Fiber Optic: Contact
There are several contact temperature sensors on the market that utilize some unique aspects of fiber optics to acheive temperature measurements in special applications.
First there are the ‘”point” fiber optic devices that have an optical material possessing known temperature sensitivity affixed to the end of a fiber or a solid light guide. The sensitivity may be polarization, a thermally sensitive phosphor or some other physical property.
Light is sent to the sensing tip and the return radiation, sometimes pulsed, sometimes steady, (according to the instrument design) contains the information that enables the control unit to indicte the temperature of the tip.
The Distributed Temperature Sensing (DTS) technique uses the Raman photo-optical-effect. It was developed at the beginning of the 1980s at Southampton University in the UK. It is based on OTDR (Optical Time Domain Reflectometry) & OFDR (Optical Frequency Domain Reflectometry) technology using techniques developed for telecommunication cable testing.
The method enables one to measure the temperature rapidly at many points along an extended length of fiber optic cable, sometimes many kilometers long.
Since there is no electrical signal, the devices are almost totally immune from electrical interference from motors, lightning strikes and radio interference. The upper temperature limit for measurement is dictated by a number of factors, but the meting point of the glass fiber is one of the considerations.
Commercially available Fiber Optic DTS technology can achieve (depending on configuration):
* Continuous measurement over kilometers
* 30 kilometers for each channel
* Spatial resolution of about 1 meter
* Thermal resolution of about 0.01 ° C
* Temporal resolution of seconds to hours depending on the desired thermal precision
Spatial, thermal, and temporal resolution are mutually dependent and depend on measurement configuration. (According to work reported at the USGA water.usgs.gov/ogw/bgas/fiber-optics/)
An informative article on the subject may be found on Wikipedia at: en.wikipedia.org/wiki/Distributed_temperature_sensing
Applications include almost any significant measurement where multiple sensors would be required to monitor the temperature distribution such as along power cables, down oil & wells, pipelines, large process or sensitive storage vessels, fire detection over large areas and similar situations.