Temperature fluctuation in cold chambers and freezers

In an ideal world, the temperature inside your fridge or freezer would be uniform in all places. Unfortunately, in the real world this is not the case.

There are high quality pharmaceutical and hospital refrigerators and freezers that achieve almost uniform air temperature inside. These refrigerators have multiple fans, cooling elements, patented defrost systems, a plethora of highly accurate calibrated temperature sensors, and have millions of dollars of research and development and decades of expertise behind them. They cost many thousands of dollars and cannot be compared to conventional refrigerators used in other industries (e.g., food supply chain) for a multitude of practical reasons, including cost. Most products that need refrigeration do not need to be stored in a highly uniform chamber, so a relatively inexpensive refrigerator will suffice most of the time.

The air temperature at a particular point within the refrigerator or freezer depends on:

• The distance from the door
• The distance from the cooling element
• The distance from the defrost resistors (if a freezer)
• The distance from the ground The hot air moves upwards, while the cold air moves downwards

Below is an image from a 2016 University of California Riverside study that illustrates a typical temperature profile of a deep freezer:

Deep Freezer Temperature fluctuation

For a domestic refrigerator, there is another study that shows similar pictures:

Temperature distribution over the packages inside a freezer at the end freezer's on-time

Temperature distribution over the packages inside a freezer at the end freezer's off-time

As is evident from the above images, it is possible to observe temperature gradients of 8-11°C even in a small chamber such as the one in the above experiment. Even in un-cooled rooms, one can easily observe a difference of 1-2°C at a distance of a few centimeters.

If the refrigerator thermometer and the sensor of the HAM ThermoSense or HAM HumiSense device are not exactly at the same point, then a deviation of a few °C is to be expected, depending also on the factors mentioned above.

The only meaningful comparison that can be made is between a temperature sensor and a handheld (ideally calibrated) thermometer, with their sensing points being (approximately) exactly at the same point after they have settled for a few minutes.

Why the Temperature Inside a Refrigerator or Freezer Is Not Uniform

When you open your refrigerator or freezer, you might expect a consistent temperature throughout, but in reality, the temperature inside these appliances is often not uniform. Let’s explore the reasons why this happens:

1. Airflow and Circulation: These appliances use fans and vents to circulate cold air, but the distribution of this air isn’t always perfect. In most refrigerators, cool air is generated from the back or bottom, and then it’s blown into the main compartments

2. Thermal Load and Placement of Items: The placement of food items inside the refrigerator or freezer also affects temperature uniformity. Dense or large items, like big blocks of meat, absorb and radiate cold differently than smaller, less dense items, which can disrupt airflow

3. Defrost Cycles: Refrigerators and freezers periodically enter a defrost cycle to melt any ice buildup on the evaporator coils. During this cycle, the temperature inside the appliance can rise sharply

4. Cooling Zones and Design: Modern refrigerators often feature multiple cooling zones with different temperature settings, which can create further temperature variation inside

5. Opening the Door Frequently: Each time you open the refrigerator or freezer door, warm air enters, causing temperature fluctuations. The areas closest to the door tend to experience the most variation since they are more exposed to outside temperatures.

I see sharp temperature increases in the HAM ThermoSense / HAM HumiSense charts, but the local cooler temperature reading is well below that. Why?

The response of the temperature sensor depends on the shape and material of the sensor. A larger sensor has a slower response compared to a smaller sensor. A sensor made of plastic or other thermally insulating material is less sensitive to temperature changes, compared to a stainless steel sensor, due to the higher thermal conductivity of the latter. In order to be able to even compare temperature readings, both sensors must have settled for a few minutes. Remember the last time you used a body thermometer: you had to wait for a few minutes before you could read your temperature. Of course, waiting for settling is much more complicated when you have two thermometers with different sensitivities in a chamber that is constantly changing temperature.

When you open the refrigerator door, a sudden temperature rise occurs due to the interaction of the room temperature air interacting with the cool air coming out of the refrigerator.

Freezers also need to be defrosted to melt the ice so that they can continue their cooling function efficiently. During defrosting, there is a rise in the temperature of the chamber above the normal average, which can take 10-35 minutes. This usually occurs almost on a scheduled basis, every 8 hours for example. The defrosting process almost never causes damage to the stored goods. You can see below an example of a refrigerator that defrosts:

The steep peaks illustrate the thawing, the small "hill" was created during the supply of goods. Neither of these events are of concern.

Most fridge testers/thermometers stop showing the true temperature during defrosting to avoid causing any concern to the owner. They may, in addition, have a period of suspended measurement when the door is opened, during which the temperature is also shown as unchanged.

If you have set rules to alert you when the temperature is out of range, you should include a "FOR" time period longer than the defrost duration. For example, "IF the temperature is greater than -10°C FOR more than 30 minutes THEN notify me". In this rule, the critical temperature point is above normal (or above the safe limit) and the duration it is exceeded is set as longer than the defrost duration. This way you will avoid most false alarms, if not all. You can also set an additional condition to alert only after business hours.

Summary

The main conclusion here is not to panic when there is a difference in the way the temperature is displayed when monitoring freezers! Variations in temperature and differences reflected between freezers and monitoring systems do not equate to failure in either system. Knowledge of these complex issues can help alleviate anxiety, determine appropriate alarm points to eliminate alarm fatigue, and generally provide peace of mind for staff and owners.

Do you want to know some quick tips for the correct maintenance of fridge and freezers?