Why Do Thermometers and Sensors Have Different Readings?


If you’re using a temperature sensor or logger, you might notice something strange if you also have a thermometer in your cold storage. Your thermometer is telling you one thing, while the temperature logger is telling you another. So, which one is right? And is your temperature logger faulty? Can both be accurate, and working?

We have all the answers you’re looking for right here. 

How Thermometers Work vs. How Temperature Loggers Work

Thermometers measure temperature by taking more or less constant ‘readings’ of the temperature. While it’s a little more complicated than that, wherever they are, they’re basically always measuring the temperature. 


Temperature loggers take readings frequently, although most don’t measure the temperature constantly. That might lead you to believe a thermometer is more reliable, since it’s always measuring the temperature. However, that’s not entirely true. Temperature loggers only take readings every few minutes, because they don’t need to do more than that. Most cold storage is just fine minute to minute. A large gap between readings is what you need to worry about, because that’s where temperatures can rise without you noticing it. 


Thermometers don’t transmit their readings to any sort of data backup system. Temperature loggers do. If you want to record temperature readings from a thermometer, someone has to visually check it, record it, and put it in your logs. That also opens up the opportunity for human error. Temperature loggers automatically transmit their readings, so there’s no data lost in the process. Not only that, the readings are entirely objective, unlike measurements from a thermometer, which can be subject to your viewpoint, eyesight, and more. 


Why Doesn’t the Logger Say the Same as the Thermometer?

So, you have a temperature logger and a thermometer in the same general area (for example, a refrigerator for medical storage). You might notice that the thermometer says one thing, while the logger says another. If they’re both working, and accurate, shouldn’t they say the same thing?


Not necessarily. 


While temperature loggers are highly accurate, it’s important to remember that they measure temperatures at set intervals. Thermometers just tell you what the temperature is, at the current moment. Then, there’s also the issue of placement, and how the other equipment you use can affect either of them. 

Here are some quick explanations for why your logger and thermometer show different readings:


As we mentioned, thermometers are constant, while loggers read the temperature at set intervals. Within any cold storage there are slight temperature variations throughout the day. If your logger shows its most recent reading at three minutes ago, the temperature might have changed since then. The thermometer won’t show you what the temperature was three minutes ago. It’ll only show you what the temperature is right now

Cooling cycles

This reason goes hand in hand with timing. Blasting cold air in a fridge isn’t just expensive, it’s also inefficient to the point of being wasteful. Refrigerators and freezers are designed to stay cold. They have special insulation built in, and they’re designed to close with a tight seal. If you look at our article on proper food storage, you’ll see how well it works. In fact, most fridges and freezers can maintain their temperature for about four hours without any power. 

Because they seal, have insulation, and maintain cold temperatures well, they only cool when needed. Once the ‘cool’ cycle begins, temperatures within your fridge and freezer drop. Likewise, before the next cool cycle, temperatures tend to be higher. Naturally, if your temperature logger has a reading right before the cooling cycle, it will be significantly higher than afterwards. 


A thermometer reading after the cooling cycle looks a lot lower than a logger reading right before.


In a perfect world, all cool storage would have a perfectly uniform temperature. No cold spots, no warm spots, just the same temperature regardless of where you measure it. Unfortunately, that’s not the world we live in. Temperature can vary throughout a refrigerator, and depending on where you take your readings, it really shows. 

Placement matters. It’s the same reason you’ve heard to keep items like eggs, cheese, or other perishables in the main fridge shelves, rather than the door. 

The main interior of the fridge is more likely to stay cool. However, how your refrigerator cools also plays a part. Wherever the cold air enters is going to be the coldest part of your fridge. So, if you have a temperature logger by the door, and a thermometer by the cold output, it’ll look different. It’s best to avoid placing your logger in the coldest part just to get a lower reading. Instead, place your logger near the products you need cold. You can then see what temperature your stock is at more accurately. 

A good example is the thermometer a lot of banks have on display, a temperature reader in your car. If it’s a really hot day, a bank thermometer in the sun might say 98 degrees. Just a block over, the other bank’s thermometer says 95 degrees. Why? Because it’s in the shade. Likewise, your car might say it’s 99 or even 100 degrees when you’re driving on the highway facing the sun. When you turn the other way, your car might say it’s 96 degrees, because you’re no longer facing the sun.

It works very similarly when you compare temperature loggers and thermometers. 

Are They Both Right or Wrong?

Which temperature reading should you rely on? Is the thermometer your best bet, or is it the temperature logger? Here’s the thing: they both work in much the same way. 


You’re still seeing accurate readings from both of them. However, the important thing about temperature loggers is in how you receive the readings. While they only log the temperature every few minutes, that’s sufficient for most purposes. If you need to know you’re maintaining the proper temperature, you don’t need a second by second update. 

Temperature loggers help you keep an eye on your cold products throughout the day. Basically, you’re making sure they always stay in safe temperature ranges. If you’re ever unsure, you can look up the logs to see a history of the readings (and some clients may even request to see these). 

Both your thermometer and your temperature are working exactly as they’re meant to. Remember, temperatures can fluctuate because of placement, when you log temperatures, and cooling cycles. So, put your temperature logger near your products, and not near warm or cool spots.