For decades, industrial plants have used thermocouples (T/Cs) and resistance temperature detectors (RTDs) for temperature monitoring applications.

The principle operation of thermocouples is based on the fact that when two dissimilar metals are joined together, a temperature-dependent voltage is created. Thermocouples are recommended for applications with high temperature (over 500 degrees F) and when a fast response is required. Using different metal combinations, T/Cs are able to measure a wide range of temperature. RTD and T/C technologies easily measure temperature gradients (multipoint temperature measurement), due to size and cabling requirements (2 to 4 wires per point). But RTDs began taking over the temperature marketplace because they have an advantage in accuracy and stability advantage over T/Cs. The principle behind RTDs is that the resistance of a conductor will change with temperature. When the temperature goes up, so does the resistance. Measuring this resistance allows the temperature to be determined.

A new technology has recently entered the field and focuses on fiber optic temperature detection via Fiber Bragg Gratings. Fiber Bragg Gratings (FBGs) are reflective structures that can be inscribed at individually defined points on optical fibers, called measuring lances. Each FBG on the lance reflects light from a laser at a specific wavelength, acting as a narrow-band filter. A change in temperature causes a shift in the reflected wavelength. A transmitter converts these reflected signals into a temperature value for each point of measurement

Siemens’ SITRANS TO500 is a multipoint measuring system that consists of a transmitter and up to 4 lances. Each lance can support up to 48 FBGs, making it possible to measure 192 points of temperature per system. Due to the low thermal mass of the optical cable, these measuring points can positioned much closer together than other sensor technologies would allow.

Applications of temperature detection with fiber-optic sensors are becoming increasingly common in the chemical industry. The sensors are not sensitive to electromagnetic interference and are also chemically resistant. With RTDs and T/Cs, sizing restrictions limit the number of measuring points. A fiber optic lance has a diameter of less than 0.1 inch (<2mm), allowing for measurement in very narrow protective tubes. Using this technology, facilities are able to rapidly capture precise temperature profiles in tight spaces.

To read more about fiber optic temperature, please click here.

To learn how your system would integrate with fiber-optic technology, please contact our Engineered Instrument Solutions team.