There are a few things that could explain a difference in why another measuring device is reporting a different temperature reading than a Sonicu temperature sensor.
1. Sensor placement: Ensure that the sensor is placed in the centermost part of the fridge/freezer. There should not be any fans blowing on the sensor, nor should the sensor be placed against anything that will change its temperature reading, such as newly stocked items that are at room temperature. Note that if the Sonicu sensor is not right next to the built-in sensor, they will read differently because no location in the fridge/freezer will be at exactly the same temperature.
2. Sensor type: Sonicu recommends using glycol-buffered temperature sensors for any fridge or freezer with a standard range of -40°C to 12°C, (-40°F to 50°F). Glycol buffered sensors more accurately represent the temperature of the contents of a fridge or freezer, compared to a non-buffered sensor, because non-glycol-buffered sensors can change temperature much faster, depending upon the air temperature change (the door being opened and closed for example) and contents being placed next to or touching the sensor.
The volume of glycol can also affect the reading. Many glycol sensors are smaller than a Sonicu sensor; some as small as 1" square. Sonicu purposefully designed and built our glycol sensors to match the requirements set by the CDC, for the Vaccines For Children (VFC) program. These dimensions can be found in the VFC Toolkit issued by the CDC.
3. Calibration: All Sonicu sensors are calibrated and include NIST traceable certificates before leaving our facility, ensuring documented accurate temperature readings. The sensor or device that is showing a different reading than the Sonicu equipment may not have been calibrated for quite some time, and could reflect some "drift" in its readings. A good example of this is a non-glycol buffered sensor that is placed in the wall of a fridge, with a LCD displayed readout on the front of the unit. A sensor like this may not have been calibrated since the fridge was manufactured, and it is displaying the reading of a non-glycol buffered sensor, that is not ideally located in the center of the unit.
4. Temperature Sample Timing: Sonicu devices, by default, take a reading once every 15 minutes. Other equipment may display real-time readings or have a longer time between readings. In either case, be sure you're looking at readings taken at the exact same time if you plan to compare.