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Sensitech Q&A to Improve Cold Chain: Question Two

Thermometer next to open tractor trailer loaded with pallets

Sensitech® is well known in the industry for its TempTale® monitoring devices. We have a team of experts in various disciplines including refrigeration, food science, life science, logistics and process improvement, who provide services and support to our customers to help them continually improve their cold chains. Throughout the years, the Sensitech Food Global Professional Services and Project teams have received numerous questions from our customers regarding how to determine the root causes of temperature issues.

Last time, we addressed what steps should be taken when a TempTale monitor indicates temperature problems have occurred. This blog in the series addresses another one of the questions we commonly receive from our customers about the difference between the temperature measured by a TempTale monitor and the temperature read by the refrigeration (reefer) unit in a trailer.


Why are there differences between the temperatures measured by the refrigeration unit and the TempTale?


Trailer refrigeration units are designed to circulate an envelope of air around the load, with the cold air delivered from the refrigeration unit at the front top of the trailer via an air chute or an unobstructed airway above the product. Air flows down around the load and, depending on the packaging and trailer loading pattern, may also flow through the load. As the cold air passes over the interior surfaces of the trailer and the product, heat energy entering the trailer from the exterior is absorbed from the trailer surfaces and food products. The air is returned along the floor back to the refrigeration unit. The heat energy absorbed by the circulating air is then expelled to the outside of the trailer by the refrigeration unit.

Diagram of airflow inside refrigerated trailer

A block or disruption in the air circulation 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. The exact location of the warmest spot can vary due to a number of factors, including the nature of the disruption to the airflow, pallet loading patterns, condition of the trailer, position of the trailer relative to the sun, etc. The recommended TempTale placement location is on the rear pallet, right side (facing the door) at eye level. This suggested placement is based on the above tendencies in combination with some practical considerations around ease of placement and retrieval, providing consistent SOPs, and avoiding damage to the device.

Poor and optimal airflow and monitor placement inside refrigerated trailer

The 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, 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 TempTale readings are typically taken. The readings from the refrigeration unit generally will not reflect this problem. The TempTale data will alert the user to temperature problems that are caused by both improper air delivery temperature and inadequate air circulation.

Chart: sample temperature readings inside trailer

We welcome your feedback and look forward to answering any other questions. Contact one of our Sensitech experts today.

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