1. Alejo Bejemino
  2. équipement de la chaîne du froid
  3. mercredi 18 octobre 2017

Dear Technet members,

I would like to share my thoughts on the temperature monitoring devices and thermostal for vaccine refrigeration system.

In the early days of EPI, temperature of the refrigerator was measured and displayed by a simple analogue thermometer which was either a bi-metal or stem type alcohol thermometer.  Twice daily the health worker records these temperature on the temperature monitoring sheet and posted on the door of the fridge.  Filled temperature records are collected, kept on the file at the end of each month and replaced by a new one.  Temperature recorded was the temperature obtained during that time when it was read.  There was no information on the temperatures during the night or between the time interval when temperatures were obtained.  These old thermometers need to be calibrated more often which was a big challenge at the remote health facility.

To rectify the problem an electronic thermometer that could continuously record and display the temperature of the fridge was developed.  It does not need calibration till its lifetime of 2-3 years when it has to be replaced.  Average temperature readings could be read for the past 30 days.  The device was further developed to include a usb connector and with temperature alarm alert and temperature data could be downloaded for record and review purposes.

The latest temperature monitoring and recording device has a temperature sensor placed inside the fridge and could send the temperature readings on a computer/cellphone and could further be accessed via cloud etc.

All of the above temperature monitoring and recording devices are passive devices, in the sense that it does not take any corrective action if temperature excursions are encountered.  It is simply used to inform or alert the health worker that something is wrong with the fridge.  Corrective actions at health facility level relies on the health worker who is alone and frequently busy and not specifically trained to do the job of diagnosing the cause of the problem.  With shortage of the technical staff at the national/provincial/distric level, it is not expected to rectify the problem as early as it is needed thus causing potency problem to the vaccines, missed vaccine sessions and increase unopened vial vaccine wastage.

Allow me to give information to each one who are not familiar with refrigeration system, that every vaccine refrigerator/freezer (with the exception of the absorption kerosene refrigerator) has a temperature control which is commonly called a “Thermostat”.  Thermostat is an active device (compared to a thermometer which is a passive device) which is connected in the electrical circuit component of the refrigerator.  Its main job is to regulate or control the temperature of the refrigerator according the temperature setting by the user.  The old thermostat control switch is numbered starting from 0 to say 5.  Each number has a corresponding cut-in and cut- out temperature.

How does thermostat work?

The purpose of all refrigeration system is to lower the temperature of the confined space below that of its immediate surrounding.  The confined space of the refrigerator is the vaccine storage compartment. The refrigerant compressor of the refrigeration system once connected to the grid or solar panel will continuously run to lower the temperature of the vaccine storage compartment thus putting the freeze-sensitive vaccines at risk.   Running compressor motor can be stopped by any of the following actions:  a) if power from the grid is manually disconnected by the user or accidental power shutdown; b) the overload protector of the compressor is automatically activated due to extreme heat and voltage fluctuation; and c) the thermostat which was set at a desired temperature is automatically activated. Please note that once the compressor is stopped by either of the above ways the temperature inside the vaccine storage compartment will continue to rise.  Operation of the refrigerator will commence again if either the refrigerator is manually connected to the grid, the overload protector cools down and voltage is back to the normal range or the thermostat automatically reach its cut-in set point.  Therefore, THERMOSTAT is an active device that automatically regulates the temperature between the cut-in temperature of +8°C and the cut-out temperature of +2°C of the vaccine storage compartment.  Cut-in temperature of +8°C is the temperature when refrigerator compressor motor will start and cut-out temperature of +2°C is the temperature when refrigerator compressor motor will stop to protect freeze sensitive vaccines from freezing.

It is therefore very important call to action for the refrigeration system equipment manufacturer to further conduct a very thorough study to equip the vaccine refrigerator with a very accurate and precise thermostat control device that will work at various conditions in the field.

WHO PQS has introduced new standards for refrigerators to have a sealed thermostat. This effort is highly appreciated hoping to reduce unopened vaccine vial wastage due to freezing temperature in the refrigerator.  If thermostat is not sealed untrained staff as in the previous refrigeration units will try to adjust the temperature setting of the thermostat thus placing the both freeze sensitive and not freeze sensitive vaccines at risk.  Electronic sealed thermostats are now used by equipment manufacturer.  Temperature settings of these thermostat can only be altered by a manufacturer trained technician.

Recently, countries are receiving refrigeration equipment under the GAVI CCEOP project.  Equipment are laboratory tested and PQS compliant, however their performance could actually be tested at different and difficult environment and situations in the field.   To be able to assist the equipment manufacturer to develop a more accurate and precise thermostat, country’s EPI national program should monitor equipment performance and share feedback to TechNet for follow up actions.

Hope this helps.

Alejo H  Bejemino

Consultant

Medical Physics and Health Physics Services, Inc

Philippines

 

 



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