Temperature Monitoring during Transit

The recorder on the refrigeration unit on older units records the temperature of the return air, but on some, particularly newer units record the temperature of the delivery air. The accuracy of the recording depends on the care taken during the pre-trip calibration and on any consequential rough handling (eg. container roughly handled). In a container with adequate air circulation, the temperature of the bulk of the fresh produce is close to the return air temperature, providing the product has been precooled to the desired carrying temperature. If the air circulation is inadequate or restricted, the bulk produce temperature may be at a higher temperature than the return air. As units now in service operate with delivery air control, then the recording of the return air temperature will show an increase of approximately 2°C (35.6°F) higher than the delivery air temperature; if the unit operates under return air control, then the recording of the return air should be fairly constant. If the delivery air temperature is recorded, then the recorded temperature should be constant and is about 0.5 to 1.5°C (32.9 to 34.7°F) below the bulk produce temperature.

To obtain an independent record of temperature throughout transport, a self-contained, single-point temperature recorder may be placed in the container. The preferred location for the recorder is in the second package down, at the door end near the centre of the stow, and clearly marked with some form of highly visible marking to ensure retrieval on discharge. If the recorder is placed on top of the stow or attached to the ceiling, then the recording will not represent produce temperature but will give information on the performance of the refrigeration unit. Alternatively, a multi-point recorder may be used with temperature sensors placed in the fresh produce as well as in the air. Some of these recorders allow interrogation of the recorded data from outside the container.

Some Containers are fitted with mechanical 31-day chart recorders but technology is rapidly moving towards controller and computer linked electronic data storage and recording and charts will become the exception rather than the rule. Modern integral containers are fitted with microprocessors capable of storing electronically, voyage information for up to 1 year, which in turn can be downloaded via a portable computer and reproduced in hard copy if required. Information that can be gathered in this manner includes supply and return air temperatures, set-point changes, alarm conditions, pre-trip condition, defrost frequencies etc. The units also have the capability of recording up to three remote sensors, with most having USDA approval for the recording of fruit temperatures in transit. For USDA and other quarantine cold treatment requirements, maximum pulp temperatures may have to be maintained below a specified temperature throughout a continuous period of days and only approved equipment may be used.

To allow external monitoring, a thermocouple may be placed in a carton of produce within the stow. The monitored carton should be located in the top layer in the centre line of the container in the tier adjacent to the doors. For integral containers, the leads should be taken out through the door seals with the loose material coiled and securely taped to a door handle on the right-hand side door. More detailed information may be obtained by using several thermocouples. The use of external 19 monitoring allows corrective action to be taken if a fault is found, in contrast to internal monitoring where a fault is discovered only at the end of the journey. However, care needs to be taken in interpreting measured temperatures and in any corrective action taken. eg. If the measured temperature of a carton in the top layer at the doors indicates that the carton is too warm, the set-point of the unit must not be reduced if there is no measurement of the coolest carton.

Shippers should inform both the shipping company and the receivers if they choose to install their own recording devices within the cargo.