Characteristics of PT100 sensors

The PT100 is a temperature sensor that at 0 °C has 100 ohms and its electrical resistance increases as the temperature increases. This PT100 sensor is the temperature sensitive heart of any resistance thermometer. Apart from the way of mounting, it is its characteristics that basically determine the technical measurement properties of the sensor.

The increase in the resistance of the PT100 is not linear but increasing and characteristic of platinum in such a way that by means of tables it is possible to find the exact temperature to which it corresponds. Normally, industrial PT100 probes are manufactured encapsulated in the same way as thermocouples, that is, inside a stainless steel tube or other material (sheath).

At one end is the sensitive element (RTD Sensor) and at the other is the electrical terminal of the cables protected inside a round aluminum box (head).

Construction and operation

Depending on the type of construction, the wound part of the PT100 in the form of a platinum wire or ribbon, which is sensitive to temperature, is encapsulated in a ceramic or glass body, or is found as a thin layer on a ceramic insert. The terminals of the measuring element are attached to the active resistive part in such a way that they resist vibrations. In the case of multiple PT100s, the respective coils are identified by the different length of the pairs of terminals. The active principle of the PT100 is the modification of its electrical resistance, which varies according to the temperature to which they are subjected. As an extended element, the PT100 collects the average value of the existing temperature over its entire length.

Rules

All platinum PT100s supplied and assembled by SRC comply with the basic values and permissible deviations of DIN IEC 751. DIN values apply to a nominal value of 100 Ohms.
The temperature stability of the PT100 with respect to its measured values likewise meets or exceeds DIN IEC requirements.

PT100 probes – Types and mounts

To obtain a safe temperature measurement with platinum PT100, it is necessary that its mechanical characteristics and measurement techniques (size, shape, resistance to vibrations, temperature range, response time, insulation resistance, etc.) are adequate for the class of measurement and the conditions of the place of application. The most standard models are made up of a stainless steel metal protection tube, in which the the sensor and has an output via cable, connector or headhe.

Apart from the temperature range of the PT100, the mounting will determine the temperature range of the PT100 probe.
Apart from the need to select the appropriate PT100 model, mounting in the place where you want to measure is of special importance. This requires a high degree of knowledge, so we recommend the advice of our applications technician.

Advantages of PT100 probes

PT100s being slightly more expensive and mechanically not as rigid as thermocouples, outperform them especially in low temperature applications. (-100 to 200°). PT100 sensors can easily deliver accuracies of one tenth of a degree with the advantage that the PT100 does not break down gradually giving erroneous readings, but rather opens normally, whereupon the measuring device immediately detects sensor failure and gives warning.

This behavior is a great advantage in all types of devices where an undetected temperature deviation could cause some serious damage. In addition, the PT100 can be placed at a certain distance from the meter without much problem using conventional copper cable to make the extension. It will depend on the type of connection to minimize signal losses, we will show them below.

Precautions

It is common in very humid environments for cables to deteriorate and for current to flow through them via condensed moisture. Even if it is minimal, this "leaked" current will make the reader display a lower temperature than it actually is. These leaks can also occur due to rust, moisture or dust covering the terminals.
From the description made of the measurement methods, it is clear that unlike thermocouples, it is not possible to connect 2 reading units to the same Pt100 since each one supplies its excitation current.

When purchasing a Pt100 sensor, it should be borne in mind that there are different qualities and prices for the sensor element that goes on the end of the Pt100 sensor. The best quality ones are made with a true platinum wire, while there are some cheap sensors made on the basis of a conductive paint on an alumina (ceramic) substrate. The latter are less accurate.
In general, a Pt100 should not be mounted in places subject to a lot of vibration as it is likely to fracture.

PT100 – Connection

PT100 2 wires

The easiest way to connect (but least recommended) is with just two cables. This is only recommended for measuring up to 10 meters from the temperature regulator since from there the pt100 sensor may have signal losses.

PT100 3 wires

The 3-wire connection mode is the most common and most widely used for industrial processes and it solves the problem of errors generated by cables well. The only requirement is that the three cables have the same electrical resistance because the measurement system is based (almost always) on the "Wheatstone bridge". Of course, the temperature reader must be for this type of connection. Most industrial equipment is prepared for a 3-wire connection.

PT100 4 wires

The 4-wire method is the most accurate of all and is used in the laboratory. The 4 wires can be different (different resistance) but the reading instrument is more expensive.