Temperature Sensors

Sensores de Temperatura
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Know the main features and specifications of a platinum temperature sensor.

Platinum Temperature Sensors

One of the most used physical properties for measuring temperature is that of the resistance increment due to the temperature. The sensors using this property are those 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) Variação da Ohmic resistance variation
Sensor cost Moderate to low Moderate to low Low Low
Hardware cost Moderate Moderate to lo High Low
Temperature Range -200ºC to +1000ºC (660ºC max. tip.) -100ºC to +300ºC, special types up to +500 ºC (125ºC max.tip.) -270ºC to +1800ºC, depending on the type. -55°C a +300°C
Valores fundamentales 50 a 10.000W 1KW a 1MW 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 tip. ±0.5%, ±2ºC ±1%, ±3ºC
Stability Excellent Moderate Low Moderate
Sensitivity 0.39% /ºC -4% /ºC 40mV /°C 10mV /°C
Linearidade Excellent Low (logarithmic) moderada moderada
Coefficient (α) Positive Negative Positive Positive
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):

α = (R100 - Ro) / Rºx100°C

As a convention, we write the platinum sensor temperature coefficient as:

α = 3.851 x 10-3 °C-1 where R100 is the resistance at 100°C and Ro the resistence at 0°C.


The standard curve is constituted from the Callendar Van-Dussen equation, which defines the resistance in view of the temperature as follows:

For t < 0°C:

Rt = Ro [1+At+Bt2-C(t-100°C) t3] Ω

For t ≥ 0°C:

Rt = Ro (1+At+Bt2) Ω

Where:

Rt = Resistance (Ω) at temperature t (°C)

Ro = Resistamce (Ω) atà 0°C

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

In order to perform the reverse calculation, that is, to calculate the temperature (°C) in view of the resistance, we should use the equations provided by Standard ASTM E1137/E1137M:

Equation operating principle of the platinum sensors

Where:

t = temperature in °C

Rt = resistance to temperature t

Ro = resistance a 0 °C

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


From mathematic relations described above, Standard IEC 60751 defines a reference value table for platinum sensors, expressed in Ohm, for the range from -200 ºC to +850 ºC.


Temperature Range (°C)
Tolerance Wire wound resistors Film resistors Tolerance values
AA -50 to +250 0 to +150 ± (0.1 + 0.0017 |t|)
A -100 to +450 -30 to +300 ± (0.15 + 0.002 |t|)
B -196 to +600 -50 to +500 ± (0.3 + 0.005 |t|)
C -196 to +600 -50 to +600 ± (0.6 + 0.01 |t|)

|t|, temperature module in °C without considering the signal


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:

Wire Wound Resistors Film Resistors Tolerance Value (°C)
Tolerancia Clase Rango de Temperatura[°C] Tolerancia Clase Temperatura Rango [°C]
W0.1 -100 to +350 F0.1 0 to +150 ± (0.1 + 0.0017 |t|)
W0.15 -100 to +450 F0.15 -30 to +300 ± (0.15 + 0.002 |t|)
W0.3 -196 to +660 F0.3 -50 to +500 ± (0.3 + 0.005 |t|)
W0.6 -196 to +660 F0.6 -50 to +600 ± (0.6 + 0.01 |t|)

|t|, temperature module in °C without considering the signal


Special Tolerances

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 Ɩ t Ɩ ) and W0.03 ± (0.03 + 0,0005 Ɩ t Ɩ ).

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.


Measuring Current:

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:


Tolerancias Especiales RTDs

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:

Temperature °C Insulation resistance MΩ
Up to 250 20
251 to 450 2
451 to 650 0,5
651 to 850 0,2

Manufacturing technologies an RTD:

The technologies of construction of platinum RTDs most used industrially basically can be divided between two categories: Ceramic (CWW - Ceramic Wire Wound) or Movie Plan (Thin Film).
There are other fabrication technologies used unless such as:
Slot sensors
Sensors glass (GWW - Glass Wire Wound)
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.

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