Platinum RTD Elements
For over 100 years, due to its high purity and very special physical and chemical characteristics, platinum has been used as the best base material for temperature sensors. Sensors using this precious metal almost ideally meet the requirements needed for perfect and reliable operation a long lifespan.
Among the advantages of using platinum as a sensor material, we can highlight its high stability, repeatability and the possibility of operating in a very linear way within a wide working range (-200°C to + 850 C and reaching +1,000°C in some special cases).
Technical Information
The advantages of the special properties of platinum, when used in temperature sensors, are evident when compared to sensors that use other materials such as semiconductors (KTY®) or Thermistors (NTC):
Among the advantages we can mention:
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High precision
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Low drift
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Low hysteresis
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Extremely long service life
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High output signal and, consequently, easy electronic handling
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Wide temperature ranges from -200°C to 1000°C.
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Very linear dR/dT curve
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High Repeatability
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Fully interchangeable
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High resistance to thermal shock
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Excellent stability in any operating range
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Platinum sensors are most commonly set to 100 Ohm @ 0°C.
Platinum sensors generally have a resistance value of 100 Ohm @ 0 °C.
That is the motive why platinum RTDs are commonly called Pt 100 sensors, although platinum RTDs with resistance values @ 0°C of 25, 500, 1000 and up to 10,000 Ohm are available in the market, depending on the application and manufacturing technology.
The typical curve of industrial platinum sensors has a nominal coefficient of 0.3850 Ohm/K, however, there are other coefficients that can also be used, such as 0.3916 Ohm/K, 0.3750 Ohm/K and 0.3925 Ohm/K.
Platinum sensor operating principle
Temperature sensors using platinum have the operating principle of changing the electrical resistance as a function of temperature variation. Such an increase in electrical resistance causes a specific curve that can be defined mathematically.
This specific curve is defined and accepted internationally by the IEC 60751 Standard, which determines the α of platinum (Alpha):
The most industrially used platinum RTD construction technologies can be basically divided into two categories: Ceramic (CWW) or Flat Film.
Where: Rt is the resistance at 100°C and Ro is the resistance at 0°C.
By convention, we write the temperature coefficient of the platinum sensor as:
The standard curve is formed from the Callendar Van-Dussen equation, which defines resistance as a function of temperature as follows:
For -200 ≤ t < 0°C:
Where
t = temperature ITS-90 in °C
Rt = Resistance at temperature t in Ω
ro = Resistance at 0°C in Ω
and the constant:
Fort ≥ 0°C:
To perform the reverse calculation, that is, to calculate the temperature (°C) as a function of resistance, we must use the equations provided by ASTM Standard E1137/E1137M:
For t < 0°C:
Where
t = temperature ITS-90 in °C
Rt = resistance to temperature t in Ω
ro = resistance at 0 °C in Ω
The constant:
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
Fort ≥ 0°C:
Standards and Tolerances
As stated earlier, RTDs are specified by their corresponding temperature coefficients, and as consequence there are some standards.
In recent decades, the worldwide trend was to adopt the IEC 60751 as the standard. The temperature ranges as well as the tolerance classes in the Standard are based on practical experimentation with RTD sensor elements, manufactured with two technologies:
Tolerance Values: The RTD tolerance values are classified into two separate tables according to the manufacturing technology. Each table, in turn, is divided into four tolerance classes according to the values shown below:
|t|, temperature module in °C
Special Tolerances
There are also two tolerance classes that have not been standardized yet. However, the IEC 60751 accepts these special classes as long as the manufacturer and the user agree on the RTDs with such tolerances. The most widely accepted special classes used in the global market are 1/5 and 1/10 for the values of W0.3, respectively, W0.06 ± (0.06 + 0.001 | t |) and W0.03 ± (0.03 + 0.0005 | t |).
In addition to tolerances, special classes can also define wider working temperature ranges, which can cover values from -200°C to +900°C, depending on the manufacturer and manufacturing technology.
Current Measurement :
The current measuring of a RTD, according to the IEC 60751 standard, must be limited to a value whose self-heating (expressed in °C/mW) of a maximum of 25% of the class tolerance value under the conditions of maximum current.
The usual measurement of current is not greater than 1mA for a 100Ω ceramic sensor.
Connection wires configuration:
Sensors mounted with a tolerance class of W0.3 or F0.3 must be connected to the electronics with a 3-wire or 4-wire configuration. Mounted sensors can also feature a construction with one or two RTD sensors.
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For both cases, the IEC 60751 standard defines the following connection settings:
Insulation of mounted temperature sensors:
The insulation of mounted sensors must follow a minimum standard; otherwise, poor insulation may cause instability in sensor reading. The values versus temperature required by the standard are as follows:
Manufacturing and RTD Technologies:
The most industrially used platinum RTD construction technologies can be basically divided into two categories: Ceramic (CWW) or Flat Film.
Each technology has its typical characteristics and applications. The main feature of the ceramic sensor is the classic construction, in which a platinum coil is housed inside a high purity ceramic tube. On the other hand, the sensor flat film is characterized by a thin meander-shaped platinum layer, applied over a high purity ceramic substrate.