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Use of fiber optic technology for Relative Humidity monitoring in RPC detectors

This paper discusses the use of fiber optic technology, specifically Fiber Bragg Grating (FBG) sensors, for monitoring relative humidity (RH) in RPC detectors. FBG sensors provide a distributed monitoring system with long-term stability, radiation hardness, and tolerance to harsh environments. The paper also presents previous applications of FBG sensors in HEP experiments and showcases a prototype sensor for RH monitoring in RPC detectors.

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Use of fiber optic technology for Relative Humidity monitoring in RPC detectors

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  1. Use of fiber optic technology • for Relative Humidity monitoring in RPC detectors • L. BENUSSI1, S. BIANCO1, M.A. CAPONERO1,2, S. COLAFRANCESCHI1, • M. FERRINI1,3, F. FELLI1,3, L. PASSAMONTI1, D. PIERLUIGI1, • A. POLIMADEI2, A. RUSSO1, G. SAVIANO1,3 , C. VENDITTOZZI3 • 1 Laboratori Nazionali di Frascati dell'INFN • 2 Centro Ricerche ENEA Frascati • 3 Università di Roma I • Presented at RPC2012 by Michele Caponero • michele.caponero@enea.it

  2. Fibre Bragg Grating (FBG): fundamentals Fibre Bragg Grating (FBG) optical sensors: a novel technology that can allow developing distributed monitoring sensing systems for gas-based detectors in large HEP experiments Basic features of FBGs ‘Intrinsic’ optical sensor, directly integrated in the fiber: no power supply at the sensing point. Many sensors can be connected in series on the same optical fiber: reduced cabling. Underlying spectroscopic technique: long term stability for quasi-static measurements; no electro-magnetic disturbances. Hostile environment endurance, Radiation Hardness. Array of n.6 FBGs (strain gauge) f 3mm sheath f.o. (patchcord) FBGs f 0.9mm sheath (wiring f.o.)

  3. FBG Fibre Bragg Grating (FBG): fundamentals Fibre Bragg Grating (FBG) sensor: diffraction grating in the core of an optical fiber. Diffraction grating is produced by modulation of the refractive index of the core.

  4. Fibre Bragg Grating (FBG): fundamentals Light propagating along the optical fibre is diffracted (back-reflected) by FBGs Probe signal before diffraction Probe signal after diffraction Probe: Dl60nm Response:Dl 0.3nm @-3dB Response (diffracted) signal FBG Many FBGs at different lBRAGG can be arranged along one optical fibre

  5. Ds = 1 meDl = 1 pm DT = 0.1 °K Dl = 1 pm l l(T0;s0) Fibre Bragg Grating (FBG): fundamentals • Diffracted wavelength ldepends on: • grating pitch • affected by Temperature T and Stress s • effective refraction index Fibre ‘at rest’ (T0;s0) Fibre ‘excited’ (T0+DT;s0+Ds) Dl(DT;Ds) l l(T0;s0)

  6. FBG signal l l(T0;s0) Dl(DT;Ds) FBG signal l l(T0;s0) Fibre Bragg Grating (FBG): fundamentals • Fibre Bragg Grating (FBG) sensor: optical Temperature and Strain Gauge • FBG in thermal/structural contact with the component to be monitored FBG sensor

  7. FINUDA Fibre Bragg Grating (FBG): previous applications for HEP by the INFN-Frascati collaboration FBG sensors already used at INFN – Frascati National Laboratory FINUDA Experiment Monitoring deformations of vertex detector mechanical structure

  8. Fibre Bragg Grating (FBG): previous applications for HEP by the INFN-Frascati collaboration Microstrip OFF Microstrip OFF Massive ‘trips’ of Microstrip Microstrip ON Microstrip ON 11/Dic/2003 - 03:08 Massive ‘trips’ for some time due to e+ injection 10:10 DAFNE machine development: Microstrip and Tofino off 16:15 Beam stable: Microstip and Tofino on 18:50 Microstrip (OSIM1) ‘trip’: recovery procedure 19:15 Microstrip stable: take data

  9. Fibre Bragg Grating (FBG): previous work by the INFN-Frascati collaboration Use of FBG for monitoring RPC gas temperature presented @ TIPP2011 Array of many FBG sensors inserted in gas conduit Monitoring temperature gradient of gas fluxed in RPC detector _______ Early detection and localisation of overheating RPC chambers FBG_1 FBG_n

  10. Fibre Bragg Grating (FBG): previous work by the INFN-Frascati collaboration FBG Temperature sensor gas IN FBG temperature sensor Reference Thermocouple gas OUT Reference Thermocouple RPCs

  11. FBG for RH measurement: hygroscopic features of the polyimide recoating FBGs are commercially available with either acrylate or polyimide recoating FBG production procedure 1) Stripping the original coating of the fiber 2) Writing the Bragg grating 3) Recoating

  12. FBG for RH measurement: hygroscopic features of the polyimide recoating FBGs are commercially available with either acrylate or polyimide recoating Polyimide recoating for RH measurement: swelling due to hygroscopic properties of the polyimide recoating induces stress and modifies the pitch of the Bragg grating • Models for polyimide hygroscopic properties are available • Hysteresis can be present • Hysteresis usually comes from large voids in the polymeric structures and subsequent absorption of water clusters • Water clusters can also deform the polymer structure and modify its hygroscopic properties FBG Acrylate (standard fiber cladding) Polyimide (FBG recoating)

  13. Sensor prototype Polyimide recoated FBG sensor in sealed box to be installed in gas conduit Commercial polyimide recoated FBG sensor RH TC Pipe joint gas inlet • RH reference measurement by capacitive sensor • Temperature measurement for compensation by Thermocouple FBG Pipe joint gas outlet

  14. Monitoring RH by FBG in RPC detector INFN - Frascati National Laboratory Experimental Hall ‘ASTRA’ RPC Test Facility Intended application: monitoring RH of gas fluxed in RPCs Thermocouple and Hydrometer conditioner RPCs RH TC FBG FBG Interrogation System gas IN gas OUT

  15. Sensor prototype testing Preliminary test of sensor prototype in 15% ÷ 60% RU range Test measurement worked out downstream RPC chambers Use of humidifier to control humidity of gas

  16. Dl DT RH Sensor prototype testing To work out RH value FBG signal must be conditioned (Temperature compensation) ? ?

  17. Sensor prototype testing

  18. Sensor prototype testing

  19. Sensor prototype testing

  20. Sensor prototype testing

  21. Conclusions and Future Work • Results show the feasibility of use of commercial polyimide recoated FBG sensors to monitor RH … …but precision and resolution must be improved • Planned work to test use of custom recoating on commercial uncoated FBG, either in selected polyimide or other hydroscopic polymer • Prospetive: development of custom ‘sensorised gas joint’ Integration with FBG Temperature sensor prototype (previous work), to develop all-in-fiber distributed sensing system for Temperature & RH FBG fiber pigtails and connector optical fiber FBG embedded in custom gas joint RPC gas flow

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