Transit Temps – Reconciling Recorders

We are often asked how we approach situations where the temperature readings recorded by a portable recorder, usually in the tail of the trailer, and the temperature readings logged by the reefer unit in the nose of the trailer, appear to conflict.

We’ve seen a tendency, by some, to jump to the conclusion that because both readings came from the same trailer, one of the recordings must be wrong.

But in the absence of a clear indication of malfunction (e.g., jammed tape), we think the better approach (at least as a starting point in most situations) is to presume the readings taken from both recorders are reasonably accurate, and to consider whether the different temperature readings may be attributable, not to malfunction, but to the different locations within the trailer where the readings were taken.

To illustrate, imagine that the air delivery chute was badly torn or detached. In this situation we’d expect to see the cool air “short-circuit,” i.e., return to the reefer unit without having circulated the length of the trailer. Here it would be no surprise to see warmer readings from a portable recorder placed in the tail of the trailer, and cooler readings recorded by the reefer unit in the nose of the trailer.

Similarly, we would expect to see warmer temperatures in the tail of the trailer in situations where the outside air temperature is very warm and the insulation around the rear doors of the trailer (a known vulnerability) is in poor condition. Although reefer units are designed to compensate when warm temperatures are detected, it must be remembered that the reefer unit resides 53’ away from the rear doors. Consequently, while air temperatures in the majority the trailer may be preserved, a warm spot may exist in the tail of the trailer.

More generally, when attempting to reconcile recorders it is important to recognize that portable devices are typically affixed to the outside of the shipping cartons to measure the temperature of the air surrounding the product. And because fresh produce gives off heat (field heat and/or heat from respiration) readings from portable recorders may be affected by this close proximity to fresh product.

Conversely, the reefer unit and its return-air temperature sensor are separated from the product by the bulkhead wall in the nose of the trailer. (The bulkhead wall is open at the bottom to allow the reefer unit to draw the air back to the unit for recirculation.) Given the return-air sensor’s separation from the product, and its proximity to the refrigeration coils, some variance between the temperature readings recorded by the reefer unit and those recorded by a portable recorder affixed to fresh product can be expected.

All this to say, in most cases, we’re reluctant to simply dismiss a temperature recorder reading as inaccurate. Typically, we believe it’s more constructive to presume temperature recorders are reasonably accurate, and to consider how the available information might fit together to give us a more nuanced picture of the temperature conditions within the trailer during the trip in question.

Doug Nelson is Vice President of Trading Assistance for Blue Book Services Inc.