SIMULTANEOUS MEASUREMENT OF TEMPERATURE AND PRESSURE SENSOR USING BRAGG GRATINGS

Introduction Research Approach Temperature sensor head Pressure sensor head Combined sensor Conclusions Applications Presentation Outline

Conventional SensorsFBG Sensors  large number sensors such as thermocouples, RTDs, thermistors, manometers, vacuum gauges etc. need to be used to monitor the thermal state of complex systems.  Easily Multiplexed 1000s of FBGs in a single fiber and Efficiently installed into desired system  Slow response time to temperature and pressure change  Fast response times to temperature and pressure change  The dynamic range is limited  Good dynamic range is observed.  Linearity and thermal stability over wide measurement range is limited  FBGs sensors are very stable and can reach linearity up to 99.99%  The output voltage doesn’t change linearity with temp and pressure  Reflected wavelength is linear with applied temp.  Conductive  Non conductive  Chemically and electrically active  Chemically and electrically inert  Low accuracy and sensitivity  High accuracy and sensitivity  Difficult for distributed sensing  Can be easily used for distributed sensing Introduction

Fabrication of Fiber Bragg Gratings using phase mask method Fiber gratings are made by laterally exposing the core of a single-mode fiber to a periodic pattern of intense ultraviolet light. The exposure produces a permanent change in the refractive index of the fiber core, creating a fixed index modulation according to the exposure pattern. This fixed index modulation is called a grating. At each periodic refraction change a small amount of light is reflected Introduction

Bragg wavelength Refractive index wavelength Effective refractive index Structure of Fiber Bragg Gratings Contd…

 The effective Bragg wavelength shift due to strain and temperature is given by  The strain response arises due to both the change in the fiber index due to photo elastic coefficients and the elongation of the sensor.  The thermal response due to the temperature depends of the refractive index and inherent thermal elongation of the fiber material. Sensing principle of FBG Introduction

1.To design sensor heads for temperature and pressure measurement. 2. To study the individual response of Temperature and Pressure sensors. 3. To design combined sensor head and study the characteristics of FBG. Research Approach

A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper, or in some cases brass instead of copper.steelcopperbrass The strips are joined together throughout their length by riveting, brazing or welding.rivetingbrazingwelding The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled.coefficient of thermal expansion Temperature Sensor Head

Deflection of Bimetallic Cantilever

Experimental Setup using single Bimetallic strip

Experimental Setup using two Bimetallic strips

Theoretical support

Results

 The sensor head is a hallow cylinder with dimensions of 4cm x 2cm which can be held in palm.  It is made of stainless steel so that sensor head is anti-corrosive.  The diaphragm is made of silicon rubber with maximum Young’s modulus of 5Mpa.  A small copper foil is placed to fix the fiber.  The 0.3cm hole made to allows water inside the sensor head and sensor is maintained under a varying pressure.  Due to the pressure variations FBG gets strained and shift is noticed. This shift is used to measure the pressure at that particular place. Pressure Sensor Head

A 4cm x 2cm Sensor head with 0.3cm hole : Design

For more sensitivity :-

When water enters into hallow cylinder, silicon rubber gets elongated Mathematical Support

Analyzing the above fig. we get : Now the length of the arc of the circle is: Now strain produced in silicone rubber is calculated as follows:

If tangential strain is assumed to be zero everywhere: Here residual stress is neglected as it is very low for silicone rubber and if it seems to matter we can eliminate it by coating with very small amount of copper and the equation is reduced to:

Finally the equation for pressure is equal to: Therefore elongation is equal to: where

Bragg wavelength shift

The temperature sensor is combined with pressure sensor in series and we can monitor temperature and pressure change simultaneously. Combined Sensor Head

Questions??