INNO fibre optic temperature sensors ,temperature monitoring systems.
To meet the requirements of continuous drag measurement of ocean temperature and depth profiles, the fiber Bragg grating (FBG) pressure sensor for measuring seawater depth uses FBG temperature compensation sensors to solve the problem of cross sensitivity. Due to the inconsistency in temperature response time between the two, there is a deviation in testing seawater pressure in areas with temperature fluctuations such as mesoscale eddies and fronts. A new type of dual fiber Bragg grating pressure sensor has been designed to address this phenomenon. By encapsulating temperature compensation and pressure fiber Bragg gratings at the center and edge of the pressure sensor (the edge grating does not contact the elastic membrane and is only affected by temperature), its temperature response characteristics are close to consistency. The experimental test results show that the temperature compensation and pressure fiber Bragg grating of the sensor have a temperature response time of 1.45 s and 1.52 s, respectively, with good consistency in response. Through sea trials, it has been verified that the correlation coefficient between the FBG pressure sensor and the reference pressure sensor ALEC-TD is as high as 0.9906, which basically eliminates measurement errors caused by inconsistent temperature responses and can achieve accurate pressure measurement.
The temperature and depth of seawater are important parameters in marine environmental monitoring. The acquisition of this parameter is often affected by changes in environmental factors. To obtain the temperature and depth profile information of various cold water masses and mesoscale eddies in seawater, the traditional abandoned temperature profile measuring instrument XBT is prone to calculation errors due to the risk of water leakage and leakage in its sensing probe, and the depth data is also easily affected by seabed waves and temperature changes. The shipborne towed fiber optic grating sensor has advantages such as strong anti-interference ability, high sensitivity, small size, intrinsic insulation, continuous measurement, and multi-sensor distributed measurement. It can accurately and meticulously depict the temperature and depth profile information of cold water masses and mesoscale eddies, making it suitable for application in marine environments.
We conducted drag tests on fiber Bragg grating (FBG) pressure and temperature sensors in the North Yellow Sea region, and completed comparative testing between FBG pressure sensors and reference pressure sensors ALEC. Through data fitting processing, it was found that when there is a sudden change in temperature in sea areas such as mesoscale eddies and fronts, the measurement deviation between the FBG pressure sensor and the reference pressure sensor ALEC will immediately increase. However, when the temperature change is not significant, there is no such phenomenon. The reason for the analysis is that the response time of the FBG pressure sensor and the FBG temperature sensor to temperature is inconsistent, resulting in measurement errors of the FBG pressure sensor.
In response to the issue of inconsistent temperature response of sensors, this article mainly conducts research from three aspects:
1) Design a new type of dual fiber Bragg grating pressure sensor, encapsulating the temperature compensation and pressure fiber Bragg gratings parallel to the edge and center of the sensor, so that they are uniformly affected by temperature;
2) Calibrate the temperature and pressure sensitivity of the packaged sensor to determine the pressure coefficient after temperature compensation;
3) Conduct temperature response time testing on the sensor in the laboratory and verify its consistency with the reference pressure sensor ALEC through sea trials.
Design and packaging of sensors
In order to meet the requirements of high sensitivity, water pressure resistance, and response characteristics, the new dual FBG pressure sensor adopts a diaphragm type structure sensitization technology. Compared with traditional packaging methods, membrane packaging has disadvantages such as poor stability, unsuitable for dynamic measurement, easy aging at high temperatures, and difficulty in serial connection. However, membrane packaging has good performance in achieving large range and high sensitivity, and can be used for dynamic drag measurement. The dual FBG pressure sensor uses metallized fiber Bragg gratings, which are welded in parallel to the center and edge positions of the membrane using laser welding (temperature compensated fiber Bragg gratings do not contact the membrane, only welded to the base).
Schematic and physical representation of fiber optic grating sensors
After special packaging, the thermal optical coefficient of the FBG pressure sensor did not change in theoretical calculations, but its thermal expansion caused a change in stress. The relationship between temperature and wavelength after packaging is Δλ B= λ B [ α + ξ + (1-Pe)（ α Sub – α) ］Δ T (1)
The FBG pressure sensor converts the change in water pressure into FBG axial strain, and restores the information of seawater pressure signal by detecting the corresponding wavelength change.
Changes in resonant wavelength of FBG and axial strain of optical fibers ε The relationship of f is [11] Δλ = (1-Pe) λ B ε F (2)
In the formula λ B is the resonant wavelength, and Pe is the elastic optical coefficient of the fiber.
Assuming that the thermal balance is not disrupted, the temperature distribution on the tube wall of the sensor packaged with a diaphragm cylinder is uniform, and the differential equation of temperature over time is [10] d Tdt= Γ A (Tf-T) Vcp ρ (3)
In the formula, Tf is the ambient temperature, and T is the temperature of the metal tube wall, Γ The heat transfer coefficient between water and metal surface, where A is the surface area of the metal diaphragm tube, ρ， Cp and V are the density, specific heat capacity, and volume of the metal shell tube, respectively.
3 Experimental testing
3.1 Sensor temperature testing
The FBG sensor experimental setup is used to determine the sensitivity of the FBG pressure sensor to temperature, and temperature sensitivity calibration is performed on the packaged sensor. Calibration is carried out in a constant temperature water bath, using SBE56 as the reference temperature sensor. Select 8 temperature points in the range of 2-35 ℃, and ensure that the stable time at each temperature point is not less than 1 hour. Take the average of each stable temperature point for 2 minutes to determine the corresponding relationship between temperature and wavelength changes. Figure 3 is obtained through quadratic fitting using Origin data processing software. The temperature compensation and pressure fiber Bragg grating temperature sensitivity of the dual FBG pressure sensor are 29.11 and 28.80 pm/℃, respectively, and the fitting linearity R2 is 0.999 99 99.
Temperature wavelength quadratic fitting curve
3.2 Sensor pressure testing
3.2.1 Sensor Temperature Compensated Fiber Bragg Grating Voltage Withstand Test In order to verify whether the center wavelength of the sensor’s temperature compensated fiber Bragg grating is affected by external pressure, pressure calibration tests are conducted on the sensor in the laboratory. In the experiment, a pressure tank was used for pressure calibration, and the SBE56 temperature sensor was used as the reference temperature. A total of 9 pressure points were selected for pressure and pressure reduction tests, with a pressure range of 0-0.8 MPa and each increase of 0.1 MPa.
It can be seen that after removing the influence of temperature changes, the temperature compensated fiber optic grating of the sensor is within the pressure range of 0-0.8 MPa, with a heart wave length drift of only 0.01 pm. The temperature compensated sensing fiber optic grating is not on the membrane, which is caused by the measurement error of the reference sensor SBE56. It is determined that the temperature compensated fiber optic grating is not affected by external pressure. Perform a voltage withstand test on the temperature compensation optical fibers of two sensors.
Sensor pressure calibration test: Due to the parallel packaging of the pressure and temperature compensated fiber Bragg grating of the sensor on the sensor, their center wavelengths are affected by temperature without being subjected to pressure. The pressure resistance testing of the temperature compensated fiber Bragg grating in the pressure sensor is consistent. Therefore, when subjected to external pressure, the sensor can compensate for the temperature of the pressure fiber Bragg grating by the change in the center wavelength of its own temperature compensated fiber Bragg grating.
In order to determine the sensitivity of the pressure sensor, that is, the correspondence between the measured pressure value and the center wavelength of the temperature compensated pressure fiber Bragg grating, a pressure calibration test is required, and the pressurization process is the same as above. The fitting using Origin data processing software shows that the sensitivity reaches 959.017 pm/MPa, with a linear fit R2 of 0.999 9. It has good repeatability and is suitable for measuring high seawater pressure.
The wavelength pressure quadratic fitting curve of FBG pressure sensors is generally used for FBG pressure sensors in ocean testing, where 1 MPa corresponds to a depth of approximately 100 meters in seawater. When the FBG pressure sensor is not temperature compensated, for every 1 ℃ change in ambient temperature, its own wavelength drift is 28.80 pm. The sensitivity of the FBG pressure sensor is 959 pm/MPa, and the corresponding pressure change is 0.030 MPa. The depth error can reach 3.0 m. Therefore, in the process of pressure measurement, it is extremely necessary to perform real-time and accurate temperature compensation on FBG pressure sensors in order to reduce measurement errors, and solving the problem of inconsistent response time is the main research objective of this article.
The temperature response time test of the sensor quickly moves the FBG pressure and its temperature compensation sensor from the cold water tank to the high temperature water bath, and monitors its temperature change in real time through a temperature demodulator. According to the method of dynamic response calibration for temperature sensors, the response time is 63.2% of the time required to reach a stable temperature. As shown in Figure 6, the response time of the temperature compensating fiber of the sensor is 1.45 s, while the response time of the pressure fiber is 1.52 s, with a response time difference of 0.07 s between them, which is basically consistent. This indicates that the newly designed dual fiber Bragg grating pressure sensor has good temperature response characteristics and basically eliminates the impact of sensor measurement errors caused by inconsistent sensor responses.
In July 2017, after towing experiments were conducted in the Yellow Sea area, Figure 7 was obtained through Original data processing software. The temperature compensation fiber Bragg grating and pressure fiber Bragg grating of the sensor have the same temperature response time. Even in the case of sudden temperature changes in Figure 6, the FBG pressure sensor can accurately compensate for temperature in real time. The measurement error caused by inconsistent temperature response has been basically eliminated, and the correlation coefficient between the sensor and ALEC is as high as 0.9906.
Data curves of FBG pressure sensor and ALEC
This article investigates the issue of inconsistent temperature response of FBG pressure and temperature compensation sensors. By designing and packaging a new dual fiber Bragg grating pressure sensor, the temperature response time is close to consistency. First, calibrate the temperature and pressure sensitivity of the sensor to determine the pressure coefficient after temperature compensation.
After response time testing, the temperature compensation fiber Bragg grating and pressure fiber Bragg grating of the sensor have temperature response times of 1.45 s and 1.52 s, respectively. Through sea trials, it has been verified that the sensor has good dynamic response characteristics to temperature, basically eliminating the impact of strain temperature cross sensitivity in pressure sensors. Meeting the requirements of ocean temperature and depth profile measurement is of great significance for the study of marine environment.