Why is there a difference between the temperature reported by the Tive device and the temperature that the refrigeration unit is reading?

Temperature tracking devices measure the ambient temperature (average surrounding air temperature) of a particular place inside of the trailer/container. Many factors influence the temperature of the ambient air inside a trailer or container.  Some of these variables include:

1. The integrity of the trailer air chute.
   A block or disruption in the air circulation, like a tear in the air chute,  will result in warm areas within the trailer. Generally, the warmest spots will be at the door end of the trailer, since it is furthest from the refrigeration unit.
2. Where the tracker was placed inside the trailer or container.
   Firstly, the location inside the trailer/container that the device is placed in, affects the device’s temperature reading. For example, devices placed directly on the product will read closer to the temperature of the product itself, while devices placed on the sidewall (inside walls of the trailer) will read closer to the ambient temperature (in most instances.)  
3. How well the product was precooled (if applicable). In most cases, product precooled to the ambient temperature of the trailer will still need 1.5-2 hours to equilibrate after loading. If the product being loaded has more than a two-degree delta from the reefer set point in either direction, the product will take significantly longer, possibly days to equilibrate to the ambient temperature in the trailer, if it equilibrates at all.  Because the tracker reads ambient temperature, tracker readings may differ from the temperature of the product itself.
4. The way the product was loaded.
5. If the product was stacked too high, or if the product was loaded sideways, this will cause poor air circulation inside of the trailer or container.  
6. Materials used to package the product.

Reefer units temperature measurement

The purpose of refrigeration is to remove heat, take it from one place, transport it, and release it to another.  Temperature measurements recorded by the refrigeration unit are typically the supply air and the return air.   If there is poor airflow in the trailer caused by pallets being stacked too high or a tear in the chute, for example, the air may be short-circuited at the front of the trailer around the area where the return air measurement is taken.  As a result, there will not be sufficient airflow to remove the heat penetrating the trailer at the rear of the trailer, where the temperature readings are typically taken. The readings from the refrigeration unit generally will not reflect this problem. 

Slight deviations in transit temperature are inevitable.  In order to get an accurate reading on the temperature of the product itself, a pulp thermometer (thermometer with a probe) must be used to verify the product's internal temperature reading.