Nortech EasyGrid V1, User Manual

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EasyGrid User Guide
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14.1.2 The Temperature Sensor
The TPT-62 sensor is based on direct contact temperature measurement. Traditional temperature
sensors like thermocouples and RTDs (resistance temperature devices) work on the same principle. In
other words, the semiconducting material must be touching the object or immersed in the liquid or gas
to be measured. The more intimate the contact and the smaller the thermal mass of the sensing tip, the
faster the semiconductor will respond to changes in temperature. We then want to be able to deliver
light to the semiconductor and measure what is absorbed. That is the function of the optical fiber.
A tiny semiconductor of GaAs
semiconductor is bonded to one end
of a well polished optical fiber. On
one side of this semiconductor, a
reflective dielectric film has been
deposited. Dielectric means it does
not conduct electricity; all of the materials share this property (“high dielectric strength”), which is one
of the principal advantages of our sensor technology over traditional temperature sensors like
thermocouples and RTDs (which use wires to convey an electrical signal).
The length of the optical fiber is covered with a PTFE (Teflon) sheath, making it very resistant to
aggressive chemical environments as well as an additional perforated PTFE sheath (spiral wrap) for
added ruggedness and even oil circulation. The entire end assembly (semiconductor and end of the fiber)
is then embedded in high temperature adhesive to protect the sensor (the semiconductor) from
chemical and mechanical aggression. Consequently, the only barrier to direct contact is this adhesive.
The position of the absorption shift is determined using a proprietary signal analysis algorithm and is
then correlated to temperature. The computation of the absorption shift does not depend on signal
intensity for this particular instrument; essentially only the wavelengths of the light are of interest.
Consequently, the various factors that contribute to attenuation on optical fiber (fiber length, number
and quality of connections, fiber diameter and composition, bending) do not impose any serious
constraints on our system. Furthermore, since the semiconducting semiconductor’s response is
universal, all our sensors are interchangeable with no need for calibration or entry of factors when
swapping sensors. FISO
’s approach gives reliable, repeatable temperature measurements without the
errors that may result from a loss of power in the connectors or a curve in the optical fiber.