A decrease in temperature downstream of a heater would indicate intrusion of gas.
A relatively simple thermal technique has been proposed to enable detection of bubbles of gas or bulk gas in a tube that is meant to contain a flowing liquid only. In a system in which gas cannot be tolerated, the technique could be used to trigger a shutoff valve downstream of a location where gas is detected.
The technique would involve the placement of a circumferential electrical heating element around the tube and a circumferential thermometer around the tube a suitable short distance downstream of the heating element. The thermometer reading would be indicative of the rate of transfer of heat from the heating element to the thermometer; this rate would depend on the temperature of the heater, the rate of convective transfer of heat between the tube and the fluid at the heater and thermometer locations, and the thermal capacity of the fluid in the tube. These variables and thus the thermometer reading would be affected by any gas that might be present.
A device to implement the technique could be fabricated by placing a thin layer of a dielectric material around the tube, sputtering a thin platinum coat over the dielectric, and patterning the platinum into narrow rings around the tube at two (or more for redundancy) positions along the tube. The upstream platinum ring would be operated as a heater and resistance thermometer, and the downstream platinum ring would be operated as a resistance thermometer. For a specific combination of heater temperature and rate of flow, the temperature sensed by the downstream resistance thermometer would include a component proportional to the transport of heat by thermal mass flow in the tube. Intrusion of gas into the tube would reduce the effective thermal mass density of the fluid in the tube, and the temperature read by the downstream resistance thermometer would decrease accordingly.
This work was done by Frank T. Hartley of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category.
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