3. Transducer
variations in a
physical quantity
another physical
quantity in
measurable form
Thermocouples 3
Transducers are used in instrumentation systems to
interface physical quantities with analytical ones.
4. Thermocouple
• 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.
Thermocouples 4
5. The Seebeck Effect
temperature
difference between
two junctions
voltage difference
between them
which drives the
current
Thermocouples 5
Eemf = - S T
where, S is the Seebeck coefficient
= T1 – T2
6. Basic Thermocouple Circuit
Thermocouples 6
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 )
7. How it looks like ?
Thermocouples 7
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.
8. 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
Thermocouples 8
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.
9. Thermocouple EMF vs Temperature
Thermocouples 9
This shows that the various thermocouples differ in their respective sensitivities.
11. Advantages
• Rugged
• Inexpensive
• Quick response time
• Very simple in construction
• Used to measure temperature ranges
spanning thousands of degrees, around
-200 C to 2200 C
Thermocouples 11
12. Disadvantages
• Low accuracy
• Vulnerable to corrosion
• Thermocouple operation is relatively
complex with potential sources of error
Thermocouples 12