Gap Discharge Transducers in a Transit Time Flow Measurement System Designed for Harsh Environments

Sammanfattning: In this thesis, the potential of the gap discharge transducer is investigated as transducers in a time-of-flight gas flow measurement setup designed for harsh environments. In this thesis the gap discharge transducer is operated in two different modes, named spark discharge transducer and glow discharge transducer. The spark discharge transducer generates a spark over a gap while the glow discharge transducer generates a continuous glow discharge. Earlier studies show that the spark discharge transducer is very durable and a potent sound pulse emitter. This thesis continues these studies by incorporating the spark discharge transducer into a flow measurement system as a sound pulse emitter. Further more, the gap discharge transducer is investigated as a potential sound pulse receiver. As an emitter in a flow measurement system the spark discharge transducer was placed in a pipe with a variable flow in a laboratory environment. The transducer was set to generate sparks to create the sound pulse and standard piezoelectric receivers were used to capture the signal. As a receiver, the gap discharge transducer was tested in two experiments. In the first experiment the spark discharge transducer was placed in a vacuum chamber to test the dependence between breakdown voltage and pressure. The pressure change from a sound pulse might cause breakdown in the gap of the transducer if an initial voltage between the electrodes is set close enough to breakdown. The breakdown leads to a spark which can be used to determine when a sound pulse arrives. In the second experiment the transducer was set to generate a glow discharge. The glow discharge was then subjected to a sound source. The voltage and current of the glow discharge depends on the environmental conditions in the gap. Pressure changes from a sound pulse will then affect the air in the gap which in turn changes the voltage and current characteristics of the discharge. The change in voltage and/or current can then be used to determine when a sound pulse hasarrived. The investigation shows that the spark discharge transducer is a potential sound pulse emitter in a flow measurement system and could be capable of determining a flow quite accurately. As a receiver, the spark discharge transducer show limited potential. The pressure change of a spark generated sound pulse requires the receiver to be set very close to the breakdown voltage. But that close to the determined breakdown voltage will very often cause the gap to breakdown spontaneously. The glow discharge transducer, on the other hand, show promising potential. The transducer is proven to be able to receive sound waves and has a sensitivity that is deemed compatible with the spark discharge transducer as a sound pulse emitter.