One of the most used physical properties for temperature measurement is the increase in resistance as a function of temperature. The sensors that use this property are called RTDs (Resistance Temperature Detectors) or thermoresistances. Materials with such property and which are most used in the manufacture of temperature sensors are: platinum (Pt), nickel (Ni), copper (Cu) and some semiconductors.
For over 100 years, because of its high purity and its very special physical-chemical characteristics, platinum has been used as the best base material for temperature sensors. Sensors using this precious metal contemplate almost ideally the necessary requirements for a perfect and reliable operation for any years in service. Among advantages in the use of platinum as sensor material we can point out its high stability, repeatability and the possibility to operate on a quite linear way within an extensive operating range (-200°C to +850°C and may reach +1.000°C in some special instances).
The advantages of planinum special properties, when used on temperature sensors, are evident when compared to sensors which use other materials such as semiconductors (KTY®) or Thermistors (NTC):
|Sensor Type||Pt RTD||Thermistor (NTC)||Thermocouple||KTY|
|Sensor element material||Platinum||Ceramic semiconductor||Joining of two metals||Silicon semiconductor|
|Operation Principle||Ohmic resistance variation||Ohmic resistance variation||Joint joint (FEM – Electromotive Force))||Ohmic resistance variation|
|Sensor cost||Moderate to low||Moderate to low||Low||Low|
|Hardware cost||Moderate||Moderate to low||High||Low|
|Temperature Range||-200ºC to +1000ºC (Usually 660ºC maximum)||-100ºC to +300ºC, special types up to +500 ºC (Usually 125ºC max.)||-270ºC to +1800ºC, depending on the type.||-55ºC to +300ºC|
|Core Values||25 a 10.000Ω||1KΩ a 1MΩ||10 mV @ 25°C||750mV @ 25°C|
|Accuracy||0,03 °C||Typical ± 1% in resistance||For class 2 depending on the type ± 2.5ºC or 0.0075 |t|||Typical ± 5%, best accuracy in resistance ± 0,5%|
|Interchangeability||±0.06%, ±0.2ºC||±10%, ±2ºC||±0.5%, ±2ºC||±1%, ±3ºC|
|Sensitivity||0.39% /ºC||-4% /ºC||40mV /°C||10mV /°C|
|Noise sensitivity||Very low||Low||High||Low|
|Special requirements||-||Linearization||Reference joint||-|
Among the advantages we can mention:
- High accuracy
- Low drifting
- Low hysteresis
- Extremely long lifetime
- High output signal and, consequently, of an easy electronic handling
- Broad temperature range from -200 to 1000°C.
- Quite linear dR/dT curve
- High Repeatability
- Fully interchangeable
- High resistance to thermal shock
- Excellent stability on any operating range
The platinum sensors have as most common definition the value 100Ohm @ 0°C.
For this reason, platinum RTDs are usually called Pt 100 sensors, although there are platinum thermoresistances in the market with resistance values @ 0 from 25, 500, 1000 and up to 10,000 Ohm, depending on the application and the manufacture technology. The typical curve of industrial platinum sensors presents a rated coefficient of 0.3850 Ohm/K, however, there are other coefficients which can also be used, such as 0,3916 Ohm/K, 0.3750 Ohm/K and 0.3925 Ohm/K.
Platinum sensor operating principle
The temperature sensors using platinum have as operating principle a change of the electrical resistance in view of the temperature variation. Such increment in the electric resistance causes a characteristic curve which may be mathematically defined.
This characteristic curve is defined and internationally accepted by Standard IEC 60751 which determines the platinum α (Alpha):
where Rt is the resistance at 100°C and Ro the resistance at 0°C.
As a convention, we write the platinum sensor temperature coefficient as:
The standard curve is constituted from the Callendar Van-Dussen equation, which defines the resistance in view of the temperature as follows:
For -200 ≤ t < 0°C:
For t ≥ 0°C:
t = temperature ITS-90 in °C
Rt = Resistance at temperature t in Ω
Ro = Resistance at 0°C in Ω
and the constants:
A = 3.9083 x 10-3 °C-1
B = -5.775 x 10-7 °C-2
C = -4.183 x 10-12 °C-4
t = temperature ITS-90 in °C
Rt = resistance to temperature t in Ω
Ro = resistance a 0 °C in Ω
and the constants:
A = 3.9083 3 10−3 °C−1
B = −5.775 3 10−7 °C−2
D1 = 255.819 °C
D2 = 9.14550 °C
D3 = −2.92363 °C
D4 = 1.79090 °C
Standards and Tolerances
As previously said, the RTDs are specified by their corresponding temperature coefficients and, for such, there are some standards.
In the last few decades, the world’s trend was to adopt Standard IEC 60751 as standard. The temperature ranges as well as the tolerance classes in the Standard are based on the practical experimentation of the RTD sensor elements, manufactured with two technologies:
Tolerance values:The RTD tolerance values are classed as two distinct tables, according to their manufacture technology. Each table, in turn, is divided into four tolerance classes as per the values shown below:
|Tolerance class (CWW)||Temperature range (CWW)||Tolerance class (Thin Film))||Temperature range (Thin Film)||Tolerance (°C)|
|W0.1||-100 to +350 °C||F0.1||0 to +150 °C||± (0,1 + 0.0017 |t|)|
|W0.15||-100 to +450 °C||F0.15||-30 to +300 °C||± (0,15 + 0.002 |t|)|
|W0.3||-196 to +660 °C||F0.3||-50 to +500 °C||± (0,3 + 0.005 |t|)|
|W0.6||-196 to +660 °C||F0.6||-50 to +600 °C||± (0,6 + 0.01 |t|)|
|t|, temperature module in °C
There are also two tolerance classes which so far have not been standardized. However, IEC 60751 accepts these special classes provided that the supply of RTDs with these tolerances is agreed upon between manufacturer and user. The most accepted special classes used in the world market are 1/5 and 1/10 of the W0.3 values, respectively, W0.06 ± (0.06 + 0.001 |t|) and W0.03 ± (0.03 + 0,0005 |t|).
Besides the tolerances the special classes can also define more extensive working temperature ranges, which may covers values from -200 to +900 ºC, depending on the manufacturer and the manufacturing technology.
An RTD measuring current, according to Standard IEC 60751, should be limited to a value, which self-heating (expressed in °C/mW), reaches maximum 25% of the class tolerance value on the maximum current conditions.
The usually agreed measuring current is not higher than 1 mA for a ceramic sensor of 1000 Ω.
Configuration of connecting wires:
The sensors assembled with a tolerance class W0.3 or F0.3 should be connected to the electronic circuit with a 3- or 4-wire configuration. The assembled sensors may also present a construction with one of two RTD sensors.
For both instances, Standard IEC 60751 defines the connection configurations:
Insulation of the assembled temperature sensors:
The insulation of assembled sensors has to follow a minimum pattern; otherwise, the low insulation may cause an unstability on the sensor reading. The values versus temperature required by the standard are as follows:
|up to 250 °C||20 MΩ|
|251 to 450 °C||2 MΩ|
|451 to 650 °C||0,5 MΩ|
|651 to 850 °C||0,2 MΩ|
Manufacturing technologies an RTD:
The most industrially used platinum RTD construction technologies can be divided basically into two categories: Ceramic (CWW) or Thin Film.
Each technology has its characteristics and typical applications. The ceramic sensor has as its main characteristic the classical construction, in which a platinum coil is housed within a ceramic tube of high purity. Already the sensor flat film is characterized by a thin layer of platinum meander-shaped, applied on a ceramic substrate of high purity.