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Best Temperature Monitor Placement: Inside Box vs. In-Trailer Air

Learn why air temperature is the controllable factor influencing product temperature

Refrigerated tractor trailer on road next to lake

At Sensitech® we are often asked, “Why monitor trailer air temperature instead of product temperature?” or in other words, why place the TempTale® monitor on the outside of a pallet rather than inside a box?

The answer involves a combination of factors and practical considerations that can strongly influence the thermal behavior of the product during transit:

  • Temperature at loading is often not uniform between pallets or even between boxes on the same pallet.
  • When trailer air temperature changes, significant variation in temperature occurs between more exposed locations (like the surfaces and corners of boxes and pallets) and the centers of boxes and pallets. For product loaded at the correct temperature, the quality and food safety risks tend to be in those areas which are highly exposed. Insertion of monitors or probes into the core of a product, box or pallet often masks temperature changes occurring in more exposed locations, providing a misleading view of what is actually happening to the product overall.
  • Location of the product within the box and pallet.
  • Location of the box within the pallet and location of the pallet within the trailer.
Diagram of TempTale monitor and air flow inside refrigerated tractor trailer

Food industry professionals are trained to look carefully at product or core temperature to avoid insufficient or incomplete cooling or freezing. It is important to ensure that the proper product temperature has been reached throughout the load prior to loading a trailer (see Sensitech’s Keeping the Cold Chain Unbroken with Four Simple Steps), however, focusing on product temperature alone during transport results in missing opportunities to detect temperature problems and missing opportunities for process improvement.

Product temperature is important, but measuring one or a very limited number of product locations will generally provide a misleading and ultimately confusing impression of what is happening. Measuring trailer air temperature tells us more than the temperature of a specific box of product on that load. The air temperature is the primary factor that affects product temperature. Measuring it gives us an understanding of what is causing product temperature changes and better reflects the overall temperature performance of the load. Examples of incidents which are easier to detect and understand when measuring air temperature are extended door openings, refrigeration unit malfunctions, incorrect refrigeration unit settings, etc.

Carrier refrigeration unit control module

A temperature probe or monitor inside a box indicating good product temperature does not necessarily mean that the rest of the load is OK. Understanding how product temperatures change in response to air temperature changes will help managers become more comfortable in using air temperature information effectively. People are often surprised to learn how closely the product temperature in exposed locations follows the air temperature under many circumstances.

Temperature probe and products stacked on multiple pallets

Air temperature is the controllable factor influencing product temperature, which means the trailer air temperature information is more actionable than product temperature. If the loading temperature is good and the air temperature and circulation is maintained properly, the outcome for the entire load will be good. When measuring trailer air temperature we are careful to make allowances for reasonable door openings and defrost cycles in the setting of the monitor alarm specifications and in the interpretation of the graphs.

Thermal heat map and temperature scale

Contact one of our Sensitech cold chain specialists today with any questions, or if you would like our project team to work with your business to assess risk and find solutions.

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