In brief
A temperature sensor detects the temperature of media or components; in industry, the Pt100 resistance thermometer is the most widely used type.
Contents
What is Temperature sensor?
Temperature sensors convert temperature into an electrical signal. Besides thermocouples and semiconductor sensors, industrial plants mainly use platinum resistance thermometers. In a Pt100, the resistance at 0 °C is exactly 100 ohms and rises almost linearly with temperature; the characteristic curve and tolerance classes are defined in IEC 60751. A Pt1000 has 1000 ohms at 0 °C.
The measuring insert is usually housed in a protective tube or thermometer stem, which is installed directly in the medium or in a thermowell in the pipeline. It is connected using two-, three- or four-wire technology; more wires compensate better for lead resistance. Key ratings are sensor type, tolerance class, measuring range, insertion length, protective tube diameter, process connection, connection head and any integrated transmitter. At high temperatures or with strong vibration, thermocouples are used instead.
Failures are caused by vibration, moisture in the connection head, mechanical stress on the protective tube or drift after long service. It is often sufficient to replace only the measuring insert while the protective tube remains in the plant. Measuring points with high accuracy requirements are calibrated at fixed intervals.
In practice: maintenance and procurement
For a replacement, sensor type, tolerance class, wiring configuration, insertion length, diameter, process connection and head form are needed. Clarify in advance whether the complete probe or only the measuring insert is to be ordered – this reduces downtime because the protective tube does not have to be removed.
Synonyms and abbreviations: Pt100 · resistance thermometer · temperature probe · RTD · Pt1000
Frequently asked questions
What is the difference between Pt100 and Pt1000?
Both are platinum resistance thermometers with the same shape of characteristic curve. The Pt100 has a resistance of 100 ohms at 0 °C, the Pt1000 of 1000 ohms. With a Pt1000, lead resistance has less effect, which is why it is often used with two-wire connection. The evaluation unit or transmitter must be set to the relevant type.
Which is better: Pt100 or thermocouple?
A Pt100 measures more accurately and stably in the medium temperature range. Thermocouples cover higher temperatures, respond faster and are mechanically more robust, but less accurate. The choice depends on temperature range, required accuracy and installation conditions. For measurements up to medium temperature ranges with high accuracy, the Pt100 is usually preferred.
What do two-, three- or four-wire connections mean?
The connection describes how many conductors link the sensor to the evaluation unit. With a two-wire connection, lead resistance enters the measurement and distorts it. A three-wire connection largely compensates for this effect, and a four-wire connection almost completely. For accurate measurements, three or four wires are therefore used.