Objective: The quality management of small volume (30-73L) LN2 dewar cryostorage tanks has historically been maintained by routine (i.e., at least weekly) internal dipstick measurements and re-filling. Meanwhile, alarm systems, if used, have been based on a designated internal temperature threshold (<-180°C) or LN2 level set point (e.g., upper canister level). The goals of our investigation were to evaluate the prospective value of real-time pressure sensitive, weight measurements of mobile dewar tanks for operational qualification (OQ) and performance qualification (PQ).
DESIGN: Real-time weight measurements were correlated to changes in LN2 volume and temperature under new tank validation (i.e., OQ) and standard tank use (i.e., PQ). Evaporation usage rates were calculated at time of fill up (t0) minus measurement prior to next fill (t1wk; usage rate=t0-t1wk), based on weight (Ew) or volume level (EL). An evaporation rate index (Evap) was calculated for in-use tanks (T) using new tanks as the control (C) group (Evap =T0-T1/C0-C1). Differences in Evap were compared among tanks to determine if an objective measure for cryotank retirement was possible?
MATERIALS AND METHODS: Using a novel Wi-Fi based weight cart device system (TrustGnosis; Brea, CA) several new (n=6) and aged (>18 years old; n=5) 35-36L Taylor-Wharton/Worthington dewar LN2 storage tanks were routinely monitored for 3-6 weeks to correlate pre- and post-fill dipstick measurements to weight changes. All tanks were hardwired into our Xiltrix (Netherlands) biphasic temperature/canister level continuous monitoring alarm system. All tanks were filled weekly, and weekly usage rates determined by weight recordings and dipstick measures prior to filling. Manual and remote online data were correlated by ANOVA and the Evap indexing calculated to assess OQ/PQ determinations.
RESULTS: The mean LN2 usage rate (Ew/L) of new VHC-35 tanks was 3.1 lbs/3 cm, yielding a direct correlation of 1.0. In contrast, aged tanks varied in their Ew (6.4 to 9.8 lbs) and EL (4-10cm). The evaporation rate of aged tanks (19 to 27% Ew/week) was greater (p<0.05) than Group 1 new tank controls (8.5 to 10% Ew/week). The Evap index of the aged tanks by levels ranged from 2.0 to 3.0. Greater (p<0.05) precision was verified using weight measurements (Evap index=2.4-2.9).
CONCLUSIONS: Remote monitoring of LN2 dewar tank weights can be an effective and more precise method to measure daily and weekly usage/evaporization rates. Manual dipstick measures are subject to user error and complacency in QC practices, whereas remote weight measurements are not. Additionally, a weight-based Ew threshold alarm may represent an improved early warning alarm system for the potential detection of a failure scenario. Overall, external quality measurements and device systems represent a promising future offering greater precision, labor efficiency, and improved specimen security and safety. Furthermore, our data shows that new tank validations (OQ) and weekly performance (PQ) can be objectively evaluated by weight and used to formulate a useful threshold measure assessing dewar tank retirement.