Thermal or Heat Detectors. ... There are two common types of heat detectors—fixed temperature and rate of rise. Both rely on the heat of a fire incident to activate the signal device. Fixed-temperature detectors signal when the detection element is heated to a predetermined temperature point.

1. thermal detector & its type

2. History
The first automatic electric fire alarm was patented in
1890 by Francis Robbins Upton an associate of
Thomas Edison George Andrew Darby patended the
first Europeon electrical heat detector in 1920 in
Birmingham England
Definition : thermal detectors are light detectors
which is used to rise induced temperature . they are
suitable for wide wavelength infrared spectral region

3. Type of thermal detectors
1.Thermocouple
2.Pyroelectric detector

4. Thermocouple (Thermopile):
Definition : thermocouple is a sensor used to
measure temperature.
Principle :
thermocouple consists of two wire legs made
different metals. The wire legs are welded
together at one end created a junction. This
junction is where the temperature is measured
when the junction experience a change in
temperature, a voltage is created.

5. DIAGRAM

6. Seeback effect: it is a phenomenon in which the temperature
difference between the two different metals induces the potential
differences between them.
Peltier Effect: the temperature difference can be created between
any two different conductors by applying the potential difference
between them.
Thompson Effect – The Thompson effect state that when two
dissimilar metals join together and if they create two junctions
then the voltage induces the entire length of the conductor
because of the temperature gradient.

7. Advantages of Thermocouple
 The following are the advantages of the
thermocouples.
 The thermocouple is cheaper than the other
temperature measuring devices.
 The thermocouple has the fast response time.
 It has a wide temperature range.

8. Disadvantages of the
Thermocouples
 The thermocouple has low accuracy.
 The recalibration of the thermocouple is difficult.
 Nickel-alloy, platinum/rhodium alloy, Tungsten/rhenium-alloy, chromel-gold,
iron-alloy are the name of the alloys used for making the thermocouple.

9. Measurement of
thermocouple:

10. Pyroelectric Detector:
Pyroelectric detectors are thermal dedectors .
Temperature fluctuations produced a charge. The
surface pyroelectric crystal which produces a
corresponding electrical signal.This temperature
gradient can be created by the absorptionbof
light.
Pyroelectrical material:
-Single Crystals
-Ceramics
-polymers

11. Diagram

12. WORKING:
When IR radiation is put on it the
temperature is rise out the surface
basically. When T is rises and due to
the presence of polyelectric
material.The charge is generated
when the charge is moved to
electrode the voltage is generated
and flow of electricity is start.

13. ADVANTAGES:
Uses at room temperature
Low power requirement
Relatively fast response
Low cost material
Faster than thermocouple

14. DISADVANTAGES:
Good solution are complex
Difficulty of scanning elements
Used for short distance
applications
Devices cannot move around
while transmission is in progress.