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Temperature Sensors
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Resistance Temperature Detectors (RTD) : consists of a small
coil of platinum or nickel wire protected by a sheath of
stainless steel. Its resistance increases with the temperature.
The range of these transducers extends from 00 C to 6000 C
(11120 F).
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R[T] = R[To] [ 1 + αo (T – To]
Where T is temperature at which the resistance is
desired
To is the mid-point of valid temperature
R[T] is the resistance at T predicted
R[To] is the resistance at To from the table
αo is the fractional change in temperature per
unit of temperature at To
Cont.
The value α all metals will be on the order of 0.4%
of the RTD resistance for every change of 10 C, in
other words, the value is 0.004 / 0 C .
RTD has drawbacks:
Power dissipation factor
Self heating effect
Delay time constant
non-lineariarity
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Thermistors
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Thermistor is a semiconductor based transducers.
It has very good sensitivity. It is 10%. It is good
for small temperature variations.
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Drawbacks:
Self heating effect
Time constant
Power Dissipation
Time Delay
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Thermocouple (TC)
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The thermocouple (TC) is a temperature
sensor that produces a voltage directly as a
function of temperature. The range is from
-150 0 C to 2000 0 C
 Interpolation
T(V) = T1 + [T2 - T1][V – V1]/[V - V1]
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Cont.

There are types E, J, K, C, R, S
R and S are high temperature sensors used
to measure temperature in the
semiconductor furnace as the one in the
Microelectronic Engineering clean room.
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Thermocouple Comparison Data
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Type Elements
mv/0 C Range0C Error(%FS)
E
Chromel/Constantan 0.044 0 to 800
0.5
J
Iron/constantan
0.033 -250 to 700 0.75
K
Chromel/Alumel
0.020 -250 to 1200 0.75
R & S Pt-Rh/Platinum
0.010 0 to 1400
0.25
T
Copper/Constantan 0.040 -250 to 350 1.00
C
Tungsten/rhenium 0.012 0 to 2000
0.5
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Appliction
High output
Reducing atm
Oxidizing
Corrosive
Moist
High temp.
Displacement Sensors
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Potentiometric: Its resistance varies as its
wiper position changes.
Inexpensive
Physical contact wears out after long hours
of use, generating noise as a result.
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Linear Variable Differential
Transformer (LVDT)
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LVDT is based on the electrical transformer
The difference of output voltages of two
secondary winding changes according to the
position of the core. AC signal is used,
typically 1000 Hz signal is used as the
source signal. Phase sensitive detector can
be used to convert AC to DC output.
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Strain Gage
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Its resistance changes as the platinum wire
changes its length and cross-sectional area.
GF = (δR/R)/(δL/L)
Where
GF is gage factor
δR/R is resistance ratio
δL/L fractional change in length, which is
called the strain.
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