Early detection of impending cryostorage tank failure using a weight-based monitoring system

Objective: To study the ability of custom-built, web-enabled scales to monitor liquid nitrogen (LN2) levels in cryostorage dewars. Design: Laboratory study Setting: A large academic fertility center in New York City. Interventions: Cryostorage dewars were placed on top of the custom-engineered scales with continuous real-time monitoring, and weight and temperature data were recorded in the setting of slow, medium, and fast rate-loss of LN2 designed to mimic models of tank failures. Main Outcome Measures: Weights were continuously monitored and recorded, with a calculated alarm trigger set at 10% weight loss. Temperature within the tanks was simultaneously monitored with probes placed near the top of the tanks, with calculated alarms using a -185 degrees Celsius as the threshold. For the slow rate-loss simulations, tanks were left intact and closed in usual operating conditions, and LN2 was allowed to evaporate at the normal rate. For the medium rate-loss simulation, the foam core of the tank neck was removed and the insulating vacuum was eliminated by making a 1/16 inch hole in the outer tank wall. For the fast rate-loss simulation, a 1/16 inch hole was made through the outer tank wall and LN2 was released at a rate of 0.15 L/second. All simulations were performed in duplicate. Results: With an intact and normally functioning tank, a 10% loss in LN2 occurred in 4.2-4.9 days. Warming to -185 degrees Celsius occurred in 37.8 - 43.7 days, over 30 days after the weight-based alarm was triggered. Full evaporation of LN2 required 36.8 days. For the medium rate-loss simulation, a 10% loss in LN2 occurred in 0.8 h. Warming to -185 degrees Celsius occurred in 3.7 - 4.8 hours, approximately 3 hours after the weight-based alarm was triggered. For the fast rate-loss simulation, a 10% weight loss occurred within 15 seconds and tanks were completely depleted in under 3 minutes. Tank temperatures began to rise immediately and at a relatively constant rate of 43.9 degrees Celsius/hour and 51.6 degrees Celsius/hour. Temperature alarms would have sounded within 0.37 and 0.06 hours after the breech. Conclusions: This study demonstrates that a weight-based, automated alarm system can detect tank failures prior to a temperature-based alarm system, in some cases over a month in advance. In combination with existing safety mechanisms such as temperature probes, a weight-based monitoring system could serve as a redundant safety mechanism for added protection of cryopreserved reproductive tissues.