Why does my temperature logger differ to the vaccine fridge display?
Why the Vaccine Fridge display differs from a Temperature Logger inside the Fridge
Table of Contents
A vaccine fridge’s displayed temperature will often differ from the temperature recorded by an independent logger inside the fridge. This does not automatically mean either device is faulty. This article outlines the main reasons why this happens, and discusses when this is acceptable and when it is an issue.
Note: It is expected to have a difference in readings. Both devices, however, need to be within the 2°C to 8°C range.
Temperature Varies within the Fridge
The temperature inside a fridge is not perfectly uniform. Warm air enters from outside while cold air is blown in from the cooling unit. While most people like to think of a concept as “the fridge temperature”, it does not exist. Instead, at any one time the fridge will have a range of temperatures within it.
In a recent discussion with one of the country’s leading vaccine fridge manufacturers, they said it is normal for a “normally stocked” fridge to have a two degree variation at any given moment.
A purpose-built vaccine fridge uses a fan to circulate air and minimise these differences. This is one of the key reasons vaccine fridges perform better than domestic fridges. The fan helps move cold air around the cabinet so that the temperature is more even across the storage area. However, even with fan circulation, the temperature will not be identical everywhere inside the fridge. Some areas will still be slightly warmer or colder depending on airflow, stock placement, door openings, compressor operation, and how quickly different parts of the fridge recover after a temperature change.
The variation can become greater when the fridge is heavily stocked. Products, boxes, baskets, or shelves can restrict airflow and prevent cold air from circulating evenly. Some areas may be directly in the path of cold air from the cooling unit, while other areas may receive very little recirculated air. “Heavily stocked” can also refer to a single shelf that is solidly filled and blocks all airflow. We would actually argue that there should be a small gap, around 2 cm, between every row in a vaccine fridge. If there is not, then it is “heavily stocked”. And if you are having difficulty jamming more stock in, then your fridge is “way overstocked”.
When airflow is blocked, a logger may record the temperature of a local warm or cold spot rather than the average condition of the fridge. For example, a logger placed directly in the path of cold air may read lower than the fridge display, while a logger placed behind tightly packed stock may read higher because that area is not receiving enough recirculated air.
In practice, loggers will typically record a temperature lower than the fridge because they are often in the path of colder air than what is being returned and controlled.
Location of the Logger versus the Fridge sensor
The fridge display normally comes from a built-in sensor mounted in a fixed location chosen by the manufacturer. That sensor is usually positioned where it can control the refrigeration system effectively but also remain protected. This means that it is often not in the actual fridge. It will also be in the path of the returning air and so will always be warmer than the cold air being pumped back into the fridge. The independent logger, however, measures the temperature at the location where it has been placed. If the logger is near the door, near the back wall, close to the cooling outlet, inside a basket, or surrounded by stock, it will see a different temperature from the fridge’s built-in sensor.
A very simple test is to move the logger between the top of your fridge and the bottom. This will typically prove that you have a temperature variation in your fridge and so the positioning of your logger is critical.
Strive for Five recommends that you place the logger in the middle of the fridge. It also recommends taking the opportunity to move the logger around to create a temperature profile of your fridge.
Some fridge manufacturers recommend placing the logger in a specific location to minimise the variation between the logger and the fridge display. While that can be useful for avoiding confusing conversations with auditors, it can also reduce the logger’s ability to highlight that part of the fridge may be too cold, or possibly even freezing.
Air Temperature / Product Temperature / Calculated Temperature
A fridge can have several different “temperatures” depending on what is being measured. The air temperature can change quickly, especially when the door is opened or the compressor starts. The product temperature changes more slowly because vaccines, packaging, and any thermal mass take time to warm up or cool down.
The fridge sensor and the logger are taking a temperature reading of their probe or sensor. It may be quick responding and be closer to showing air temperature, or it can be slower and more like the product temperature.
The displayed fridge temperature may be different again, because it may be calculated, averaged, delayed, or filtered by the fridge electronics before it is shown on the screen.
Response Time and Sampling Interval
The fridge sensor and the logger may not respond at the same speed. A small exposed sensor in moving air can react quickly to a temperature change. A sensor inside a case, buffer, bottle, packaging, or slower-responding probe will change more gradually. This means one device may show a short spike or dip while the other device shows a smoother, slower change.
Neither is “right” or “wrong”.
Instead, think of them as being sensitive or insensitive to change, or responsive or unresponsive.
The question is, “how sensitive should they be?” We would suggest the answer is “slightly more sensitive than the vaccines”, which then means “how sensitive are vaccines?”
And that depends on their size, material, and packaging. There is no single answer. What we can suggest is that many loggers are more responsive than vaccines while some vaccine fridges will be less responsive. Loggers are designed to alert you to problems while vaccine fridges like to play it cool.
Sampling interval also affects what each device records. A fridge display may update frequently, while the logger may only record every few minutes. If a short temperature change occurs between logger readings, the logger may miss it. Alternatively, the logger may capture a short event that is no longer visible on the fridge display by the time someone looks at it.
Accuracy and Calibration
Both the fridge sensor and the logger have their own accuracy tolerance. One sensor may read slightly high and the other slightly low. These differences can also be in opposite directions, meaning they will add to each other. This is why there is some focus on ensuring that both the fridge and logger are calibrated. Calibration is simply the process of ensuring that the device is within an acceptable variation from the true temperature.
Note, however, that accuracy is measured in a constant temperature environment, not a continually changing environment. When the temperature is changing, thermal latency has much more impact than accuracy.
Door Openings and Recovery behaviour
Door openings can exaggerate the difference between the displayed temperature and the logger temperature. When the door is opened, warm room air enters the fridge. A logger near the front of the cabinet or in free air may show a rapid temperature rise. The fridge’s built-in sensor may be further back, shielded, or influenced by circulated air, so it may show a smaller or slower change.
After the door is closed, the fridge begins recovering. During this recovery period, some areas may cool faster than others, particularly areas directly in the path of cold airflow. Other areas may take longer to return to the normal storage temperature.
Compressor and Defrost Cycles
The fridge’s compressor cycles on and off as part of normal operation. This causes small rises and falls in temperature throughout the day. The logger may record these changes more clearly than the fridge display, especially if the display is averaged, delayed, or filtered. Defrost cycles can also cause differences. During a defrost cycle, some fridges stop updating the displayed temperature. The fridge may hold the previous displayed value while the defrost cycle is occurring, then update the display once the cycle is complete. Meanwhile, an independent logger continues recording the actual temperature at its location.
This can make the logger appear to disagree with the fridge display during or immediately after defrost, even though the fridge is operating as designed. We often see quite significant defrost cycle spikes, and they are totally ignored by the fridge display.
You can easily identify defrost cycles because they are incredibly regular and will have a frequency of 6, 8, 12, or 24 hours. If they are causing a problem, ask the fridge technician to adjust the settings so they do not last as long.
Practical interpretation
A difference between the vaccine fridge display and an independent logger is expected. The key question is not whether the two readings are identical, but whether both readings remain within the required 2°C to 8°C range.
Small differences are normal. Larger differences may indicate poor airflow, overstocking, poor logger placement, a warm or cold spot, calibration drift, or a fridge that is not performing evenly across the cabinet. Strive for Five still requires you to record the fridge’s min/max readings, and those readings continue to be important. The logger, however, gives you an impartial view of the temperature within the fridge.
When both the fridge readings and the logger readings are within range, you can be confident that your vaccines are being stored safely. When the logger is out of range, however, you need to investigate whether the fridge is overstocked, whether the logger is in a problematic location, or whether the fridge may be malfunctioning.