Most National Health Departments and Councils seem to agree that the recommended fridge temperature should be between 2 °C and 7 °C (35 °F to 44 °F). The lower the temperature, the slower the growth of bacteria and the longer perishable foods will keep fresh. You can check the temperature with an ordinary thermometer, but that only tells you what the temperature is at that particular time. But what happens to the temperature during the whole day?
To get a good idea of the temperature over a period of time the DS1921Z made by Maxim comes in very handy. It is an autonomous temperature logger in an iButton package . This is a strong metal button the size of about four small coins put on top of one another. The DS1921Z has an internal temperature sensor (range: –5 °C to +26 °C, accuracy: ±1 °C), 4 Kbit memory, a real-time clock and a battery, which lasts between 2 and 10 years, depending on the log frequency. The iButton can log temperatures at a rate between once per minute up to once every 255 minutes. The memory has room for 2048 values, which means it’s possible to store a measurement every minute for a full day (24×60=1440).
The (free) 1-Wire viewer software makes it a piece of cake to configure the iButton and to read the results after the measurements are complete. Apart from the iButton you also need a USB dongle (the DS9490 made by Maxim) to connect the iButton to the PC. In the graph you can see the result of the measurements during a 24-hour period, where one iButton was placed in the door and another at the back of the bottom shelf. It is clear that there is a temperature variation of about 2 to 3 °C in both places as a result of the thermostat in the fridge. According to the advice of the Health Department, the door wouldn’t really be cold enough to store perishable items in this case, whereas it would be safe to store them at the back of the bottom shelf.