Cryogenic Liquid Sensing Using SAW Devices

Sensing at cryogenic temperatures is required for many critical applications, but is extremely difficult. There are a wealth of problems for cryogenic sensor devices, which include the extreme cold, which makes many sensors inoperable due to freeze-out of conduction carriers, mechanical stress and strain which impacts reliability, undesirable device heat generation in the vessel, and a host of others. In addition, consideration of sensor wiring and vessel penetration is required. In principle, acoustic devices can successfully operate to extremely low temperatures without any serious performance degradation. In particular, SAW devices operate as sensors, and certain embodiments are passive, wireless, and coded for multi-sensor applications. However, few results are reported on SAW devices for cryogenic applications. Research has been performed on the use of SAW devices for operation as liquid level sensors. The initial application is for a level sensor to be used by NASA in its cryogenic liquid fuel tanks, for both ground and space vessels. The work has investigated the use of both quartz and lithium niobate devices as liquid level sensors in liquid nitrogen. In principle, the devices can operate as simple switches, with the devices turned off due to liquid SAW damping when submerged. One primary concern in using SAW devices is the survivability of the devices when shocked by rapid, large, thermal changes in the submersion and withdrawal from the liquid. Another concern is the ability of a packaged device to be reliable. Experiments were performed on bare die, commercially available packaged devices, and on UCF fabricated devices. Samples underwent a host of rapid thermal cycling by liquid submersion and data was taken under varying test conditions. Survivability of devices and the operational parameters were measured throughout the year long study. The results found that the samples are remarkably robust. All packaged devices showed no failures in the entire study. This paper will present the experimental data and results from the study on SAW quartz and lithium niobate device operational performance parameters. A review of the various test conditions and device results will be shown. The results of this study conclude that under proper conditions, SAW devices can be used as liquid sensors at cryogenic temperatures.