An all-optical pressure sensor for nuclear power plant applications

Open Access
Author:
Bednar, Jonathan Paul
Graduate Program:
Mechanical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 12, 2012
Committee Members:
  • Martin Wesley Trethewey, Thesis Advisor
  • Karl Martin Reichard, Thesis Advisor
Keywords:
  • Fiber Bragg grating pressure sensor
  • optical pressure sensor
  • nuclear power plant pressure transducer
Abstract:
An average nuclear power plant in the United States contains between 1,000 and 2,000 pressure transmitters. The use of capacitance cell nuclear pressure transmitters in these high temperature environments degrades integrated sensing electronics and limits the transmitter’s usable life. An all-optical pressure sensor would remove the transducer interface and communication electronics from the sensing environment— extending the sensor’s life and reducing sensor maintenance and lifecycle costs. This paper presents the design and experimental evaluation of a fiber Bragg grating optical pressure transducer. The primary transduction mechanism of this design relates pressure in a fluid filled pipe to sensing diaphragm strain using optical diaphragm strain measurements. Similar to capacitance cell pressure transmitter designs, the proposed pressure transducer is susceptible to measurement bias induced by temperature perturbations. To minimize measurement bias, a fiber Bragg grating temperature compensation sensor is used to calculate a temperature independent diaphragm strain measurement. The transducer design was fabricated and thermal experiments were conducted while the sensing diaphragm was mechanically unconstrained in a thermal chamber. During these experiments, the sensing diaphragm’s temperature corrected strain was calculated while the ambient temperature was perturbed. An experimentally developed coefficient of thermal expansion was found to improve the accuracy of strain temperature compensation. Testing also showed that the time dependence of strain compensation caused the fiber Bragg grating sensing diaphragm to ultimately exhibit 4x the maximum temperature error expected from a capacitance cell pressure transmitter in the same environment. The assembled pressure transducer was experimentally evaluated using a hydraulic test system. The transducer’s sensitivity between 0 and 1,200 psig was experimentally determined to be 1.64 psi/με. A multi-step pressure experiment was conducted to confirm the transducer’s measurement repeatability. During this test, the maximum difference between pressure measurements made using the optical transducer and reference instrumentation was 44.3 psi. The mean measurement difference for this test was 19.0 psi. A capacitance cell pressure transmitter in similar conditions is expected to have an overall measurement accuracy of ± 2.5 psi. The largest source of measurement error from the fiber Bragg grating pressure transducer is likely to be the time dependent, transient response of diaphragm strain temperature compensation measurements. Future development of the pressure transducer concept should focus on reducing temperature compensation errors. Given this improvement, the proposed fiber Bragg grating pressure transducer has significant potential for nuclear power plant applications.