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Temperature Measurement Sensors
Temperature measurement sensors are common in the process control industry. Because of this , many
methods have been developed for temperature measurement sensors. Common types of temperature
measurement sensors include Resistance Temperature Detector (RTD), Thermocouple, Thermistors,
Pyrometers and Infrared.
Resistance Temperature Detector (RTD)
Function
A change in temperature will cause an RTD to heat or cool, producing a proportional
change in resistance. The change in resistance is measured by a precision device that is
calibrated to give the proper temperature reading. RTDs are like electrical transducer,
converting change in temperature to voltage signals by measuring resistance. RTDs are the
most accurate temperature sensors used for any industrial application below 600 deg. C
Construction
The RTD are made of pure metals or certain alloys that increase in resistance as temperate
increases and decreases in resistance as the temperature decreases. These metals should be
pure, of uniform quality and supposed to be stable within a given range of temperature.
Platinum, Copper or Nickel are most used metals to construct RTDs as they have a linear
resistance-temperature characteristics. They are highly able to withstand repeated
temperature Cycles.
Types of RTDs
• 2-Wire RTD: This type of circuit provides one connection to each end of the sensor.
• 3-Wire RTD: This type of circuit provides one connection to one end of the sensor
and tow connections to the other end of the sensor.
• 4-Wire RTD: Two connections connected at each end of the sensor. This type of
circuit is the most effective way to eliminate lead wire effects.
RTD with protective well & terminal head

Thermocouples
Function
A device that converts thermal energy into electrical energy. Thermocouples cause an
electric current to flow in the attached circuit when subjected to change in temperature.
The amount of current produced depends on the temperature difference between the
measurement and reference junction, the characteristics of the two metals used and the
characteristics of the attached circuit.
Construction
Thermocouple is constructed of two dissimilar metal wires joined at one end. They may be
constructed of several different combinations of materials.
Dissimilar Metal Wire 1 +
Dissimilar Metal Wire 2 -
The performance of a thermocouple material is generally determined by using that
material with platinum. The leads of the thermocouple are encased in a rigid metal sheath.
The measuring junction is formed at the bottom of the thermocouple housing.
Hot & Cold Junction
The junction that is put into the process in which temperature is being measured is called
the HOT JUNCTION. The other junction is at the last point of thermocouple material and
which is almost always at some kind of measuring instruments, is called COLD
JUNCTION.
Types of Thermocouples
Type Positive Wire Negative Wire Temp. Range

J Iron Constantan 32 to 1400 Deg. F
K Chromel Alumel 32 to 2300 Deg. F
T Copper Constantan -300 to 700 Deg. F
E Chromel Constantan 32 to 1600 Deg. F