknown

The volume of the compressed gas is the area of the capillary multiplied by the displaced height.

Manometer equation:

Pc = Pu + hpg

To solve problems we must consider: height travelled by volume,

31
Q

Measurement HIGH pressures. What instruments?

A

High pressure = pressure above 7000 bar

Carried out by monitoring electrical signal, such as how certain wire materials will change resistance when large pressures are applied [linear resistance vs pressure].

32
Q

What are pressure regulators? What is the purpose? What are two examples?

A

Pressure needs to be regulated to avoid very high pressure values or very low (safety and operability). Regulators can interrupt flow.

Relief valves:

switch can turn on and off pressure for overpressure or vacuum situations. operates according to THRESHOLD that opens to atmosphere or auxiliary line when too high or too low.

= reversible

Rupture disks:

Rapidly increase or decrease pressure in a vessel. Disks BURST when pressure is below or above limit thresholds

= NOT reversible

CHEE 291 TEST 2 (4 decks)

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