Conventional practice in vaccine storage is to insert a temperature probe into

Common practice in the monitoring of cold chain conditions for temperature sensitive products is to employ a physical thermal buffer into which the temperature probe is inserted. This buffer may be a bottle of glycol or other liquid, a container of glass beads, aluminum block or nearly any other media the user feels appropriate. The purpose of the buffer is to simulate the experience of the stored product rather than the air temperature. Obviously, this mission will not be accomplished to the extent that the physical buffer is not matched to the thermal properties of the stored product and its container. Cold chain managers are faced with a complex problem if they attempt to take this issue into account. Furthermore, a match is not possible with a single physical buffer when the cold storage unit contains different products or size containers. This paper quantifies the results of this mismatch from various factors and suggests possible solutions to this dilemma.   Type: Original Research

The CDC’s publication entitled, "Guidelines for Storage and Temperature Monitoring of Refrigerated Vaccines" recommends using a glycol bottle for refrigerator applications. As the glycol acts as a thermal buffer, it reduces probe sensitivity to air temperature fluctuations within the refrigerator cabinet. NIST studies have demonstrated that temperature probes in glycol-filled bottles can more closely approximate vaccine vial temperatures when placed in the same refrigerated area where the vaccine is stored. Current CDC guidance does not specify a specific volume or geometry for the buffer. This leaves open to interpretation the specifics of the buffering methodology. Our review of physical thermal buffers currently utilized in the general industry shows little consistency in the geometric shape, with volumes ranging from 10 to 300 ml. It was also observed that pre-filled syringes as small as 0.25 ml were being stored, which is significantly smaller than the volumes of physical buffering being used. This is especially important with respect to freezing, as the volume of physical buffering would not provide accurate notification of compromised vaccines. The use of telemetry in the measurement and monitoring of equipment is not only necessary but essential to the successful prediction of the unit operation as well as the demonstration of a close approximation of the contents. The telemetry available from the raw air temperature probe will provide valuable information on the unit’s performance both near and long term. One of the multiple telemetry streams that can be derived from the raw air temperature is virtual temperature buffering. That is an algorithmic based representation of the volume to be represented. A fixed buffer assumes the entire content is a single volume, where this is rarely accurate. The application of a virtual temperature buffer, one can accurately simulate any volume and shape that is maintained in the storage unit. With virtual buffering, the effect of an excursion can be accurately applied to each volume rather than a blanket assumption that all volumes are affected the same. This presentation describes the development and evaluation process of a virtual buffering process that has been demonstrated to be highly accurate and repeatable.