Pressure measurement
From Wikipedia, the free encyclopedia
•Manometer: a pressure measuring
instrument, usually limited to measuring
pressures near to atmospheric. The term
manometer is often used to refer specifically
to liquid column hydrostatic instruments.
Zero reference
• Absolute pressure is zero referenced against a
“perfect vacuum” (it-the value-is equal to gauge
pressure plus atmospheric pressure).
• Gauge pressure is zero referenced against
ambient air pressure; it-the value-is equal to
absolute pressure minus atmospheric pressure.
Negative signs are usually omitted; often
expressed as “inches of vacuum” or some such.
Examples?
• Differential pressure is the difference in
pressure between two points. Examples of uses?
Zero reference
•
•
•
•
Absolute pressure
Gauge pressure
Differential pressure
Are these distinctions:
– Meaningful?
– Accurate?
– Useful?
– Explain and discuss
Pressure conversion table
Error? In Wikipedia???
• “In American and Canadian engineering,
stress is often measured in kip. Note that
stress is not a true pressure since it is not
scalar. “
• Really! “kip” stands for kilopounds. Is this
a unit of stress or pressure?
Is pressure related to stress?
• Pressure is a property of fluids, which, by
definition cannot support a shear.
• Stress comes in three forms:
– Tensile/compressive stresses are related to forces
normal to a surface
– Shear stresses are in the plane of the surface
– The bulk modulus is related to hydrostatic forces
(pressure)
• Except for the fact that the bulk modulus is
measured by applying hydrostatic pressure,
stress relates to properties of solids
• Stress is not relevant to discussions of pressure
Hydrostatic Gauges
• Hydrostatic gauges (such as the mercury
column manometer) compare pressure to the
hydrostatic force per unit area at the base of a
column of fluid. Hydrostatic gauge
measurements are independent of the type of
gas being measured, and can be designed to
have a very linear calibration. They have poor
dynamic response.
• Why?
Piston
• Piston-type gauges counterbalance the
pressure of a fluid with a solid weight or a
spring. For example dead-weight testers
used for calibration and Tire-pressure
gauges.
• How do you get from spring displacement
to pressure?
• How do you “calibrate” a tire gauge?
Liquid column
ΔP = mg Δh
Bourdon Gauge (Mechanical)
Key concept: pressure
difference across different
areas of inner and outer
surfaces causes crescent to flex
Diaphragm (Ancient, mechanical)
If each element is sealed
with a known, fixed internal
pressure, flex will depend
on pressure change on
outside. Stacking amplifies
effect!
Diaphragm (modern, capacitance)
• resistive (strain gauge)
• inductive
• capacitive - The deflection of the piston is often one half
of a capacitor, so that when the piston moves, the
capacitance of the device changes. This is a common
way (with proper calibrations) to get a very precise,
electronic reading from a manometer, and this
configuration is called a capacitive manometer
vacuum gauge.
• “This is also called a capacitance manometer, in which
the diaphragm makes up a part of a capacitor. A change
in pressure leads to the flexure of the diaphragm, which
results in a change in capacitance. These gauges are
effective from 10−3 Torr to 10−4 Torr.” [Nonsense! MKS
sells capacitance gages over the range 0.01 – 155,000
Torr!]
• piezoelectric/piezoresistive
Capacitance Manometer
•
•
•
•
A = Annular electrode
D = Disk electrode
S = Substrate
G = Getter (in vacuum
space)
• Differential capacitance
between annulus and
disk depends on
pressure difference
between Test Chamber
and “Getter”.
Thermal conductivity gauges
• Thermal conductivity of gases depends on density
• Gas density is directly proportional to pressure at a given
temperature.
• A thermocouple or Resistance Temperature Detector
(RTD) can then be used to measure the temperature of
the filament.
• This temperature is dependent on the rate at which the
filament loses heat to the surrounding gas, and therefore
on the thermal conductivity.
• Common variant: Pirani gauge
– uses a single platinum filament as both the heated element and
RTD. These gauges are accurate from 10 Torr to 10−3 Torr, but
they are sensitive to the chemical composition of the gases
being measured.
Heat Transfer of Gases
• Conductivity is
linear in
pressure over
about 2 orders
of magnitude.
• Viscous flow
regime
• Pirani and
thermocouple
gauges
Ionization gauge
• Most sensitive gauges for very low
pressures (high vacuums, AKA "hard"
vacuums).
• Sense pressure indirectly by measuring
the electrical ion current produced when
the gas is bombarded with electrons.
• Ion density will be proportional to gas
density.
• The calibration of an ion gauge is unstable
and dependent on the nature of the gases
being measured, which is not always
known.
Ionization gauges
• Useful range: 10-10 - 10-3 Torr (roughly 10-8 - 10-1
Pa)
• Hot cathode version: an electrically heated
filament produces an electron beam. The
electrons ionize residual gas molecules. Ion
current current depends on the number of ions,
which depends on the pressure in the gauge.
• Cold cathode version: the same, except that ions
are produced by electrons from a high voltage
electrical discharge.
Making vacuum: pumps
• Two kinds:
– Displacement
• Remove gas from system by putting it somewhere
else
• Works like a water pump
– Entrapment
• Remove gas molecules from vapor
• “Gas” remains in the system
• Works like a garbage can? Better example?
Rotary vane pump
Exhaust
Intake
The workhorse of
physics labs for the
last…70?... years!
It was invented
by Charles C. Barnes
of Sackville, New Brunswick
who patented it on
June 16, 1874.[1]
Note that the vanes
are spring-loaded,
so the expand and
contract while traversing
the eccentric hole.
Scroll Pump
Exhaust port in center
Intake
Blue area rotates
counterclockwise
with silver area sprung
against it. Volume
between parts decreases.
Oil Diffusion Pump
Water
cooling
Text
Tex
t
Boiler
Vapor jet entrains residual
gas molecules.
Molecular flow required.
Hence must be backed.
“It’s all about momentum
transfer!”
Turbomolecular Pump
Oilfree
Must be backed
Vacuum to 10-7 Torr
Expensive
Finite lifetime (5 years)
Cryopumps
Basically just
two cold fingers:
77 K traps water
10 K traps
everything else
but hydrogen
Activated carbon
adsorbs hydrogen
Must be backed
SEM’s run at 10-10 Torr!