TECHNICAL BACKGROUND > How Our Sensors Work
Pressure (Model 60/61) and Temperature (Model 120/121) Sensor designs are based on the Fabry-Perot interferometer, in which two parallel, minimally separated, partially reflecting surfaces form an optical reflecting cavity. If one of these parallel surfaces is a pressure sensitive diaphragm, changes in external pressure cause a change in depth of the optical reflecting cavity, which in turn alters optical cavity reflectance spectra. For Temperature, the outer surfaces of a thin silicon layer define the optical reflecting cavity; as temperature changes, the refractive index of silicon changes and this in turn alters optical cavity reflectance spectra.
The Sensor Instruments (for Pressure and Temperature) contain an 850 nm LED whose emissions reach the sensor via an optical fiber. In the sensor's optical reflecting cavity, the spectral distribution of LED light is modified as a function of cavity depth, and this spectrally altered light is reflected back down the fiber to the instrument. Light returning to the instrument is optically split into two spectral components; the photocurrents from these two components form a ratiometric signal which in turn correlates with changes in the measured parameter. For pressure measurements, differential pressure is obtained by referencing an internal solid state barometric transducer.
The Sensor Instruments (for Pressure and Temperature) contain an 850 nm LED whose emissions reach the sensor via an optical fiber. In the sensor's optical reflecting cavity, the spectral distribution of LED light is modified as a function of cavity depth, and this spectrally altered light is reflected back down the fiber to the instrument. Light returning to the instrument is optically split into two spectral components; the photocurrents from these two components form a ratiometric signal which in turn correlates with changes in the measured parameter. For pressure measurements, differential pressure is obtained by referencing an internal solid state barometric transducer.
The combination Oxygen/Temperature Sensor (Model 160) consists
of two luminescent compounds in a polymer matrix applied to the
sensing end of an optical fiber. The phosphorescence decay time
of one of the compounds is dependent on the temperature and
oxygen concentration at the sensor location. The decay time of
the second compound is only dependent on the temperature at the
sensor location. A UV light source is briefly turned on to
induce phosphorescence in both compounds. The UV light source is
then turned off and the emitted light is spectrally separated
and the decay time for each compound is individually measured.
The temperature at the sensor location is then calculated from
the decay time of the compound which is only sensitive to
temperature. This calculated temperature along with the decay
time of the oxygen sensitive compound is then used to calculate
the oxygen concentration at the sensor.