Lesson[2] .pdf temperature transduders

temperature

1. TEMPERATURE
TRANSDUCERS
Process and Instrumentation
Lerotholi Polytechnic
©2012

2. Learning outcomes
•Sketch the construction and plot the
characteristics for:
•Thermocouple
•The platinum RTD
•The IC temperature transducer
•The NTC thermistor
•State ranges application and
advantages/disadvantages of the devices
mentioned above.

3. Thermocouples
•Thermocouples cover a range of temperatures, from –262
to +2760 °C and are manufactured in many materials
•Relatively cheap
•Have many physical forms, all of which make them a
highly versatile device
Defects
•The output voltage generated is too small
•The second is their non-linearity

4. Construction and Principle of operation of
a thermocouple
•The thermocouple is essentially comprised of
two dissimilar metals joined together at one
end
•With this arrangement, when the ends that are
joined together are heated, an output voltage
is obtained between the other two ends

5. •The ends that are joined together are referred
to as the “hot” junction and the other ends are
referred to as the “cold” junction
•Most thermocouple metals produce a
relationship between the two temperatures
and the e.m.f. as follows:
e = α(θ
1 – θ
2) + β(θ
1
2
– θ
2
2
)
α and β are the constants for the type of
thermocouple

6. Cold junction compensation
•Be held at a constant known temperature,
usually 0 °C, and called a ‘Cold Junction’
•The temperature of these junctions should
be measured and the measuring instrument
takes this into consideration when
calculating its final output

7. Example
•Problem: Find the Seebeck voltage for a thermocouple
with α = 3.5 × 10
-2
and β = 8.2 × 10
-6
if the junction
temperatures are 40°C and 80°C.

8. The Platinum RTD
The construction
•Consists of a thin film of platinum deposited on a ceramic
substrate
•The gold contacts at each end make contact with the film.
•The platinum film is trimmed with a laser beam such that
its resistance is 100Ω at 0
o
C

9. Principle of operation of RTD
•The resistance of the film increases as the
temperature increases
•The increase in resistance is linear
•The relationship between resistance change
and temperature rise being 0.385Ω/
o
C for the
unit
R
t = R
o + 0.385T

10. Example
•Problem: A platinum RTD transducer has resistance of
100Ω at 0
o
C, what temperature would be represented by
115.2Ω?

11. The NTC Thermistor
The construction
•Consists of an element made from sintered
oxides of metals such nickel, manganese and
cobalt with contacts made to each side of the
element.

12. Principle of operation
•As the temperature of the element increases, its
resistance falls
•The resistance/temperature characteristics being non-
linear
•The relationship between resistance and temperature is
given by the formula:-
??????
2=??????
1??????
??????
??????2
−
??????
??????1
•Where R
1 = Resistance at temperature T
1
o
K
•R
2 = Resistance at temperature T
2
o
K
•B = characteristic temperature
• = 4350
o
K

13. The IC temperature transducer
•Integrated-circuit temperature transducers are available in
both voltage and current-output configurations.
•Both supply an output that is linearly proportional to
absolute temperature.
•Typical values are one microampere of current per one-
degree temperature change in Kelvin (1 µA/
o
C) and ten
millivolts per one-degree change in Kelvin (10 mV/
o
C).
•The LM35 is rated to operate over a −55° to +150°C
temperature range

14. •V
out = βT
Where β is the sensitivity of the IC detector
T is the temperature being measured.

LM35
+5 V O / P