Understanding Why Freezer Temperatures May Not Be Uniform
Factors to consider when monitoring freezers in life science labs when temperature discrepancies and fluctuations arise
The display on your -80℃, -20℃, or 4℃ cold storage units will almost always say just that - -80℃, -20℃, or +4℃. However, it is not uncommon for discrepancies between the temperature reported on your freezer and lab monitoring systems to arise. For instance, a freezer in the lab will display -80℃, however the monitoring solution reflects -75℃. This blog highlights how fluctuations in freezer temperature are common and other considerations to keep in mind for properly monitoring freezers in R&D labs.
A brief overview of how freezers work
Most freezers and refrigerators have a compressor (or compressors) that cool the unit. The compressor runs when the temperature gets too high, and turns off once the temperature has gotten low enough. The freezer insulation keeps the temperature from rising quickly, even when the compressor is off.
What does uniformity mean?
Uniformity generally refers to a state of consistency. In this context, we’re interested in temperature uniformity throughout different locations in the freezer. When analyzing temperature uniformity, temperature readings over time are averaged. Variation due to compressor cycles is not taken into account because while this affects temperature over time, it should affect all locations in the freezers in a consistent way.
In an ideal world, a ULT freezer averages -80℃ on all sides and in the middle. However, in reality, it might be -75℃ at the top, -78℃ at the side, and -82℃ in the middle. The University of California Riverside published a study in 2016 evaluating ULT freezer temperature uniformity and is very helpful in understanding what the temperature distribution can look like.
Image below is a figure from that study which illustrates a typical temperature profile:
What causes the temperature variability, or lack of uniformity?
Two common reasons why there may be a lack of uniformity in freezers being monitored could be:
- Convection inside the freezer
The top of the freezer will have a higher average temperature
- Differences in insulation thickness
All freezers have certain locations which are less well-insulated than the rest (e.g. around the door seal)
What factors increase and decrease this variability?
Common factors include:
- Number of racks
When a freezer has big empty spaces, it allows more air to circulate. This means more temperature change due to convection, and more temperature fluctuations overall.
- Amount of material stored
The more frozen material stored in a freezer, the more uniform the temperature stays during normal use. In thermodynamics terms, the freezer has a high thermal mass, and so it takes more heat for the temperature to change.
- Freezer make and model
All freezer models have slightly different performance characteristics. For example, a Stirling freezer and ThermoFisher TSX could both be set to -80℃, but their average temperatures and typical temperature variation could differ. This doesn’t mean that anything is wrong with either freezer!
How do discrepancies in temperature monitoring affect day-to-day lab management
There are two key take-aways to keep in mind when working with freezers in the lab and how to appropriately monitor them for your business needs:
Takeaway 1: If precise temperature matters to your samples, understand the spots in your equipment with variable temperatures and advise scientists appropriately. If you’ve ever accidentally frozen a dozen eggs because you put them on the wrong shelf in your refrigerator at home - the same problem can happen in the lab.
Takeaway 2: For installation and maintenance of your lab monitoring system, be aware of how probe placement might affect the temperature data collected. If you are a TetraScience Lab Monitoring customer, read on for specifics. If not, request a demo to find out how the solution can streamline freezer monitoring in your lab.
How does this affect my TetraScience installation?
Overall, freezer temperature uniformity should not affect the day-to-day usage of your alarm system. Your alert thresholds should be set above the highest average temperature typically seen in your freezer during normal operation.
Understanding temperature distribution is more important during installation or if you need to move, reposition, or replace the temperature probe.
If you are doing self-installation you should be aware of these temperature differences, and take the following steps to minimize issues due to normal temperature variation:
- Place the probe towards the back of the freezer; do not place it too close to the door where the average temperature is higher and there will be more variability in temperature.
- If possible, avoid placing the probe on a completely empty shelf.
- Avoid placing the probe on the back wall of the freezer, if the shelf is completely full. This can be too well insulated by the freezer racks, and it will take longer to detect temperature excursions.
Keep in mind that every model of freezer is different! If you think the temperature you’re recording is too high or low on average, you can always try moving the probe to a different location.
The key takeaway here is not to panic when there is a difference in how temperature reflects when monitoring freezers in R&D life science labs! Fluctuations in the temperature and discrepancies reflected between the freezers and monitoring systems do not equate failure in either system. Knowing these intricacies can help alleviate worry, establish appropriate alert set points to eliminate alert fatigue, and generally bring peace of mind to scientists and lab managers. The key is to employ solutions that are built and battle-tested to support best-practices for how important lab equipment is not only used, but also properly monitored.