Thermal Damping For Temperature Monitoring Vaccine & Drug Storage Applications

Buffered temperature probe for vaccines
Paul Daniel, Vaisala
Senior GxP Regulatory Compliance Expert
Life Science

In this week's blog, we are looking at temperature monitoring in refrigerators that store vaccines, drugs and biologicals, which are typically maintained between 2 and 8° Celsius.

First: two key do­­cuments you need to read while figuring out how to monitor temperature for drugs and vaccines:

This is from NIST:
Thermal Analysis of Refrigeration Systems Used for Vaccine Storage: Report on Pharmaceutical Grade Refrigerator and Household Refrigerator/Freezer

Another crucial paper on temperature monitoring is from the Centers for Disease Control & Prevention titled,
Vaccine Storage & Handling Toolkit

Both of these documents (and others) recommend placing temperature monitoring probes into a thermal buffer, such as a bottle filled with glycol, to dampen temperature fluctuations and more closely mimic the temperature of refrigerated or frozen products. The CDC guidelines state: "Glycol-encased probes can provide a more accurate reading of actual vaccine temperature and are therefore recommended by CDC."

However, the guidelines also allow for options in materials used as a buffer with the temperature monitoring probe: "The CDC recognizes that some providers may use a temperature probe in glass beads to approximate the temperature of the vaccine in a vial and is currently working with NIST to evaluate their performance. Until more data are obtained, temperature probes in a buffer, like glass beads, are allowable…CDC may revise its recommendations and allowable buffered substitutes at a later date pending the outcome of the NIST evaluation."



For the Vaisala viewLinc monitoring system we use either epoxy-encapsulated NTC thermistors, or RTDs connected to our data loggers to monitor temperature in these applications. Our thermistors and RTDs often require thermal buffering to prevent rapid temperature fluctuations associated with opening a refrigerator door. Instead of fluid filled bottles, we designed a small aluminum cylinder for thermal buffering. It's simple, unbreakable, and of similar thermal mass to the standard glycol-filled bottle.

However, as noted in the NIST paper (link above), not all data loggers are created equal in terms of usability. It's worth nothing that the researchers at NIST found that temperature data loggers that use external probes don't need to equilibrate after being placed into a refrigerator. They wrote:

"To download data, we simply detached the main unit from the cable, leaving the probe inside the refrigerator. The main unit was then connected to a USB data collection cradle, which allowed us to transfer the data to a computer… "

We've found that this type of probe + logger configuration has worked best for our customers and the thermal block is a small, elegant solution for thermal buffering in temperature monitoring applications. Simpler is (usually) better.

Many customers also use the viewLinc Continuous Monitoring software for temperature monitoring. The system supports real-time monitoring and alarming, with audit-friendly reporting for historical data.

One last thing… the main objective of the NIST and CDC researchers was to evaluate the suitability of various types of refrigerators for storing temperature sensitive products. To do this, they used an array of temperature monitoring sensors to map the thermal conditions in the refrigerators and evaluate temperature uniformity. It's probably no surprise that a typical dormitory refrigerator is not the best choice for storing sensitive products. If you want to map your own refrigerators, big or small, see our Mapping Solutions. 





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Romulo Patungan

Mar 26, 2019

What would you recommend for -80C Freezer thermal buffer?

Paul Daniel

Mar 27, 2019
Hi Romulo -

Personally, I wouldn't recommend a thermal buffer in any controlled storage. I think measuring air temperature is a better practice because we see the worst case condition. If the reason for wanting the thermal buffer is to avoid nuisance alarms, there are better ways to do this. For instance, use an alarm delay on your monitoring system to filter out the short-term excursions caused by door openings.

I know many do not agree with me. That's fine, and it is always good to have many opinions to choose from as we determine the course that is right for our applications. Personally, I think thermal buffering obstructs our view of the data. I'd rather see the true data, and control the symptom (ex: nuisance alarms) through other pathways.

If you want to do thermal buffering in ultra-low freezers, I recommend an aluminum block. This is what we supply with our Vaisala data loggers for customers who want to use a thermal buffer. The aluminum block acts as a heat sink, and holds the RTD in a specifically drilled hole in the block. The size of the block is sufficient to simulate the thermal response of a 40mL bottle of glycol.

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