Temperature probes by WIKA
For many years, WIKA has been among the leading manufacturers of high-quality industrial temperature measuring instruments. Our experience and know-how in the construction of temperature sensors sets us apart.
What is a temperature sensor?
The term temperature sensor, with electrical thermometers, describes a unit from one or more temperature sensor elements and a usage-specific armour, which can consist of, for example, connection head, neck tube, thermowell or a hand grip. The sensor element built into the temperature sensor takes the actual measurement and converts the measured temperature into an electrical signal.
With WIKA, temperature sensors in industrial temperature measurement are essentially divided between the following groups:
Temperature probe of a thermocouple
Temperature sensors with thermocouples are suitable for the measurement of high temperatures of up to +1,700 °C.
Thermocouples consist of two different metals which are bound together to form the 'thermocouple'. The connection point (hot junction) represents the actual measuring point of the temperature sensor, the wire ends are designated as the cold junction. When there is a temperature change at the measuring point of the temperature sensor, due to the different electron densities of the metals and the temperature difference, a voltage is generated between the hot and cold junctions. This is approximately proportional to the temperature of the measuring point (Seebeck effect).
The low diameter of the sheathed thermocouple of a temperature sensor enables a response time that is faster than is possible with a resistance thermometer.
Temperature probe of a Pt100/Pt1000 resistance thermometer
A resistance thermometer has its strengths in the low and medium temperature ranges of -200 … +600 °C. In industry, mainly temperature sensors with Pt100 or Pt1000 measuring resistors are used. If the temperature sensor detects an increase in temperature, then its resistance also increases (positive temperature coefficient). The resistance of a Pt100 resistance thermometer at 0 °C is 100 Ohm, for a Pt1000 resistance thermometer it is 1,000 Ohm.
Types of measuring resistors in temperature probes
Basically there are two types of measuring resistors differentiated in temperature sensors: Thin-film measuring resistors and wire-wound measuring resistors. The advantages of thin-film measuring resistors are their small overall size and high vibration resistance with the appropriate construction of the temperature sensor. Thin-film measuring resistors have come to represent the standard sensor design, provided that they are not ruled out as a result of their temperature range (measuring ranges for accuracy class B temperature sensors: Thin-film measuring resistors -50 … +500 °C, wire-wound measuring resistors -200 … +600 °C).
Surface thermometers for vessels and pipelines
Surface thermometers are suited to the non-contact measurement of temperature on the surfaces of vessels or pipes. They are designed as resistance thermometers (TR50) or thermocouples (TC50). The latter enable an exact measurement of extreme temperatures between -40 to +1,200 °C. Both variants are available with explosion protection.
Surface thermometers feature problem-free installation without thermowells. The mounting to vessels is achieved using a welded-on or screwed-on contact block which contains the temperature probe. For fixing to pipelines, a variant with tightening strap is suitable. Centrally bored washers and metallic weld-pads are available as further process connections. For specific ambient conditions, the connection leads of the surface thermometers can be delivered with special insulation materials. The cable end can be delivered ready for connection or with fitted plug (optional) or with a field case.
What is the value of the current at the output limits in accordance with NAMUR?
Lower output limit in accordance with NAMUR is 3.8 mA (other value possible)
Upper output limit in accordance with NAMUR is 20.5 mA (other value possible)
