variations in a
Transducers are used in instrumentation systems to
interface physical quantities with analytical ones.
• A thermoelectric transducer that converts
heat directly into electricity, according to
the Seebeck effect.
• This can be used to generate electricity,
measure temperature difference or change
the temperature of objects.
The Seebeck Effect
which drives the
Eemf = - S T
where, S is the Seebeck coefficient
= T1 – T2
Basic Thermocouple Circuit
If the SA and SB are constant for the
given temperature range, then
From Seebeck effect, the voltage
gradient at any point in the circuit,
Total measured end-to-end voltage
between the junctions,
Vb -Vc = (SA – SB) (Th -Tc )
= SAB (Th -Tc )
How it looks like ?
Its construction consists of two conductors, welded together at the measuring point
and insulated from each other along the length, inside an outer protection sheath.
Types of Thermocouples
Type K : Chromel-Alumel
Type J : Iron-Constantan
Type T : Copper-Constantan
Type E : Chromel-Constantan
Type N : Nicros-Nisil
Type S : Platinum-Rhodium
Type B : Platinum-Rhodium
Type R : Platinum-Rhodium
There are many thermocouples, but only 8 of them are standardized internationally.
BASE METAL NOBLE METAL
Each thermocouple type has a specific range of temperature and environment.
Thermocouple EMF vs Temperature
This shows that the various thermocouples differ in their respective sensitivities.
• Temperature Measurement
• Power Generation
• Radiation Sensors
• Gas Safety Equipments
• Manufacturing Industry
• Quick response time
• Very simple in construction
• Used to measure temperature ranges
spanning thousands of degrees, around
-200 C to 2200 C
• Low accuracy
• Vulnerable to corrosion
• Thermocouple operation is relatively
complex with potential sources of error