Temperature Sensors

1
Temperature Sensors

• 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).
• RT RTo 1 ao (T To
• Where T is temperature at which the
  resistance is desired
• To is the mid-point of valid temperature
• RT is the resistance at T predicted
• RTo is the resistance at To from the table
• ao is the fractional change in temperature
  per unit of temperature at To

2
Cont.

• The value a 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

3
Thermistors

• Thermistor is a semiconductor based transducers.
  It has very good sensitivity. It is 10. It is
  good for small temperature variations.
• Drawbacks
• Self heating effect
• Time constant
• Power Dissipation
• Time Delay

4
Thermocouple (TC)

• 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 - T1V V1/V - V1

5
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.

6
Thermocouple Comparison Data

• Type Elements mv/0 C Range0C Error(FS)
  Appliction
• E Chromel/Constantan 0.044 0 to 800 0.5
  High output
• J Iron/constantan 0.033 -250 to 700 0.75
  Reducing atm
• K Chromel/Alumel 0.020 -250 to 1200 0.75
  Oxidizing
• R S Pt-Rh/Platinum 0.010 0 to 1400
  0.25 Corrosive
• T Copper/Constantan 0.040 -250 to 350
  1.00 Moist
• C Tungsten/rhenium 0.012 0 to 2000
  0.5 High temp.

7
Displacement Sensors

• Potentiometric Its resistance varies as its
  wiper position changes.
• Inexpensive
• Physical contact wears out after long hours of
  use, generating noise as a result.

8
Linear Variable Differential Transformer (LVDT)

• 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.

9
Strain Gage

• Its resistance changes as the platinum wire
  changes its length and cross-sectional area.
• GF (dR/R)/(dL/L)
• Where
• GF is gage factor
• dR/R is resistance ratio
• dL/L fractional change in length, which is
  called the strain.