There are various types of temperature sensors, including:
• Resistance Temperature Detectors (RTDs): Pt100, Pt1000 – based on the principle of resistance change with temperature.
• Thermocouples: Consist of two different metals and generate a temperature-dependent voltage.
• Semiconductor Temperature Sensors: Utilize the temperature-dependent properties of semiconductor materials.
• Infrared Temperature Sensors: Measure temperature without contact via thermal radiation.
• Thermistors (NTC, PTC): Resistance-based sensors with a highly nonlinear characteristic curve.

A Pt100 or Pt1000 is a resistance temperature detector (RTD) used for measuring temperature. The difference between them lies in their resistance value at 0°C: a Pt100 has 100 ohms, while a Pt1000 has 1000 ohms. The resistance changes with temperature in a defined manner.

A thermocouple consists of two different metallic conductors joined at one end. Due to the thermoelectric effect, an electrical voltage is generated at the open end, which correlates with the temperature. Thermocouples are robust and particularly suitable for high-temperature applications.

The decision depends on several factors:
• Temperature range: Thermocouples are suitable for very high temperatures (>1000°C), while RTDs are better for lower temperatures with high accuracy.
• Accuracy: RTDs are more precise than thermocouples.
• Response time: Thermocouples respond faster to temperature changes.
• Environmental conditions: In harsh environments or where strong electromagnetic interference is present, thermocouples are often the better choice.

A failure of a resistance sensor or thermocouple can manifest in various ways depending on the type of sensor and its application. Here are some common symptoms and methods for testing such failures:⸻Failure of a Resistance Sensor (e.g., Pt100 or NTC/PTC)Symptoms of failure:
• Open circuit: If the resistance sensor (e.g., a Pt100) has a broken wire (due to cable break or corrosion), the measuring device may display “infinite” resistance or freeze at a constant value, indicating no resistance is being measured.
• Short circuit: If the sensor is short-circuited (e.g., due to damaged insulation), the device may show a resistance close to zero ohms or produce incorrect temperature readings.
• Incorrect values: If the sensor shows values significantly different from other temperature sensors or is outside the expected temperature range, it may be defective.How to test the resistance sensor:
• Resistance measurement: For a Pt100, measure resistance at a known temperature (e.g., 0°C). It should be about 100 ohms. Known resistance values exist for other temperatures. Deviations may indicate a defect.
• Continuity check: Use a multimeter to check continuity. An open circuit shows no resistance (infinite), and a short circuit shows 0 ohms.⸻Failure of a ThermocoupleSymptoms of failure:
• Open circuit: A broken wire (due to stress or corrosion) can cause the reading to freeze or display an error message indicating an open circuit.
• Short circuit or contamination: A short between the conductors (e.g., at bend points or connectors) or oxidation due to high temperatures can cause false readings, since a new thermoelectric junction may be created at the fault location. This can result in readings that are too high, too low, or simply implausible.
• Temperature deviations: Aging or physical damage can cause inaccurate readings.How to test the thermocouple:
• Voltage measurement: A thermocouple generates a small voltage proportional to the temperature difference between the junctions. A precision multimeter can measure this voltage. For example, a Type K thermocouple produces approx. 0 mV at 0°C and approx. 4.1 mV at 100°C.
• Continuity check: Like a resistance sensor, a continuity test can indicate whether the thermocouple is broken (no voltage or infinite resistance).
• Temperature comparison: Use a known accurate temperature measuring device (e.g., calibration thermometer) to compare with the thermocouple reading.⸻Summary
• Resistance sensors (e.g., Pt100): Failures often appear as open circuits (infinite resistance) or short circuits (close to 0 ohms). Test by measuring resistance at known temperatures.
• Thermocouples: Failures often show as open circuits or incorrect temperature readings. Test by measuring voltage and comparing with standard reference values.

Thermistors are temperature-sensitive resistors that are divided into two main types:
• NTC (Negative Temperature Coefficient): The resistance decreases as the temperature increases. They are commonly used for temperature measurement, control, and as thermal protection in electronic devices.
• PTC (Positive Temperature Coefficient): The resistance increases as the temperature rises. They are often used for overcurrent protection or in self-regulating heaters.
Thermistors are characterized by their high sensitivity but are less linear than Pt100 or Pt1000 sensors. They are particularly suitable for applications that require a compact design and fast response time.

The measuring range is defined to cover the expected temperature values. The following aspects must be considered:
• The sensor must operate within its specified temperature range.
• An appropriate resolution and accuracy must be chosen. The measuring range should be as large as necessary and as small as possible.
• Environmental conditions and factors such as voltage drop over long cables must be taken into account.
• For transmitters that are set to a specific measuring range, if the zero-point value (e.g., 4 mA or 0 V) is above room temperature, the transmitter may output implausible values to the downstream control system during process downtime.

2-wire: Simplest configuration, but resistance changes in the leads affect the measurement result.
3-wire: Compensates for lead resistance, commonly used in industrial applications.
4-wire: Highest accuracy, as lead resistance is fully compensated. Used in high-precision measurements.

Transmitters can provide various output signals, including:
• 4…20 mA (current signal): Robust signal, resistant to voltage drops and electromagnetic interference, ideal for long cable runs and industrial applications.
• 0…10 V (voltage signal): Easy to measure, but more susceptible to interference and voltage drop, suitable for short distances.
• 0…5 V or 1…5 V: Commonly used in electronic control and automation systems.
• Modbus, Profibus, HART: Digital interfaces for communication with control systems.
• PWM (pulse-width modulation): Signal with variable pulse width for digital processing.
• CAN bus: Serial communication protocol for industrial and automotive applications.
The choice of output signal depends on the application, environmental conditions, and requirements for signal transmission.

• 4…20 mA (current signal): Insensitive to voltage drops and electromagnetic interference. Ideal for long cable runs and industrial applications.
• 0…10 V (voltage signal): Easier to measure, but more susceptible to interference and voltage drop. Suitable for short distances.

A linear output signal means that the output value (voltage or current) changes proportionally to the measured temperature. For example, doubling the temperature results in a doubling of the output signal within the defined measuring range. This simplifies the processing and interpretation of the measured values.

• Analog output signal: A continuous signal that is directly proportional to the measured temperature. Examples include 4…20 mA or 0…10 V. Analog signals are easy to process and are commonly used in industrial applications.
• Digital output signal: Transmits measured values in discrete (binary) form. Examples include Modbus, Profibus, or CAN bus. Digital signals offer high resistance to interference, allow complex communication, and are frequently used in modern control systems.

• Resistance sensors: Pt100, Pt1000, Ni100, Ni1000
• Thermocouples: K, J, L, T, U, E, N, S, B (in almost all designated products)
• Semiconductor temperature sensors: Various sensors communicating via I²C bus are currently compatible with some of our products (Temperature – TMP116 / Humidity – SHT31, SHT41 / Air pressure – MPL3115A2 / Light intensity – ADPS9300)
• Thermistors (NTC, PTC): on request

• Analog signals: 0…10 V (and special solutions like 0.5…4.5 V), 4…20 mA
• Digital signals: Modbus

At this point, a distinction must be made. Analog transmitters are usually fixed to a specific sensor and measuring range. In some cases, the transmitter automatically detects the connected sensor (Pt100/Pt1000), and DIP switches allow selection between preset measuring ranges. Most digital transmitters, however, can be programmed or parameterized via software.
For this purpose, we provide free software for LKM transmitters so that you can define values such as sensor type, measuring range, output signal, switching point, and hysteresis (depending on the product used) yourself.