Examples and characteristics of pressure gauge

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Examples and characteristics of pressure gauge
Examples of pressure sensor packaging
Temperature characteristics of a piezoresistive
pressure sensor. Transfer function at three
different temperatures (a) and full-scale errors
for three values of compensating resistors (b).
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Pressure measurement Pirani Gauge
Pirani vacuum gauge is a sensor that measures
pressure through thermal conductivity of gas. The
simplest version of the gauge contains a heated
plate. The measurement is done by detecting the
amount of heat loss from the plate that depends
on the gas pressure. When an object is heated,
thermal conductivity to the surrounding objects
is governed by
Gs is thermal conductivity via the solid
supporting elements Gr is the radiative heat
transfer a is the area of a heated plate k is a
coefficient related to gas properties PT is a
transitional pressure that is the maximum
pressure that can be measured.
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Pressure measurement Pirani Gauge

• If the solid conductive and radiative loss is
  accounted for, the gas conductivity Gg goes
  linearly down to absolute vacuum. The trick is to
  minimize the interfering factors that contribute
  to G0.
• This can be achieved by the use of both the
  heated plate that is suspended with a minimal
  thermal contact with the sensor housing and the
  differential technique that to a large degree
  cancels the influence of G0.

Thermal conductivities from a heated plate (a).
Transfer function of a Pirani vacuum gauge (b)
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Pressure measurement Pirani Gauge
Pirani vacuum gauge with NTC thermistors
operating in self-heating mode

• The sensing chamber is divided into two identical
  sections where one is filled with gas at a
  reference pressure, say 1 atm, and the other is
  connected to the vacuum that is to be measured.
• Each chamber contains a heated plate that is
  supported by the tiny links to minimize a
  conductive heat transfer through solids.
• Both chambers are preferably of the same shape,
  size, and construction so that the conductive and
  radiative heat loss would be nearly identical.
• The bridge automatically sets temperature of Sr
  on a constant level Tr that is defined by the
  bridge resistors and is independent of the
  ambient temperature.

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Acoustic Sensors Classifications

• Condenser Microphone Produces output voltage
  proportional to distance if the charge is kept
  fixed
• Fiber-Optic Microphone Measures diaphragm
  deflection by comparing reflected beams
• Piezoelectric Microphones Converts deflections
  directly into voltage
• Electret Microphones No external voltage high
  sensitivity, high frequency of operation high
  dynamic range one of the most widely used.
• Dynamic microphones Voltage induced by coil
  moving in a permanent magnet filed.
• Solid State Acoustic Detectors Measures
  vibrations to perform detection etc. Examples are
  SAW devices.

Please refer to the handout for detailed
information
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Radiation sensors scintillating detectors

• Scintillating detectors (example NaI)
• Widely used for sensitivity and ease of us
• Poor energy resolution
• Scintillating film can have reliability issues
• Uses a photomultiplier tube

Scintillation detector with a photomultiplier
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Radiation sensors Ionization sensors

• Ionization Detectors
• Oldest and most widely used
• Ionization in gas happens at 10 20 eV energy
• The positive and negative charged ions drift to
  the electrodes under application of an electric
  voltage
• Could be realized using ionization or
  proportional chambers

Simplified schematic of an ionization chamber (a)
and a current vs. voltage characteristic (b)
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Radiation sensors Ionization sensors

• Under application of an electric voltage the
  current increases in a very different manner for
  different voltage region as shown below

• Region 1 Not all ions are swept away some
  recombine
• Region 2 (saturation) All ions are swept away
  continuous mode operation no secondary
  ionization can be used for energy resolution
• Region 3 (proportional region) Avalanche
  (Townsend) multiplication can occur as the
  accelerating voltage is large enough for ions to
  collide and create more ions. Used in pulsed
  mode can give energy resolution of the incoming
  radiation
• Region 4 (Geiger-Muller counters) In this
  region, the energy of incoming radiation dont
  matter. No energy resolution.

Various operating voltages for gas-filled
detectors