VTB™ Series: Probe Placement

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The following is found in the Centers for Disease Control and Prevention (CDC) Vaccine Storage and Handling Toolkit:

Temperature Monitoring Device Placement

Prior to storing vaccines in a unit, determine where the most reliable and consistent temperature readings are and store your vaccines there. The probe should be near where vaccines are being stored. This should be in the main body of the storage unit, away from walls, ceiling, cooling vents, doors, floor, and back of the unit.

Upon reading this for the first time, I questioned why? What difference does probe placement make? I am certain that others have come to the same conclusion, does it really matter?

Upon further research and evaluation, I learned that the CDC receives much of its technical input from NIST (National Institute of Standards and Technology). In my own observations, I found that there are variations in the measurement of the temperature based on location of the sensing probe, the volume of contents of the storage unit, and the quality of the storage unit itself.

Breaking down the statement from the CDC:

  • …determine where the most reliable and consistent temperature readings are

The temperatures being recorded are to represent those that the stored goods are experiencing.

  • …away from walls, ceiling, cooling vents doors, floor and back of the unit

The structural areas described above represent a thermal mass that can affect airflow. These surfaces may be much colder open areas.

  • …this should be in the main body of the storage unit

When considering placement of the temperature monitor, consistent placement is necessary. By placing the temperature probe in the center of mass of the storage unit the most “reliable and consistent” measurements will be made.

Thermal buffers will be discussed in a later article; however, these do warrant consideration in their placement. Often the thermal buffer is large and bulky. So much so, it is often placed “out of the way” in the bottom of the storage unit. Any measurement probe at or near the surfaces of the storage unit will be further buffered by those surfaces. In these cases, the measured temperature will no longer be accurate and representative of the temperature of the stored good.

For consistency sake, the procedure for the placement of the temperature monitor be placed in the approximate center of mass. That is centered from top to bottom, side to side, front to back. The probe is placed with care not to be contacting any surface including the wire shelving. Every effort is made to position the probe in free air in the center of mass.

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Wireless Vaccine Temperature Monitoring Sensor Image