Temperature Measurements ARHAM VEERAYATAN INSTITUTE OF ENGINEERING TECHNOLOGY AND RESEARCH B. E. III, Semester – V (Chemical Branch) SUB: INSTRUMENTS & PROCESS CONTROL Prepared by: Pipaliya Chirag Rakholiya Samip Roshiya Haresh Rupapara Renish Sheikh Tausif

Outline 1. Liquid-in-glass thermometres 2. Bimaterial thermometres 3. Electrical thermometres 4. IR-thermometres 5. Pyrometres 6. Summary 7. Other measurement methods

Liquid-in-glass thermometres

 The “traditional” thermometres  Measurement scale from -190 °C to +600 °C  Used mainly in calibration  Mercury: -39 °C … +357 °C  Spirit: -14 °C … +78 °C

Functionning method  Method is based on the expansion of a liquid with temperature  The liquid in the bulb is forced up the capillary stem Thermal expansion: Thermal expansion:

Structure

Causes of inaccuraties  Temperature differences in the liquid  Glass temperature also affects  The amount of immersion (vs. calibration)

Bimaterial thermometres Method based on different thermal expansions of different metals Method based on different thermal expansions of different metals –Other metal expands more than other: twisting –Inaccurary ± 1 ° C –Industry, sauna thermometres

Bimaterial thermometres

Electrical thermometres

Resistive thermometres Resistive thermometres –Resistivity is temperature dependent –Materials: Platinum, nickel

Characteristic resistances

Thermistor thermometres Semiconductor materials Semiconductor materials Based on the temperature dependence of resistance Based on the temperature dependence of resistance Thermal coefficient non-linear, 10 times bigger than for metal resistor Thermal coefficient non-linear, 10 times bigger than for metal resistor NTC, (PTC): temperature coefficient’s sign NTC, (PTC): temperature coefficient’s sign

Example of a characteristic curve

Limitations of electrical thermometres Sensor cable’s resistance and its temperature dependency Sensor cable’s resistance and its temperature dependency Junction resistances Junction resistances Thermal voltages Thermal voltages Thermal noise in resistors Thermal noise in resistors Measurement current Measurement current Non-linear temperature dependencies Non-linear temperature dependencies Electrical perturbations Electrical perturbations Inaccuracy at least ± 0.1 °C Inaccuracy at least ± 0.1 °C

Infrared thermometres

Thermal radiation Every atom and molecule exists in perpetual motion Every atom and molecule exists in perpetual motion A moving charge is associated with an electric field and thus becomes a radiator A moving charge is associated with an electric field and thus becomes a radiator This radiation can be used to determine object's temperature This radiation can be used to determine object's temperature

Thermal radiation Waves can be characterized by their intensities and wavelengths Waves can be characterized by their intensities and wavelengths –The hotter the object:  the shorter the wavelength  the more emitted light Wien's law: Wien's law:

Planck's law Magnitude of radiation at particular wavelength (λ) and particular temperature (T). h is Planck’s constant and c speed of light.

Blackbody An ideal emitter of electromagnetic radiation An ideal emitter of electromagnetic radiation –opaque –non-reflective –for practical blackbodies ε = 0.9 Cavity effect Cavity effect –em-radiation measured from a cavity of an object

Cavity effect Emissivity of the cavity increases and approaches unity Emissivity of the cavity increases and approaches unity According to Stefan-Boltzmann’s law, the ideal emitter’s photon flux from area a is According to Stefan-Boltzmann’s law, the ideal emitter’s photon flux from area a is In practice: In practice:

Cavity effect For a single reflection, effective emissivity is For a single reflection, effective emissivity is Every reflection increases the emyssivity by a factor (1-ε) Every reflection increases the emyssivity by a factor (1-ε)

Cavity effect

Practical blackbodies Copper most common material Copper most common material The shape of the cavity defines the number of reflections The shape of the cavity defines the number of reflections –Emissivity can be increased

Detectors Quantum detectors Quantum detectors –interaction of individual photons and crystalline lattice –photon striking the surface can result to the generation of free electron –free electron is pushed from valency to conduction band

Detectors –hole in a valence band serves as a current carrier –Reduction of resistance Photon’s energy

Detectors Thermal detectors Thermal detectors –Response to heat resulting from absorption of the sensing surface –The radiation to opposite direction (from cold detector to measured object) must be taken into account

Thermal radiation from detector

Pyrometres Disappearing filament pyrometer Disappearing filament pyrometer –Radiation from and object in known temperature is balanced against an unknown target –The image of the known object (=filament) is superimposed on the image of target

Pyrometres –The measurer adjusts the current of the filament to make it glow and then disappear –Disappearing means the filament and object having the same temperature

Disapperaring filament pyrometer

Pyrometres Two-color pyrometer Two-color pyrometer –Since emissivities are not usually known, the measurement with disappearing filament pyrometer becomes impractical –In two-color pyrometers, radiation is detected at two separate wavelengths, for which the emissivity is approximately equal

Two-colour pyromerer

Pyrometers –The corresponding optical transmission coefficients are γ x and γ y Displayed temperature

Measurements –Stefan-Boltzmann’s law with manipulation: –Magnitude of thermal radiation flux, sensor surface’s temperature and emissivity must be known before calculation –Other variables can be considered as constants in calibration