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Instrumentation, Measurement and Control > Pressure measurement > Flashcards

Pressure measurement Flashcards

1
Q

S.I units of pressure

A

N/m^2

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2
Q

the difference between measured pressure and a perfect vacuum

A

absolute pressure

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3
Q

usually the difference between measured pressure and local atmospheric pressure

A

Gauge pressure

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4
Q

difference between two measured pressure

A

differential pressure

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5
Q

Old fashioned pressure measurement device which is a U-shaped tube with the ends exposed to different pressures. Liquid column will move until its weight is in equilibrium with the differential pressure

A

Manometer

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6
Q

Pressure difference equation

A

density * gravitational acceleration * height

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7
Q
  • Has a oval cros section
  • Increase in pressure in inside of tube
  • Measures gauge pressure
  • Tube tries to attain a circular cross section: straightening, movement is magnified by using a curved tube, causes pointer to move and display pressure (small stresses so tube won’t bend permanently)
A

Bourdon Tube

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8
Q

Advantages of mechanical pressure gauges

A
  • display only, transmitters or switching devices
  • Instruments available for low to very high pressures
  • Very good visual display
  • Manual devices are inexpensive
  • Accurate
  • Certificate available
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9
Q

Disadvantages of mechanical pressure gauges

A
  • Bulky
  • Transmitters types are very bulky and relatively expensive
  • Over pressure damages instrument
  • Non hygienic: threaded connection, process fluid in tube, difficult CIP/SIP
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10
Q
  • flexible membrane to separate fluid from measuring element
  • deflection of diaphragm is translated into a pressure measurement
  • ## Typically 316 stainless steel: range of materials and liners can be selected for compatibility with process fluid
A

Diaphragm element gauges

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11
Q
  • These have circular shaped corrugated membranes. Typically supplied with clamp connections. Process fluid only contacts the surface of the diaphragm
  • The deflection on the diaphragm is a measure of the pressure and is indicated on the dial
  • The deflection of the diaphragm exerts a pressure on the fill fluid in the bourdon tube behind the diaphragm
A

Diaphragm element gauges

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12
Q

Diaphragm valves are … and suitable for CIP/SIP.

A

hygienic

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13
Q
  • Good if the measurement is required in an awkward location - use of a longer capillary allows the gauge to be located away from the measurement point
  • Hear can cause fill fluid to expand: need to use temperature compatible fluids
A

Diaphragm element gauges

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14
Q

Diaphragm element gauges advantages

A
  • Hygienic (3A & EHEDG certificates)
  • Suitable CIP/SIP
  • Suitable for aggressive media
  • Suitable for adhesive,crystallising and corrosive media
  • Suitable for highly viscous media
  • Suitable for environmentally hazardous, toxic media
  • Range of contact materials (SS, monel, hastelloy)
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15
Q

convert pressure directly to electrical signal and are a more modern type of gauge.
these gauges are usually supplied with flush mounted hygienic connections (CIP/SIP).

A

pressure cells

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16
Q

gauge filled with liquid such as silicone oil or glycerine

A

filled gauge

17
Q

gauge with no filling

A

dry gauge

18
Q

Advantages of filled gauges

A
  • protects against high dynamic pressure loads
  • dampens vibrations
  • prevents internal moisture
  • reduces condensation on the dial
  • protects against adverse hot & cold temperature extremes
  • more reliable and durable
19
Q

Must be chemical compatible with process fluid and non txic in food applications. Special fill fluids are also available for oxidising media such as oxygen and chlorine

A

fill fluids

20
Q

Pressure cells disadvantage

A

tend to have a small display if fitted at al

21
Q

Pressure cells advantages

A
  • have hygienic connections
  • compact
  • accurate
  • can function as transmitters or switches
  • incorporating overload protection
22
Q
  • dry sensor
  • pressure deflects ceramic diaphragm
  • this causes a change in capacitance : capacitance inversely proportional to distance
  • hygienic
  • high chemical stability due to resilient ceramic
  • high mechanical stability
  • suitable for vacuums
  • Up to 40 bar
A

Ceramic diaphragm capacitive pressure sensor

23
Q
  • pressure deflects diaphragm
  • fill fluid transfer pressure to a resistance bridge
  • pressure causes change in voltage
  • available with and without diaphragm seals
  • capillary extender to move sensor away from high temperatures
  • very high pressure capability - 700 bar
A

Metallic pressure sensor

24
Q

opens or closes an electrical contact when a certain pressure is reached

A

pressure switches

25
Q

Two options for differential pressure measurement

A
  • two sensor modules wit ha single transmitter
  • single differential pressure module
26
Q

Protection against overpressure

A
  • pressure relief
  • bursting discs/panels
27
Q

Circular discs fitted between two flanges
Expensive but very accurate
Non fragmenting
Need replacing after an over pressure event

A

bursting discs

28
Q

typically comprised of a polyamide membrane with an insulated electrically conductive path mounted between supporting rings. It is replaced along with the rupture disc

A

Burst Tel

29
Q

Magnetic switch that is activated by the fluid flow from the burst disc. It is not destroyed by over pressure and therefore does not need to be replaced

A

Flow Tel

30
Q

What presssure Bourdon tubes measure

A

gauge pressure

31
Q

Why are control signals sent as currents?

A

Overcomes issues of resistance in wires

32
Q

How are valves usually actuated?

A

Using pressurised air

33
Q
A

Instrumentation, Measurement and Control (12 decks)

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