Measuring temperatures in medicine shortage areas
An air sensor is a temperature sensor that has been suspended within the air space of a storage area and measures the air temperature.
Refrigerator and freezers
In the event of a high temperature excursion, the air in the fridge or freezer will warm first and this will then begin to warm the medicine. This means there is a short delay between the change in air temperature of the storage area and its impact on the medicine. Air sensors are therefore useful as an early warning indicator, alerting users to a problem shortly before it impacts upon products.
Rapid, significant changes in ambient temperatures are unlikely. Air temperature sensors alone are generally suitable for monitoring these areas.
A load sensor is a temperature sensor that has been placed into a mocked-up product, e.g. inserted into a liquid filled vial or bag. A load sensor provides an indication of the actual temperature of the medicines, which in the event of an excursion will take a short time to warm or cool after the air temperature has changed.
To replicate a liquid medicine, a temperature sensor may be placed in a container which is filled with a thermal buffer, such as propylene glycol (commonly referred to as glycol). The volume of the liquid should be representative of the smallest volume medicine stored in the refrigerator or freezer.
Choosing sensor type
It is recommended that at least one of each type of sensor is used to monitor a refrigerator or freezer as they each serve a different purpose in the event of an excursion.
The air sensor will be likely to initially detect the problem, and load sensor may then be consulted to determine if medicine itself is likely to have been affected.
It is possible to purchase refrigerators with an in-built air sensor and a ‘wandering’ sensor that can be placed into a mock product.
Recording data from temperature sensors
Two types of data recording devices are generally used: min / max recorders and continuous data loggers.
Min / Max recorders
Min / max recorders record the minimum and maximum temperatures reached since the device was last reset.
They are cheap to buy and reasonably easy to use, and are the minimum requirement in medicines storage locations. Users must manually check and record the data and reset the min/max recorder after each reading. This is a human process which is susceptible to failure in a busy environment.
Min / max recorders are of limited value when assessing the impact of excursions as they do not capture the exact length or profile of the excursion. When interpreting a min/max recording, it must be assumed that medicines have been exposed to the minimum and maximum temperatures recorded from the point the device was last reset. The actual excursion time period may have been much shorter.
An example form for documenting min/max temperatures may be found in chapter 3 of the Green Book. This example is intended for use with refrigerated vaccines, but may be adapted for recording of temperatures in any storage area.
Data loggers are compact devices which store an electronic record of temperature measurements at short intervals.
Data loggers are available as standalone recorders or may be linked web-based recording systems via a local network connection. The data captured can be downloaded in tabular and graph form to provide a highly detailed temperature profile for the storage area.
In the event of an excursion, a data logger can show the exact extent and length of the excursion. This allows for a much more accurate assessment of the potential impact of the excursion on the medicine’s quality.
Standalone data loggers are normally low cost, and easy to use. However, they normally have a limited capacity and must be periodically downloaded and reset. If a logger reaches its capacity it may stop recording data or may record over earlier data.
Data loggers that are battery powered may eventually run out power and recording will stop. A system must therefore be in place to either periodically replace the battery, or check the battery level indicator.
Key features required from a standalone data logger include:
- compliant with BS EN12830 standards
- UKCA / CE marked
- a display which allows for reading of current temperature, indication of active logging, battery status and alarm indicator
- user programmable logging frequency with large memory capacity, e.g. able to log temperature at 5 minute intervals for 6 months or more
- robust construction, splash resistant, and designed for operation at temperatures in which it used
- evidence of calibration supplied with the logger
Temperature monitoring systems
Data loggers may be integrated into digital monitoring systems. These systems allow a user to monitor multiple storage areas remotely via a web-based browser.
These systems have unlimited capacity to store data. They remove the need for periodic manual intervention required with standalone loggers and remove the risk the logger will stop recording if the logger’s memory is full.
Digital monitoring systems are of value if multiple store areas or refrigerators require monitoring, as data for all sensors can be easily accessed and reviewed through the software.
Calibrating temperature sensors
Ensuring sensor accuracy
Sensors which are not routinely calibrated may drift over time and display false temperature readings.
Temperature monitoring sensors should be calibrated annually to an accuracy of +/- 0.5°C by a specialist testing service with UKAS accreditation. UKAS is the National Accreditation Body for organisations that provide calibration services. You can check whether a company is UKAS accredited.
Sensors may be tested at 2 or 3 temperature points (“2 or 3 point calibration”) which cover the full range of temperatures the sensor is expected to monitor. “3 point calibration” gives the greatest confidence of accuracy at all points within the range and is a requirement in departments operating under an MHRA licence.
Assuring sensor accuracy
Certificates of calibration should be retained and held on record. Refer to our guidance on the .