Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1984-12-12
1987-12-15
LaRoche, Eugene R.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250231R, G01D 534
Patent
active
047135387
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to apparatus and method for measuring an external parameter, eg, temperature, pressure or displacement.
It is sometimes required to measure external parameters (physical variables) in relatively inaccessible places where the use of electrical transducers for remote monitoring is inappropriate. For example, inside electrical machines such as transformers, generators or motors, no conventional electrical transducer can be used because of the high magnetic fields. It has been proposed to use optical techniques in such cases.
Some optical materials are known to exhibit a temperature dependence of some feature of their absorption spectrum. An optical thermometer can thus be made by arranging to pass light through such an optical material acting as a temperature transducer, and monitoring variations in the energy of light transmitted through the material with temperature. One such device is proposed in "An Optical Temperature Sensor for High Voltage Applications" by Saasky and Skaugset, in a paper given at the 7th IEEE/PES Transmission and Distribution Conference and Exposition, Apr. 1 to 6, 1979. This paper disclosed that selected materials appear to exhibit useful temperature dependent absorption spectra, but a preferred material is selenium ruby glass which exhibits a relatively sharp edge in its absorption spectrum where the absorption co-efficient rises from a relatively low value to a very high value rapidly with decreasing light wavelength. The location in the spectrum of this edge is temperature dependent so that the absorption co-efficient of such ruby glass to light at a specific wavelength on the absorption edge can be highly temperature dependent.
The above paper describes a possible temperature sensor comprising a pellet of the ruby glass sandwiched between a reflective coating and the end of a fibre optic bundle. Light from a suitable diode source is fed to the pellet along some of the fibres of the bundle passes through the pellet is reflected back through the pellet by the reflective coating and into the other fibres of the bundle to be fed to a photo-detector. The intensity of light detected should provide an indication of the temperature of the pellet.
This device has various drawbacks in that there may be other variables effecting the output for which the device does not compensate. Compensation is described in the paper only for any variations in the original light source.
A somewhat similar temperature sensor is described in "Fibre-optic Instrument for Temperature Measurement" by Kyuma, et al. in IEEE Journal of Quantum Electronics, Vol. QE-18, No. 4, April 1982. Again the temperature dependent absorption edge of a selected optical material is used as the sensing device. Two semi-conductor materials, CdTe and GaAs are disclosed. Two lengths of single optical fibre are used to convey light from a source through a body of the temperature sensitive material and then back again to the photo-detector. However, in this arrangement, the light source is arranged to emit two pulses of light at different wavelengths, one wavelength selected to be on the absorption edge of the sensing material, and the other wavelength selected to be substantially away from the absorption edge, in the transmission region of the material. The intensities of the light of the two wavelengths transmitted through the sensor material are then measured and compared. In this way it is said that the resulting device is made relatively insensitive to changes in the light detector output resulting from effects other than the temperature dependency of the absorption edge of the sensor material. For example, the device can have other temperature dependent losses in the light path from the light source to the detector including optical connector loss at the interfaces between the optical fibres and the body of sensor material, and the scattering and other loss co-efficient in the optical fibres themselves. If the two wavelengths of light used in this device are of the same o
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Central Electricity Generating Board of Sudbury House
LaRoche Eugene R.
Lee James C.
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