L.3 - Pressure Flashcards
Define Hydrostatic Pressure:
pressure exerted by a column of liquid
Define Differential Pressure w/ units:
the difference between two pressures. Unit: kPad or Psid
Define Absolute Pressure w/ units:
the pressure with respect to absolute zero pressure (a perfect vacuum). Unit: kPaa or Psia
Define Gauge Pressure w/ units:
pressure with respect to atmospheric or barometric pressure. Unit: kPag or Psig
Define Vacuum Pressure w/ units:
pressure less than atmospheric pressure (negative gauge pressure). Unit: kPag or Psig (negative sign)
What is a Manometer?
a pressure measuring device that uses the height of a column of liquid if known density to determine magnitude of applied pressure
What is a U-Tube Manometer?
it is a pressure measuring device that measure the difference in pressure between two ports (labelled P1 and P2)
What does the accuracy of a manometer depend on?
1) the accuracy of the measuring scale
2) the density of manometer fluid (temp. of, pressure of)
3) Ability of operator to accurately read the scale
What are some requirement for a manometer fluid to be usable?
1) it needs to be clean with few contaminants
2) it needs to have a precisely know density
3) it needs to have relatively low viscosity
4) little tendency to wet manometer tubes
5) in some cases low vapour pressure
What are pressure transducers?
sensors that measure pressures ranging from really low absolute pressures to really high gauge pressures
How do mechanical pressure transducers work?
they rely on elastic deformation of a material in response to a change in an applied pressure
List some mechanical pressure transducers:
1) Bourdon Tubes (C-type, Spiral, Helical)
2) Diaphragm
How do Bourdon Tubes Work?
> when the pressure applied to the inside of the tube is greater than the outside pressure, the tube tends to straighten because area of the outer circumference is > area of inside circumference
resultant force creates an outward movement/deflection at the tip of the tube
What material are bourdon tubes made of, and why?
who cares
> can be made up of stainless steel for ex.
material selection is based on the instrument service requirements, such as measurement range and material compatibility
What is “drift” in a bourdon type?
it is permanent deformation resulting from long-term use of a bourdon type
What affects the deflection of the bourdon tube tip?
1) Material Properties
2) Length of Tube
3) Thickness
4) Cross-Sectional Area
What is a Diaphragm, how does it work?
> a thin plate made up of metal/silicon supported along it’s outer edge by rigid housing
> diaphragm produces (small) linear deflection relative to applied differential pressure
What does a safety blowout do?
it vents process fluid away from the front of the gauge to prevent gauge glass from breaking (away from glass and blowout from back)
What does an oil filled case do?
filling the gauge with oil dampens mechanical vibrations, making the gauge easier to read and extending its service life
What does a snubber do?
used to dampen/restrict pressure fluctuations which could wear out the gauge and make it difficult to read
What is a chemical seal/Diaphragm seal used for?
it is placed between the gauge and the process fluid to help extend the gauge service life in case where the fluid is corrosive or to prevent fluid from plugging the sensing element
What does a DP cell do?
a differential pressure cell can be used to measure pressure, level, flow etc.
