Temperature measurement with a Pt100 (or Pt1000) sensor
Electronics Engineering
Instrumentation - Systems - Softwares - Test benches
Specialized in the low-cost development of electronic instrumentation, we use our proprietary pre-developed technical building blocks to be able to rapidly answer to specific requests and to propose complete solutions ready for use.
The lead time to make the products available is often about 3 to 4 weeks.
English version under development, please refer to the french version for the complete information
Platinum temperature sensors (PRTD - Platinum Resistance Temperature Detector) are very commonly used in the industry because of their great reliability. Their principle is based on the variation with temperature of the electrical resistance of a platinum conductor (wire or foil), in accordance with well-known and very stable in time characteristics described by the standards IEC 60751, ASTM E1137, ... and the previous standards IEC 751, DIN 43760, ...
The platinum sensors can cover a large field of applications, and are sometimes even used beyond the temperature range -200°C to +800°C.
The most common models are referenced as Pt100, Pt500, Pt1000, which indicates their nominal electrical resistance at 0°C. For example for the reference Pt100 which is the most known sensor, the electrical resistance is 100 ohms at 0°C ; for a Pt500 it is 500 ohms at 0°C ; etc ...
The change in electrical resistance is nearly linear with the temperature, except for the very negative temperatures. For a Pt100, this coefficient is about +0.4 ohm/°C at 0°C and +0.35 ohm/°C at 300°C. For a Pt1000, the behavior is exactly the same, but the coefficient has to be multiplied by 10.
3-wire Pt100 sensor
Depending on the manufacturing process, the sensors are distributed among tolerance classes :
class at -100°C at 0°C at 100°C at 200°C at 300°C
B F(W)0.3 +/- 0.80°C +/- 0.30°C +/- 0.80°C +/- 1.30°C +/- 1.80°C
A F(W)0.15 +/- 0.35°C +/- 0.15°C +/- 0.35°C +/- 0.55°C +/- 0.75°C
AA F(W)0.1 +/- 0.27°C +/- 0.10°C +/- 0.27°C +/- 0.44°C +/- 0.61°C
...
Selecting a highly accurate sensor is only really useful, if the electronic interface is also of a sufficient accuracy and reliability, and above all if the thermal coupling is fully mastered between the sensor and the medium to be studied, because it can introduce by itself an error of several degrees.
Pt100 sensor 5 mm x 2 mm
Connection configurations and electronic interfaces
The selection of the connection configuration and of the type of electronic interface, depends on the ease of installation, the affordable cost and the accuracy which is expected. There is no "best solution" in the absolute, but for a given situation there is the more appropriate solution.
- The 2-wire configuration is the simplest one, but provides no measurement accuracy as soon as the effect induced by the electrical resistance of the wiring is in the same order of magnitude as the desired accuracy. Unless it is taken into account as a systematic error, a common 24AWG wire (85 ohms/km) will introduce a raw error of about 0.4°C per meter of wiring when connected to a Pt100 sensor, but only 0.04°C in the case of Pt1000.
- The 4-wire configuration theoretically represents the method providing the best accuracy, because the voltage is measured directly at the active part of the sensor with a high impedance interface so that the electrical resistance of the wires are no longer involved in the error of measurement. But in practice, it doesn't necessarily mean that this wiring configuration must absolutely be preferred.
- The 3-wire configuration very often ensures sufficient accuracy for a medium distance wiring. The principle is based on the assumption that the 3 wires have more or less the same electrical resistance, and the electronic interfaces specifically designed for this mode of connection use analog or digital methods to make the measurement error little dependent on the length of wiring.
For example, using a 18AWG wiring (21 ohms/km) is quite appropriate for a medium distance, and depending on the quality of the electronic interface, the error is usually smaller than 0.4°C for a 100 meters of wiring connected to a Pt100 sensor.
2-wire Interface
Example of a 3-wire Interface : VPt = V1 - 2 x V2
4-wire Interface
The main reasons for the inaccuracy of the measurement (Please refer to the french version)
Pt100 or Pt1000 ?
The response curves in temperature, of a Pt100 sensor and a Pt1000 sensor are similar but with a difference in scale of 10. Thus, a Pt1000 sensor has a nominal electrical resistance of 1000 ohms at 0°C and a coefficient of about +4 ohm/°C at 0°C. The Pt100 resistance/temperature calculator proposed on this page is still usable provided that the value of the electrical resistance is multiplied by 10.
Depending on what the objective is, the use of a Pt1000 sensor may prove to be very interesting in some cases.
For instance, as a source of error to be taken into account is the self-heating of the sensor due to the measurement current, if this current is divided say by 10, the voltage across the terminals of a Pt1000 sensor will be exactly the same as the voltage in a configuration with a Pt100 : UPt1000 = RPt1000 x I Pt1000 = (10 x RPt100) x (I Pt100/ 10) = RPt100 x I Pt100 = UPt100
But as a result, the power dissipated in the sensor will be divided by 10, which will lower the error due to the self-heating : PPt1000 = RPt1000 x (I Pt1000)2 = (10 x RPt100) x (I Pt100/ 10)2 = ( RPt100 x (I Pt100)2 ) /10 = PPt100 / 10
Furthermore, whether it is a 2-wire or a 3-wire configuration, it has to be noticed that the influence of the length of the wiring on the measurement error is 10 times lower with a Pt1000 sensor if compared with a Pt100.
But however as an advantage rarely comes without a drawback, depending on the length of the wiring and the electrical noise present in the measurement environment, an impedance multiplied by 10 contributes to the possibility of getting more noise on the measurements, and this increased sensitivity to electromagnetic interferences can cancel the advantage of selecting a Pt1000 sensor.
Consequently, the choice between a Pt100 sensor and a Pt1000 sensor largely depends on the type of installation and the desired accuracy. However, it has to be remembered that in the case of a digital conversion close to the measurement point (less than 2 meters), the choice of a Pt1000 sensor is generally always preferable.
Pt100 resistance/temperature calculator
°C
<=>
Ω
Out of range -200°C / 850°C
A calculator is always wellcome to determine the relation between the electrical resistance and the temperature, and is also much faster than using the conversion tables.
This tool is also downloadable here as a HTA file (HTML Application). It is pleasant to use during development works using Pt100 sensors, and appears as an unobtrusive very small window on the screen.
USB data acquisition modules for temperature measurement
Temperature is a very important physical parameter often taken into account in technical tests and physics experiments. TECHNETEA most often implements 3-wire Pt100 and 2-wire PT1000 interfaces in its data acquisition systems and test benches, but other configurations can also be considered (4-wire connection, Pt500, etc ...) for some more specific requests.
Physical aspect, number of channels, measurement range, accuracy, noise processing, response time, are parameters taken into account during the design stage as well for the electronics than the firmware, to be able to answer to specific characteristics requested by the client.
Temperatures recording carried out with a TECHNETEA module 16x Pt1000 (2-wire).
(Complete file with a recording of 2 minutes, downloadable to be viewed under Excel ®)
When several different types of electronic interfaces are routed on the same Printed Circuit Board, the resulting data acquisition module can also measure other physical parameters than only temperatures ; or it can even relay power to perform more complex functions as for example temperature controls.
USB : 4 inputs for 3-wire Pt100
USB : 3 inputs for 3-wire Pt100 + 1 isolated ouput
USB : 16 inputs for 2-wire Pt1000 -40°C/+200°C input range
Thermal analysis with 8x Pt1000 sensors Real time display software
(10 mm) Pt1000 sensor (with connector) for contact measurement
USB module with 8x 2-wire Pt1000 under development
The wide variety of possibilities leads to imagine the design of specific custom-made data acquisition modules especially compact.
Example of a temperature recording file
