Engineering, technology

1. Presently Used Industrial Thermometers.

2. Resistance Temperature Detectors
P M V Subbarao
Professor
Mechanical Engineering Department
Active Temperature Detectors……

3. History
• The same year that Seebeck made his discovery about
thermoelectricity, Sir Humphrey Davy announced that the
resistivity of metals showed a marked temperature
dependence.
• Fifty years later, Sir William Siemens proffer the use of
platinum as the element in a resistance thermometer.
• His choice proved most propitious, as platinum is used to
this day as the primary element in all high-accuracy
resistance thermometers.
• In fact, the Platinum Resistance Temperature Detector,15
or PRTD, is used today as an interpolation standard from
the oxygen point (-182.96°C) to the antimony point
(630.74°C).

4. BASIC THEORY
• The electrical conductivity of a metal depends on the
movement of electrons through its crystal lattice.
• Due to thermal excitation, the electrical resistance of a
conductor varies according to its temperature and this forms
the basic principals of resistance thermometry.
• The effect is most commonly exhibited as an increase in
resistance with increasing temperature, a positive temperature
coefficient of resistance.
• When utilising this effect for temperature measurement, a
large value of temperature coefficient is a deal.
• Stability of the characteristic over the short and long term is
vital if practical use is to made of the conductor in question.
• The relationship between the temperature and the electrical
resistance is usually non-linear and described by a higher
order polynomial.

5. 0
50
100
150
200
250
300
350
400
-300 -100 100 300 500 700 900
t, o
C
R,
Ohm
Platinum RTD
Copper RTD

7. The Birth of RTD
• The classical resistance temperature detector (RTD)
construction using platinum was proposed by C.H. Meyers
in 1932.
• He wound a helical coil of platinum on a crossed mica web
and mounted the assembly inside a glass tube.
• This construction minimized strain on the wire while
maximizing resistance.

8. Laboratory Standard
• Meyersʼ design produced a very stable element.
• The thermal contact between the platinum and the
measured point is quite poor.
• This results in a large thermal response time.
• The fragility of the structure limits its use today
primarily to that of a laboratory standard.
• Another laboratory standard has taken the place of
Meyersʼ design.
• This is the bird-cage element proposed by Evans
and Burns.

9. The Bird-cage RTD
Stainless Steel open ended fluid
probe is ideal for protecting the
temperature sensor at the same time
of reducing thermal mass

10. COMPARISON OF ELEMENT TYPES
0
50
100
150
200
250
300
350
400
-300 -100 100 300 500 700 900
t, o
C
R,
Ohm
Platinum RTD
Copper RTD

11. Platinum Sensing Resistors
• Platinum, with its wide temperature range and stability, has
become the preferred element material for resistance
thermometers.
• Platinum sensing resistors are available with alternative Ro
values, for example 10, 25 and 100 Ohms.
• A working form of resistance thermometer sensor is
defined in IEC and DIN specifications.
• This forms the basis of most industrial and laboratory
electrical thermometers.
• The platinum sensing resistor, Pt100 to IEC 751 is
dominant in many parts of the world.
• Its advantages include chemical stability, relative ease of
manufacture, the availability of wire in a highly pure form
and excellent reproducibility of its electrical characteristic.
• The result is a truly interchangeable sensing resistor which
is widely commercially available at a reasonable cost.

12. Calendar—Van Dusen equation
    
 
 
3
0 01
.
0
1
01
.
0
01
.
0
1
01
.
0
1 T
T
T
T
T
R
R 




 


• Where, , and are calibration constants, dependent on the purity of
the platinum which is country dependent.
•The dominant constant is  which has a value of either 0.003921/°C
for the so-called U.S. calibration curve, or
• 0.003851/°C for the "European" calibration curve.
•RTD sensors corresponding to either curve are available.
•For the U.S. calibration curve,   1.49,   0 for T < 0 and  = 0.11
for T>0.
 
4
3
2
0 100
1 T
C
T
C
T
B
T
A
R
R 








