Today

1
Today

• Temperature transducers
• RTDs
• Termistors
• Types and models
• Calibration
• Strain effects
• Fixing and precautions
• Bridge connections

2
RTD

• Metal resistor resistance is subject to variation
  due to temperature effects depending on the
  material.

Platinum offers a smooth linear relation in the
field-200C ? 150C with R(T) ??R0 ( 1 ?T )
0.5 linearity ??between 0?100C commonly is
?? 0.00385 (C)-1
3
RTD

• Resistance Temperature Detector are based on the
  dependence between resistivity and temperature
  
• ? resistivity L length A
  section area

? ???R? ????T?
4
RTD
Wire rolled over or drowned in glass or
ceramic Protective caps in metal, glass or clay
5
RTD

• Due to its linearity PLATINUM is the most used
• T.R.P. (ThermoResistor Platinum)
• Pt-100 that is TRP having R0100?R.T.D.
  (Resistance Temp. Detector)

6
RTD
Temperature coefficient at 0C ?/(?C)
Fusionpoint
Material
Platinum
1773
0.00392
Nickel
1455
0.0065
Copper
1083
0.00425
Platinum
Nickel
Copper
-300 -200 -100 0 100 200 300 400 500 C
7
RTD
A 2 wire connection bridge has to be balanced
thanks to a variating resitor R4 therefore
determining RX hence TX

• Its a NULL MODE measuring principle

8
RTD
RX RRTD Rwire therefore we need Rwire RRTD
which cannot be true if wiring requires long
cables
9
RTD

• A 3 wire connection automatically balances any
  variation of Rwire

10
RTD

• A common connection is the 4 wire voltmeter
  connection using Icost than reading EOUTRRTD I

11
RTD
1) Probe heating problems Q R I2?? T????
measuring error Therefore I ? ? (1??20 mA, DC o
AC) A solution is touse an impulseexcitation
andreading.
12
RTD

• 2) High time constant??
• The bigger the probe mass the bigger the thermal
  inertia (fastest RTDs have ????0.1 s)
• 3) High sensitivity to mechanical strain and
  vibrations
• Like in strain gages ??? ?R? ???T?

13
Thermistors

• Thermistors are semiconductors thermometers build
  using sintherized metallic oxides(Mn, Ni, Co, Si
  ecc.)
• Their resistance varies as a function of
  temperature with high sensitivity R f ( T )
  R?? T???

14
Thermistors
R resistance value is high, but transducer
accuracy is low, making thermistors suitable for
rough measurements
15
Temperature transducers issues

• Probe transfers heat from or to the measurand
  body both by radiation and by convection

16
Temperature transducers issues

• At tip head Tstatic lt Ttot therefore the higher
  the fluid velocity V the higher the contribution
  on the reading due to the pressure differential.
• A static calibration is needed to compensate
  this effect

17
Pyrometers

• Planck LAW
• W? emispherical radiation intensity
• ? radiation wavelength
• T absolute temperature
• The Planck law express the emission of a black
  body at a T temperature in each wavelength ?

18
Pyrometers
Black body radiation
19
Pyrometers

• Thanks to the Stefan-Boltzman law we know that
  the integral of W?(?, T) on the whole spectrum is
  given by
• Which is valid for black bodies or black body
  practical approximations

20
Pyrometers
?
21
RTD
Emissivity of the actual body is a major
uncertainty contributor that has to be accounted
for different bodies at the same temperature
will have different emissivity.
22
Pyrometers
Emissivity depends also on the viewing angle the
same body under different angles will lead to
different readings
23
Pyrometers

• The radiation, focused by the lens, heats the
  secondary transducer (a thermopile or a
  thermistor)

24
Infrared imaging
25
Infrared imaging

• A calibration is needed to account for emissivity
  ?.
• Produces monocrome images having I ( x, y) ??T (
  x, y ) than converted into false colors.
• Applications
• - Medical diagnostics
• - Civilian diagnostics
• - Environment monitoring
• - Non destructive tests
• - etc.....