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The Pt100 probe is the most widely used type of resistance temperature detector (RTD = Resistance Temperature Detector), and like all resistance thermometers, it uses electrical resistance to measure temperature. Thus, a resistance thermometer does not directly show temperatures, but the amount of resistance in ohms as a function of temperature.
Platinum is the main primary material used in high-precision resistance temperature sensors, including the Pt100 sensor. The Pt100 Resistance Thermometer is the most widely used temperature probe in manufacturing process control because it covers a wide temperature range from -200 °C to +850 °C and boasts good measurement accuracy and repeatability, which is also a prerequisite for laboratory measurements.
For these reasons, the Pt100 probe is often preferred over the thermocouple.
The Pt100 probe has a resistance of 100 ohms at 0 °C and 138.5 ohms at 100 °C. Its resistance varies linearly with temperature, i.e., as temperature increases, so does the resistance of the Pt100; therefore, if we can measure resistance, we can determine temperature.
The Pt100 type resistance thermometer takes its name from the measurement scale with which they are equipped.
The acronym “Pt” is the chemical symbol for platinum and indicates that the sensor is made of platinum (Pt).
“100” indicates that at 0 °C the sensor has a resistance of 100 ohms (Ω).
1) Basic element
Let’s look at the physical properties of platinum. Platinum is a basic element with the chemical symbol Pt. (as mentioned, this is the first part of the designation of the Pt100 resistance thermometer).
Platinum has a molecular weight of 195, giving it a lot of mass and confirming it as a rather heavy metal. The presence of free electrons in it makes it capable of conducting electricity efficiently, although its conductivity is not comparable to that of copper or silver.
2) Linear fashion
Platinum exhibits an electrical resistance that varies almost linearly with temperature. In the case of Pt100 it has a resistance of 100 ohms at zero degrees Celsius (second part of the Pt100 designation).
3) Inert property
A further characteristic of platinum, which makes it extremely advantageous in the field of temperature measurement, is its high chemical inertness. In fact, it shows a remarkable resistance in reacting with other substances, giving it a fundamental stability for this type of applications.
Let’s see how the temperature measurement process inside the Pt100 probe works.
As mentioned, the Pt100 temperature probe falls into the group of sensors called “resistance temperature detectors”, commonly called “RTD sensors ” or “resistance thermometers”.
The sensing part of the thermometer contains a resistance measurement sensor; when the temperature changes, the electrical resistance of the Pt100 sensor increases or decreases.
So, Pt100 measures temperature by using a change in resistance to indicate the temperature value.
For a Pt100, the resistance at 0°C is 100Ω, while at 100°C it is 138.5Ω. Therefore, the change in resistance for each degree Celsius change is 0.385Ω.
There are two types of pt100 which differ from each other:
Thin film resistance pt100. This type is smaller in size and has high vibration resistance. Currently, thin film Pt100 resistance thermometers are the most widely used and are suitable for use in the range of -50°C to +500°C.
Wire wound resistance pt100. Pt100 wire wound resistance thermometers are suitable for use in the temperature range -200°C to +600°C. They are often used in cryogenic or high temperature fields.
The structure of a Pt100 probe can also include several Pt100 resistors: 1, 2 or 3 × Pt-100. The most common is 1 × Pt-100.
The Pt100 resistance thermometer can be made with 2-, 3- or 4-wire connection, where the most accurate is the 4-wire connection.
In the standard version, the Pt100 probe is resistant to vibrations, but it can be made with special arrangements to make it resistant to strong vibrations and extreme circumstances.
Similarly, the Pt100 probe can be made in the ATEX version, i.e., suitable for measuring temperatures in explosion-prone areas.
A Pt100 sensor is typically constructed by wrapping a thin platinum wire around a non-conductive core that helps support the thin wire. The entire assembly, called a sensing element, is enclosed in a sheath to protect the sensor, and give it stability.
PT100 sensors can be constructed from a single platinum wire, providing a sensor with two leads, or from two platinum wires providing a dual sensor with independent outputs from each other.
The connection cables can be connected to an I/O board designed to accept RTD inputs, or they can be connected to a temperature transmitter, which will output a standard 4-20 milliamp signal proportional to the signal received from the resistance thermometer.
In either case, the I/O board or transmitter will have firmware that will determine the temperature read by the RTD from the measured resistance.
The tolerance class of a Pt100 probe indicates the accuracy with which the sensor can measure the temperature defined by the IEC 60751 standard.
The most common accuracy (tolerance) classes for Pt100 probes are class AA, A, B and C.
There are also more accurate accuracy (tolerance) classes, such as 1/5 DIN or 1/10 DIN constructed as fractions of the B tolerance class values.
However, these tolerance classes are less common and are usually more expensive than classes A, B and C. Furthermore, to ensure such a precise tolerance class they have narrow fields of use.
A resistive temperature sensor such as the resistance thermometer can be connected to the readout instrumentation with 2, 3 or 4 wires.
The choice of connection depends on the degree of accuracy required in temperature measurement and the type of process application.
The 2-wire Pt100 is the least accurate as the resistance of the connection cable adds up as an error to the measurement of the resistance at the temperature detected.
As mentioned, this type of connection does not compensate for the resistance of the connecting cable, which can greatly affect the measurement reading, to a greater extent as the cable length increases.
The 2-wire Pt100 is the simplest configuration but less accurate and reliable than the 3-wire Pt100 and 4-wire Pt100. It is usually used with short leads or where high accuracy is not required.
To compensate for the added resistance, a second platinum wire is added to the sensor at the third lead.
The third lead is used to determine the lead resistance itself, which is subtracted from the overall measurement resistance, providing the true resistance due to temperature change alone.
The requirement is that the diameter of the conductors, and therefore their resistances, are the same, as is assumed from a 3-wire connection.
In other words, the 3-wire Pt100 is a resistance thermometer that uses three wires to connect the sensor to the reading instrumentation, allowing for compensation of variations in the electric cable.
The 3-wire Pt100 is the most used in industrial applications where the temperature measurement accuracy is better than the 2-wire Pt100, but less precise and reliable than the 4-wire Pt100.
The 4-wire Pt100 resistance thermometer is very precise and is often used in laboratories, to measure the temperature of fluids and gases and where maximum reading accuracy is required.
The 4-wire Pt100 differs from the 3-wire Pt100 due to the presence of an additional wire for each pole of the sensor. These compensation wires eliminate the effect of variations in the electrical wires that carry the signal to the reading instruments.
Therefore the 4-wire Pt100 resistance thermometer is more accurate and reliable than the 3-wire Pt100 because of the compensation of the resistances of the wires used for measurement.
As mentioned, the principle of operation is to measure the resistance of a platinum element. The most common type (Pt100) has a resistance of 100 ohms at 0°C and 138.4 ohms at 100°C.
For a Pt100 sensor, a temperature change of 1 °C will cause a resistance change of 0.384 ohms, so even a small error in the resistance measurement (for example, the resistance of the wires leading to the sensor) can cause a large error in the temperature measurement.
Therefore, for precision measurements it is necessary to use 4-wire Pt100 resistance thermometers: two to carry the sensing current and two to measure the voltage across the sensor element.
It is also possible to opt for 3-wire sensors, although these operate on the (not necessarily valid) assumption that the resistance of each of the three wires is the same.
In fact, only the 4-wire connection fully compensates for the effect of the wire resistance of the entire cable circuit.
With 2 or 3 wire connections it is possible to manually program the measured cable resistance as a transmitter parameter to correct the final reading. But even in these cases the parameter is only corrected to the ambient temperature at the time of the cable measurement, losing its accuracy as the ambient temperature changes.
The 4-wire connection also compensates for changes in ambient temperature.
In Italy, Pt100 wire colors may vary depending on the standard used by the manufacturer or the specific application.
However, the most common standard in Italy is the one established by the EN 60751 standard, which provides the following colors:
There are many options to consider when choosing the right temperature probe for your needs: