After the recent blog article about buffer vials (Glycol vs. Sand), we recognize that sensor placement is equally important for temperature monitoring. Despite what we may know about the "temperature settings" for a refrigerator, the fact remains that several variables affect the average temperature within. Understanding "microclimates" within a refrigerator is important in determining ideal sensor placement. The following two variables are the most common contributors to these "microclimates":
Defrost cycles routinely affect the temperature within a commercial refrigeration or vaccine storage unit. In simple terms, a defrost cycle is triggered to prevent buildup of frost on the evaporator coil, the key component of a refrigeration system. As the refrigerator frequently prevents frost buildup, defrost cycles will account for the most extreme temperature changes in an average day.
Cooling Cycles, a common feature of any refrigeration system, can also significantly affect internal temperatures. Cooling cycles will vary to maintain a consistent temperature, preselected by you, and can account for small, periodic fluctuations in temperature.
How significant are these variations? Are these variables important to the storage of vaccines, a particularily sensitive application that has little flexibility for temperature change? The answer is yes. Defrost cycles can change temperatures by up to 9°C within a refrigerator. Kudos to NIST.gov for this visual representation of temperature change within a Refrigerator. Even for commercial-grade refrigerators, these changes still occur.
- #9: (Air-Top) shows the relative cycles, marked by the large peaks and valleys.
- Temperatures of the "Lower Wall" are affected more by the defrost cycles than the "Mid Wall".
- The "Lower Wall" maintains a higher temperature (8>7) than the "Mid Wall".
- Glycol vials maintain temperatures that are only slightly affected by standard variables.
- Overall, the only consistent 'zone' is the center of the "Middle" of the refrigerator. Buffer vials in this zone (Glycol or Sand) are relatively untouched by defrosting and/or cooling.
Monitoring temperature in the correct 'areas' is a key consideration, particularily for vaccine storage. We can see that refrigerator air varies in temperature from 1-10°C, and this can compromise the safety of a vaccine. Check out this chart (via KDHeks.gov) for helpful tips on sensor placement for vaccine refrigeration, and be sure to keep variation in mind when placing temperature sensors!