Sensornet ORYX DTS, Руководство пользователя

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Page 68 Oryx DTS User Manual v4
DOUBLE-ENDED MEASUREMENTS
INTRODUCTION TO DOUBLE-ENDED
As briefly discussed on page 57, an effect known as ‘differential loss’ within the
sensing fibre can affect the calibration accuracy of any DTS system and needs to
be corrected for. Differential loss refers to the slightly different loss (or attenuation)
in the fibre between the Stokes and the anti-stokes signals. In good fibre optic
cable, this loss difference is constant along the length of the fibre and the effect can
be corrected for using a straight forward ‘slope correction’ as described on page 58.
There are two specific cases were the differential loss may not be constant along
the whole length of the fibre cable:
1. If there are connectors or severe bends in the fibre line. Loss at these
points is sometimes unavoidable and furthermore, the loss may be
different for each of the Stokes and anti-stokes signals. This point-like
differential loss results in an artificial temperature step at the connector or
bend point.
2. If the fibre is subject to harsh environments where there is the possibility of
hydrogen ingress. This can cause the attenuation of the fibre to change
and again, the change may be different for the Stokes and anti-Stokes
signals. Generally the change in attenuation only occurs in the exposed
region and other parts of the fibre are unaffected.
The effects described above will result in the differential loss changing along the
length of the fibre, in one case at a definite point, in the other, over an extended
region of fibre.
In both cases, the effect can be corrected for by using what is known as a combined
or double-ended measurement. This relies on both ends of the fibre being
connected to the DTS and a measurement is taken from each end of the fibre
before a temperature profile is calculated.
As long as both ends of the fibre optic cable can be connected to the DTS, a
double-ended measurement can be performed. The fibre cable can be routed
around an installation and reconnected to the DTS (the fibre does not retrace its
path) or, two fibres may be inside one cable with a splice to join the ends such that
the fibre physically retraces its path back to the DTS (so-called ‘U’ installation).
NOTE: It is not necessary for the fibre to physically retrace its path for double-
ended method to work.