WO2004074821A1

WO2004074821A1 - Temperature compensation to an optical fibre sensor for measuring moisture

Info

Publication number: WO2004074821A1
Application number: PCT/GB2004/000169
Authority: WO
Inventors: Arthur H. Hartog; Adrian P. Leach
Original assignee: Sensor Highway Limited
Priority date: 2003-02-20
Filing date: 2004-01-21
Publication date: 2004-09-02

Abstract

The invention is a system and method for measuring temperature as well as moisture along the length of an optical fibre (12). By calibrating the temperature-varying loss characteristic of the optical fibre (12), a knowledge of local temperature along the length of the fibre (12) enables much more precise moisture measurements to be made. A distributed temperature measurement can be made by using optical time domain reflectometry (OTDR) techniques that measure localized changes in Raman scatter (caused by temperature changes). Such changes in Raman scatter along the length of the optical fibre (12) are independent of loss changes in the fibre induced by the presence of moisture. The system comprises an optical fibre (12), a body of material (14) which may comprise a hydrogel based polymer, a rigid containment structure (16), and an electro-optical unit (18) operating as an OTDR. Single-ended configurations or double-ended configurations may be used.

Description

TEMPERATIRE COMPENSATION TO AN OPTICAL FIBRE SENSOR FOR MEASURING MOISTURE

BACKGROUND

This invention generally relates to optical fibre sensors. More particularly, the invention relates to a system that provides temperature compensation to a distributed optical fibre sensor that measures moisture. Fibre optic distributed moisture sensors are known in the art. Several embodiments have been demonstrated in the literature, and similar concepts have been used to develop sensors that detect the presence of hydrocarbons. For example, U.S. Patent No. 5744794 assigned to the University of Strathclyde teaches a moisture sensor that uses a specialized cable design including an expanding material within its structure. The expanding material is preferably hydrogel, although other materials which demonstrate a volumetric expansion in the presence of moisture can be used. Also included in the cable structure is an optical fibre, a structural strength member, and an aramid (ECevlar) yam which is wound helically around the entire structure. As moisture is absorbed by the hydrogel material, the hydrogel expands and, because the aramid yarn is inflexible, the optical fibre is periodically constricted along its length, causing microbending in the locality of the increased moisture. The loss increase due to microbending along the length of the fibre can be detected using optical time domain reflectometry (OTDR) methods and thus a distributed moisture sensor can be realized.

Unfortunately, such moisture sensors are also sensitive to temperature (i.e.the hydrogel material will expand as a result of temperature changes as well as exposure to moisture). Where moisture measurements are required to any degree of accuracy, they are likely to be ambiguous due to local temperature changes. Thus, there exists a continuing need for an arrangement and/or technique that addresses one or more of the problems that are stated above. In particular, the prior art would benefit from an optical fibre sensor for moisture or hydrocarbon detection that can be compensated with respect to temperature.

SUMMARY The invention is a system and method for measuring temperature as well as moisture along the length of an optical fibre. By calibrating the temperature-varying loss characteristic of the optical fibre, a knowledge of local temperature along the length of the fibre enables much more precise moisture measurements to be made. A distributed temperature measurement can be made by using optical time domain reflectometry (OTDR) techniques that measure localized changes in Raman scatter (caused by temperature changes). Such changes in Raman scatter along the length of the optical fibre are independent of loss changes in the fibre induced by the presence of moisture.

BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a schematic of the present invention including a single ended measurement. Fig. 2 is a schematic of the present invention including a double ended measurement.

DETAILED DESCRIPTION

Figure 1 shows the system 10 of the present invention. System 10 comprises an optical fibre 12, a body of material 14, a rigid containment structure 16, and an electro-optical unit 18. Optical fibre 12 is in optical communication with unit 18. System 10 is used to measure a specific measurand, and the material 14 is selected so that it reacts to the specific measurand. The optical fibre 12, material 14, and containment structure 16 are arranged so that exposure to the measurand of interest leads to the mechanical perturbation of the optical fibre 12. Unit 18 operates as an optical time domain reflectometer (OTDR), as described in US

5744794 and EP 490849. Essentially, unit 18 launches optical pulses into the optical fibre 12, and backscattered light is returned from locations along the optical fibre 12 to the unit 18. Knowing the launch time of a pulse and the speed of light allows a user to correlate the received backscattered light with the relevant location along the optical fibre 12. The backscattered light, in turn, is a function of losses imparted on the optical fibre 12, such as losses caused by microbending of the optical fibre 12. Specifically, the backscattered power returning to the unit varies as a function of time. For a fibre which is uniform along its length, the backscattered power decays exponentially, the rate of decay being directly related to the loss (attenuation per unit length) of the fibre. Where the attenuation of the fibre varies as a function of position, the rate of decay changes and by calculating the derivative of the backscatter waveform with respect to temperature, the local value of the attenuation can be deduced. (The foregoing pre-supposes that other characterics of the fibre, such as the core diameter, scattering loss coefficient and the index-difference between core and cladding are uniform along the fibre, which is generally the case in good-quality, modern optical fibres). Thus, if the optical fibre 12 is mechanically disturbed, such as if the material 14 is exposed to the measurand of interest, microbending will occur on the optical fibre 12 leading to losses of the backscattered light transmitted to the unit 18. These losses may then be identified and quantified as described above and interpreted as exposure to the measurand of interest. For instance and as it taught by US 5744794, if moisture is the measurand of interest, material 14 may comprise a hydrogel based polymer. If the material 14 is exposed to the measurand of interest, the material 14 expands (while at the same time being constricted by containment structure 16) which leads to microbending of the optical fibre 12. The microbending causes losses in the backscattered light received by the unit 18, which losses are identified and quantified as exposure to moisture.

US 5744794 describes various geometric configurations of the optical fibre 12, material 14, and containment structure 18 that lead to the sought-after mechanical disturbation of the optical fibre 12 upon exposure to the measurand of interest. The present invention can encompass any of the geometric configurations described in such patent.

As previously disclosed, however, the systems described in US 5744794 are sensitive to temperature as well as being sensitive to moisture (the measurand of interest). Thus, any moisture measurement made by such system may be ambiguous based on the temperature sensitivity of the system. The present invention, as shown in Figure 1, calibrates the temperature- arying loss characteristic of the optical fibre 12, which enables a more precise moisture measurements to be made.

In the present invention, optical fibre 12, which is used for moisture sensing as described above, is also used as a distributed temperature sensor (DTS) to measure the temperature profile along its length. The technique of obtaining a distributed temperature profile along the length of an optical fibre is known in the art and is generally described in US 4823166, which is incorporated herein by reference. This temperature sensing technique is also based on OTDR. Thus, since both the moisture sensing technique and the temperature sensing technique are based on OTDR, the same optical fibre (such as 12) can be used to obtain both measurements. Moreover, since the temperature sensing technique uses the moisture insensitive Raman scattering principle (as will be described), the moisture-induced loss effect of the sensing optical fibre will not affect the accuracy of the temperature measurements.

The temperature sensing technique based on OTDR will now be generally described. In accordance with this technique, a pulse of optical energy typically is introduced to the optical fibre such as by unit 18, and the resultant backscattered optical energy that returns from the fibre to the unit 18 is observed as a function of time. The time at which the backscattered light propagates from the various points along the fibre to the unit 18 is proportional to the distance along the fibre from which the backscattered light is received. In a uniform optical fibre, the intensity of the backscattered light as observed exhibits an exponential decay with time.

Therefore, knowing the speed of light in the fibre yields the distances that the light has traveled along the fibre. Variations in the temperature alter the local scattering coefficient and thus show up as variations from a perfect exponential decay of intensity with distance. Thus, these variations are used to derive the distribution of temperature along the optical fibre. In the frequency domain, the backscattered light includes the Rayleigh spectrum, the Brillouin spectrum and the Raman spectrum. These spectra may be separated from the remaining backscattered light, for example by means of appropriate bandpass filters, and observed independently. The Raman spectrum is the most temperature sensitive with the intensity of the spectrum varying with temperature, although all three spectrums of the backscattered light contain temperature information. The Raman spectrum typically is observed to obtain a temperature distribution from the backscattered light.

In accordance with the moisture sensing technique, the OTDR measurement (for loss and thus humidity) can be effected either using the Raman backscatter, especially the Stokes channel (which is relatively temperature-insensitive) or in certain cases it may be preferred to add a third, unfiltered, channel to detect mainly Rayleigh scattering which is far less sensitive to temperature than even the Stokes Raman channel.

DTS systems can be used in single-ended configurations, as shown in Figure 1, or in double-ended configurations, as shown in Figure 2. In the former case, the ratio of anti-Stokes Raman/Stokes Raman scattering is determined and corrected for a pre-determined difference in the attenuation between the two Raman bands. A more accurate approach is to use a double- ended approach in which both Raman signals are measured in turn from each end of the sensing fibre. The temperature is obtained by combining the anti-Stokes/Stokes ratios from each fibre end; the geometric mean of the Raman ratios from the two ends gives a signal which is dependent - to a very good approximation - on temperature only. The differential loss is eliminated in the latter signal processing, including in cases where the loss is non-uniform. If, however, the backscatter traces (either anti-Stokes or Stokes) were combined by taking (for a single Raman wavelength) the product of the signals from either fibre end, then a signal which depends on the fibre attenuation and not on the temperature is obtained. Thus in the case of double-ended measurements, the four Raman backscatter traces can be combined in the normal way to obtain a loss-independent temperature profile and two separate estimates of the attenuation profile along the fibre, one at the mean of the probe and anti-Stokes Raman wavelengths and the other at the mean of the probe and Stokes Raman wavelengths. The latter may be used for the humidity measurements corrected by the temperature profile.

In summary, a system 10 is capable of providing all the necessary measurements to give a temperature corrected moisture measurement using a single moisture sensitive optical fibre 12 as follows: i) moisture sensitive optical fibre 12 is firstly calibrated for loss variations due to temperature at fixed humidity under controlled conditions (i.e.prior to installation); ii) OTDR capability of DTS is used to measure loss profile of installed optical fibre 12 which is primarily due to moisture (but also contains a second order temperature dependence); iii) DTS is used to measure temperature profile of moisture sensitive optical fibre 12 using Raman effect; and iv) temperature profile data of fibre 12 is used in conjunction with temperature vs loss calibration obtained in stage (i) to correct loss measurements made in stage (ii) so that moisture is deduced with greater accuracy.

The system 10 and fibre 12 may be used in a variety of remote locations to detect and correct moisture measurements. Appropriate remote locations include along or within pipelines. For instance, the present invention may be used to detect and correct moisture content measurements along pipelines that carry hydrocarbons extracted from the earth. Although the present invention has been described to measure moisture, it is understood that it may also be used to measure and correct other measurands. For instance, the material 14 may be selected to react when exposed to hydrocarbons (instead of moisture), in which case the system 10 would be used to detect hydrocarbons (including detection of hydrocarbons and correction using the temperature measurement). While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.

Claims

CLAIMSWhat is claimed is:

1. A method to sense moisture in a remote location, comprising: providing a sensing optical fibre; measuring the moisture content at the remote location by use of the sensing optical fibre the moisture content measurement also being dependent on temperature; measuring the temperature at the remote location by use of the sensing optical fibre; anc correcting the moisture content measurement by use of the temperature measurement.

2. The method of claim 1, wherein the measuring the moisture content step comprises: providing a material that expands when exposed to moisture; and communicating the material expansion to the optical fibre so that the material expansior causes losses in backscattered light transmitted by the optical fibre.

3. The method of claim 1 , wherein the measuring the moisture content step and the measuring the temperature step are conducted using optical time domain reflectometry techniques.

4. The method of claim 1 , wherein the material comprises hydrogel.

The method of claim 1, wherein the optical fibre is single-ended.

6. The method of claim 1 , wherein the optical fibre is double-ended.

7. A system to sense moisture in a remote location, comprising: a sensing optical fibre in optical communication with an electro-optical unit; the fibre adapted to measure the moisture content at the remote location, the moisture content measurement being dependent on temperature; the fibre adapted to measure the temperature at the remote location; and wherein the moisture content measurement is corrected by use of the temperature measurement.

8. The system of claim 7, further comprising: a material that expands when exposed to moisture; and wherein the expansion of the material is communicated to the optical fibre so that the esφansion causes losses in backscattered light transmitted by the optical fibre.

9. The system of claim 7, wherein the moisture content measurement and the temperature measurement are conducted using optical time domain reflectometry techniques.

10. The system of claim 5, wherein the optical fibre is single-ended.

11. The system of claim 5, wherein the optical fibre is double-ended.

12. A method to sense moisture in a remote location, comprising: calibrating an optical fibre for loss variations due to temperature at a fixed humidity under controlled conditions; after the calibrating step, installing the optical fibre in a remote location; after the installing step, measuring the loss profile of the optical fibre which is primarily due to moisture, the loss profile also containing a second order temperature dependence; after the installing step, measuring temperature profile of the optical fibre; and after the installing step, using the temperature profile data in conjunction with data obtained in the calibrating step to correct loss measurements made in the measuring the loss profile step.