Proveden

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Experimental methods E181101
EXM2
Temperature(thermocouples, thermistors)
Some pictures and texts were copied from
www.wikipedia.com
Rudolf Žitný, Ústav procesní a zpracovatelské
techniky CVUT FS 2010
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State variables- temperature
EXM2
Temperature is measure of inner kinetic energy of
random molecular motion. In case of solids the
kinetic energy is the energy of atom vibration,
in liquids and gases the kinetic energy includes
vibrational, rotational and translational motion.
Statistically, temperature (T) is a direct
measure of the mean kinetic energy of particles
(atoms, molecules). For each degree of freedom
that a particle possesses (rotational and
vibrational modes), the mean kinetic energy (Ek)
is directly proportional to thermodynamic
temperature where k-is universal Boltzmann
constant. For more details see wikipedia.
Thermodynamic temperature is measured in Kelvins
K, that are related to different scales, degree
of Celsius scale TC273.15, or degree of
Fahrenheit F1.8C32. Remark you can say degree
of Celcius, or degree of Fahrenheit, but never
say degree of Kelvins - always only Kelvins.
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Temperature measurement
EXM2

• Thermometers
• Glass tube (filled by mercury or organic liquid,
  accuracy up to 0.001 oC)
• Bimetalic (deflection of bonded metallic strips
  having different thermal expansion coefficient)
• Thermocouples (different metals electrically
  connected generate voltage)
• RTD (Resistance Temperature Detectors
  temperature dependent electrical resistance)
  thermistors (semiconductors)
• Infrared thermometers
• Thermal luminiscence (phosphor thermometers
  time decay of induced light depends upon
  temperature used with optical fibres)
• Irreversible/reversible sensors (labels), liquid
  crystals

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Temperature measurement
EXM2
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Thermocouples
EXM2
Leger
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Thermocouples
EXM2
Measured voltage is given by temperature T2-T1.
Cold junction temperature T2 should be 0C. Or at
least measured by different instrument (by RTD).
Seebeck effect (electrons diffuse from hot to
cold end)
Different wire has no effect if T3 is the same at
both ends
It does not matter how the connection of wires is
realized (soldered, welded, mechanically
connected)
Usual configuration Cu wires to voltameter.
Measured voltage is given by temperature T2-T1
Exposed end Insulated junction
Grounded junction
Law of successive thermocouples (next slide)
T2
T3
T1
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Thermocouple pile
EXM2
T2
T1
V 3-times greater
Example of a thermocouple pile manufactured by
lithography
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Thermocouple types
EXM2
Chromel 90 nickel, 10 chromium Alumel 95
nickel, 2 aluminium, 2 manganese, 1
silicon NicrosilNickel-Chromium-Silicon Constanta
n 55 copper, 45 nickel
Type K (chromel-Alumel) , sensitivity 41 µV/C
J (ironconstantan) has a more restricted range
than type K (-40 to 750 C), but higher
sensitivity 55 µV/C
N (NicrosilNisil) high temperatures, exceeding
1200 C. 39 µV/C at 900 C slightly lower than
type K.
T (copperconstantan) -200 to 350 C range.
Sensitivity of about 43 µV/C.
E (chromelconstantan) has a high output (68
µV/C) which makes it well suited to cryogenic use
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Resistivity thermometers
EXM2
Specific electrical resistivity (units ?m) of
materials depends upon temperature. Temperature
can be therefore evaluated from measured
electrical resistance of sensor (resistor) by
using for example Wheatstone bridge arrangement

• There are two basic kinds of resistivity
  thermometers
• Thermistors (resistor is a semiconductor, or a
  plast) high sensitivity, nonlinear, limited
  temperature
• RTD (metallic resistor, see next slide) stable,
  linear, suitable for high temperatures. R100 ?.
  
• Another classification according to sign of
  temperature sensitivity coef.
• NTC (Negative Temp.Coef) typical for
  semiconductors, R2252 ? is industrial standard
  resistance.
• PTC (Positive Temp. Coef.) typical for metals, or
  for carbon filled plastics

Cold Hot sample
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RTD platinum thermometers
EXM2
RTD Platinum thermometers Pt100, Pt1000 (nominal
resistance 100/1000 Ohms respectively) Therefore
coefficient of relative temperature change is
approximately (this value slightly depends upon
platinum purity, for example typical US standards
?0.00392, Europian standard ?0.00385). 2-wires
(reading is affected by parasitic ohmic
resistance of long and tiny wires (which need not
be negligible in comparison with 100? of RTD).
Examplegt compute resistance of Cu wire for
specific resistivity of copper 1.7E-8
?.m 3-wires 4-wires
Parazitic resistances of leading wires are added
to the sensor resistance
Parazitic resistances of leading wires are partly
compensated
The most accurate arrangement
Almost zero current flows in these two wires as
soon as internal resistance of voltameter is
high
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Systematic errors in contact measurement
EXM2

• Pt1000 is in fact a tiny heater (at 1 mA, sensor
  generates RI20.001 W) and the heat must be
  removed by a good thermal contact with measured
  object.
• RTD-2 wires connection (resistance of leading
  wires are added to the measured sensor
  resistance). Specific resistance of copper is
  ?1.7E-8 ?.m, resistance of wire is R4?L/(? D2),
  L-length, D-diameter of wire.
• Time delay due to thermal capacity of sensor
  (response time depends upon time constant of
  sensor as well as upon thermal contact between
  fluid and the sensor surface, see next slide)
• Temperature difference between temperature of
  fluid and the temperature of measuring point
  (junction of thermocouple wires, or Pt100
  spiral). This difference depends upon the thermal
  resistance fluid-sensor and thermal resistance
  sensor-wall (resistance of shield). See next
  slide

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Time constant of sensor
EXM2
Demuth
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Time constant of sensor
EXM2
Time delay of sensor follows from the enthalpy
balance
Heat from fluid to sensor W
Enthalpy accumulation
where M-mass, cp specific heat capacity of
sensor, Ts temperature of sensor, ?-heat transfer
coefficient, S surface of sensor,
Tfluid-temperature of fluid (temperature that is
to be measured). For step change of fluid
temperature solution of this equation is
exponential function with time constant ?
Time constant is the time required by a sensor to
reach 63 of a step change temperature.
Heat transfer coefficient ? depends upon fluid
velocity (more specifically upon Reynolds number
or Rayleigh number in case of forced and natural
convection, respectively). Example for a
spherical tip of a probe and forced convection it
is possible to use Whitakers correlation
Nu-Nusselt number, D-diameter of sphere, ?
thermal conductivity of fluid, u-velocity of
fluid, ? kinematic viscosity, a-temperature
diffusivity. Conclusion the higher is mass of
sensor the greater if time constant. The higher
is velocity of fluid, the better (the shorter is
the time constant).
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Example time constant of sensors
EXM2
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Tutorial time constant of sensors
EXM2
Identify the time constant of a thermocouple
A/D converter NI-USB 6281
PC Labview
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Tutorial science direct reading
EXM2
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Tutorial science direct reading
EXM2
Rabin, Y., Rittel, D., 1998. A model for the time
response of solid-embedded thermocouples.
Experimental Mechanics 39 (2), 132136.
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Heat conduction by shield
EXM2
Scheeler
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Heat conduction by shield
EXM2
Distortion of measured temperature of fluid due
to heat transfer through wires or shielding of
detector. The error decreases with improved
thermal contact (fluid-surface, see above) and
reduced thermal resistance of leading wires or
shield RT. For wire or a rod the thermal
resistance is
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Example steady heat transfer (1/2)
EXM2
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Example steady heat transfer (2/2)
EXM2
toto platí jen pro malé Re, presnejší
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Heat transfer - tutorial
EXM2
Identify heat transfer coefficient (cross flow
around cylinder)
Pt100 T C
Cylinder H0.075, D0.07 m
cp910, rho2800 kg/m3
Df0.05m
Air cp1000, rho1 kg/m3, ?0.03 W/m/K
FAN (hot air)
OMEGA data logger (thermocouples) T1,T2 , T3
Watt meter
Measured 1.3.2011 1200 W
Example Re8000, Pr1
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Heat transfer - tutorial
EXM2
Example velocity of air calculated from the
enthalpy balance is 5 m/s (Tnozzle140 0C, mass
flowrate of air 0.01 kg/s) Corresponding Reynolds
number (kinematic viscosity 2.10-5) is
Re17500 Nusselt number calculated for Pr0.7 is
therefore
This is result from the heat transfer correlation
More than 2times less is predicted from the time
constant
Probable explanation of this discrepancy Velocity
of air (5m/s) was calculated at the nozzle of
hair dryer. Velocity at the cylinder will be much
smaller. As soon as this velocity will be reduced
5-times (1 m/s at cylinder) the heat transfer
coefficient will be the same as that predicted
from the time constant (76 W/m/K)
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Thermocouple - tutorial
EXM2
Record time change of temperature of air
compressed in syringe.

Thermocouple

P-pressure transducer Kulite XTM 140

Example V2/V10.5 ?cp/cv1.4 T1300
K T2396 K temperature increase 96 K!!