Measurement of Pressures and Temperatures

Measurement of Pressures and Temperatures

Václav Uruba CTU Prague, AS CR

State of Fluids

• Liquids

• Gases – equation of state

• Any physical quantity is function of temperature T and pressure P

Ideal gas - air

PV  RT

3

3,484 10 P kg3

T m

   ^{ } ^

7 0,75

2,561 10 kg

T m s

 ^  

   

 

4 0,87

1,83 10 J

T K m s

 ^  

   

  ^{l 1,098T1,25}  ^m

 P

State Quantities

• Temperature – THERMOMETERS

• Pressure – MANOMETERS

• Both STATIC – flowing fluid

TOTAL

(stagnation)

High velocities

Adiabatic Compression

T – static temperature

T₀ – total (stagnation) temperature

1 2 0

1 1

2

T M

T

  ^

 

  

^{ 1,4}

Temperature on a sensor

• We measure recovery temperature

• For

0

1 Tr T

r T T

  



 

0 : _r 0

M  T  T  T

Tr

0 : _r 0

M  T  T

PRESSURE

Pressure Units

• Pascal [Pa = N/m²]

• mm H₂O (9.81 Pa)

• mm Hg, Torr (133.322 Pa)

• bar (10⁶ dyn/cm² = 10⁵ Pa)

• atm (1.0133 x 10⁵ Pa) – sea level

• at (kgf/cm² = 0.981 x 10⁵ Pa) – tech. atm.

• psi (Pound-force per square inch = 6895 Pa)

Manometers

• Types

– Differential pressure - PSID

– Absolute pressure (rel. to ref.) - PSIA – Gauge pressure (rel. to atm.) - PSIG – Vacuum pressure (negative gauge)

– Sealed pressure (rel. to atm. at sea level) - PSIS

• Principles

– Liquid column – Elastic parts

Liquid Column

• Water k = 9.8

• Alcohol k = 7.6

• Mercury k = 133

p h g  

   

p Pa  h mm k 

k    g

Technology

• Elastic parts

– Bourdon tubes – Diaphraghms – Bellows

• Deformation indication

– Mechanical

– Piezoresistive strain gauge – Capacitive

– Electromagnetic – Piezoelectric – Optical

– Potentiometric – …

Bourdon tubes

Sealed tubes

Elastic elements

Capacitance measuring

• Robust

• Linear

• Precise

• Stable

• Big

• Low frequency

Piesoresistive diaphragms

• High sensitivity

• High frequencies

• Small

• Temperature influence

• Nonlinear

• Fragile

Pressure scanners

• Piesoresistive

• Up to 64 sensors

• Electronics

Pressure-Sensitive Paints

• Deactivation of photoexcieted molecules of organic luminosphores by oxygen molecules (quenching).

• Discovered by H. Kautsky and H. Hirsch in 1935.

• Certain materials are luminous when excited by the correct light wavelength.

• The luminescence is dependent on air pressure.

Pressure-Sensitive Paints

• Qualitative

• Only high pressures  high velocities

Surface pressure distribution

TEMPERATURE

Methods

• Thermal expansivity

• Electrical sensors

• Optical methods

Method of thermal expansivity

• Liquid in Glass Thermometers

• Filled System Thermometers

• Bimetallic Thermometers

Electrical sensors

• Thermocouples

• Resistance Temperature Detectors (RTDs)

• Thermistors (THERMal resISTORS)

Thermocouples

+

• Cheep

• Small, small inertia

• Big range

-

• Small sensitivity

• Worse precision

• Reference

Resistance Temperature Detectors

+

• Precision, stability

• Simple

• Range (50-1000K)

-

• Bigger

• Price

platinum: 0.0038

wire

2w, 3w, 4w

 

0 1

Rt  R t

Thermistors

• Semiconductors (oxids of Mn, Ni, Co, Cu, Fe, Ti)

• Steinhart-Hart equiation

+

• Sensitivity

-

• Nonlinear

• Low temp (upto 300°C)

 

 

1/ ln ln

T  A B R  C R 

Optical Methods

• Liquid crystals

• Radiation Thermometers (RTs)

• Thermal Imaging (Thermography)

• Laser-Induced Fluorescence

• Rayleigh scattering

Temperature of surface!!!

Temperature of fluid

Liquid-crystal temperature-sensitive films

• Hydrophobic derivatives of polyvinyl alcohol and cholesteric liquid crystals

• Encapsuled (friction)

• Temperature -5 to +150°C

• Thickness 30-50m

• High sensitivity

Radiation Thermometers (infrared)

• Non-contact sensors

• Electromagnetic radiation received

• Range -40 °C to 3000 °C

Thermal Imaging

• Infrared range of the electromagnetic spectrum (9 000–14 000nm)

• Black body radiation law (Planck)

SURFACE !!!

PLIF

• Planar laser-induced fluorescence (planar-LIF) - instant whole- field concentration or temperature maps in liquid flows.

See more in „Optical methods“

Rayleigh scattering

• The Rayleigh signal is dependent on:

– Laser intensity

– Scattering cross section – Number density

• If species composition and pressure are known in the gas the gas temperature can be determined from imaging of the Rayleigh scattering.

See more in „Optical methods“