Guide to Accurate use of Secondary Thermometers in Pharmacy Refrigerators
14th February 2020
As a leading manufacturer of medical refrigeration appliances, Shoreline receives many calls through its helpline requesting guidance on the correct usage of secondary thermometers. Confusion often occurs when the secondary thermometer display does not match the refrigerator’s built-in temperature display and this can often be some 5°C difference. In these circumstances this article provides users of vaccine refrigerators with written guidance, to act as a point of reference and a benchmark for best practice advice.
To quote the NHS Green Book Chapter 3: ‘Storage, distribution and disposal of vaccines’ (page 18) updated June 2013 states: “The Department of Health provides a protocol that covers the minimum standards expected of professionals responsible for vaccination. The protocol applies to all staff involved in immunisation, and covers:
- ordering and delivery
- auditing and monitoring of stock, including checking expiry dates
- maintenance of the cold chain, including frequent and regular
monitoring of fridge performance, and
- incident reporting.”
This protocol applies to all healthcare professionals, be they in Primary Care, GP Surgery level, or as supporting ‘High Street’ Community Pharmacists who also administer vaccinations. All parties must comply with these ‘Green Notes’.
So why are we being asked to use secondary thermometers even though our pharmacy fridge already has a built in temperature display? Chapter 3, page 29 states:
“At least one maximum-minimum thermometer that is independent of mains
power should be used (as well as any integrated thermometer), so temperatures
can be measured in the event of electricity loss.”
As the temperature display on the refrigerator is powered from mains electricity a loss of power will result in the loss of temperature display. Should such a situation occur the important information that is required when power is resumed is “How long was the fridge off for?” and “What temperature did the fridge interior rise to?” Both answers are required before deciding to retain or dispose of the stored vaccines. Contact your vaccine supplier with these answers and you should be given the appropriate advice and guidance.
It is highly recommended that a temperature data logger is also used inside the fridge. This will record temperature against actual time. In the event of a power loss the data logger will continue to record and store the temperature and this information can be downloaded for reference.
Example – Temperature Data Loggers
The use of a simple secondary thermometer, usually a dry-cell battery powered type, can be used. However, if the sensing probe’s position within the fridge is not considered and correct, it can cause incorrect temperature interpretation by the user, resulting in unnecessary disposal of expensive vaccines, believing them to have been stored out of the correct temperature storage range.
These thermometers often have a small plastic coated sensor which is usually just left hanging inside the fridge in the free airspace. As the sensor is small there is no ‘mass’ and therefore changes in air temperatures can fluctuate rapidly. Air has no mass and temperatures can change very quickly. Imagine yourself in a warm room and the front door is opened to the icy cold outside. You can instantly feel on your sensitive skin the change in the air temperature. This soon recovers once the front door is shut again. Just because you felt the icy wind from the outside momentarily does not mean the room interior and furniture temperatures fell to the same level because the room and its contents has a mass. Just as you had to allow your refrigerator to run for a suitable period before it was ‘down to temperature’ when you first switched it on before loading with product, so a corresponding rise in temperature cannot happen instantly because if its mass.
Your vaccine refrigerator is probably fitted with its own integral temperature display which controls the actual internal temperature to the required +2°C to +8°C range by switching the cooling motor on/off as required. The sensing/controlling probe of the temperature probe is placed in the most suitable position to represent an average of the internal temperature which means the top part of the interior could be slightly warmer and the bottom part slightly cooler. That said, each refrigerator manufacturer would have designed it to hold the required +2°C to +8°C in any position throughout the interior and if every point in the interior could be measured simultaneously a slight stratification of temperature measurements might be observed but each point would be somewhere between +2°C and +8°C. This explains why a secondary thermometer display will not match exactly the refrigerator integral display.
Integral Refrigerator Temperator Display
Typical Battery Power Thermometer
Recommendations & Best Practice:
When using a secondary thermometer DO NOT leave the sensor end hanging in free air. This must be placed inside one of the packages so as to sense and monitor the PRODUCT temperature – which is the whole purpose of why we are asked to record such temperatures. Ideally, this probe should be immersed in a small volume of liquid to mimic that of a vaccine.
Although not specifically mentioned in the NHS Green Book, seriously consider the use of a temperature data logging system to accurately record temperature against actual time. Once again, the logger sensor should be placed in amongst the stored vaccine packages to mimic changes in actual product temperatures.
Whilst the guidance on the use secondary thermometers from the NHS Green Book is to ensure temperature monitoring is not interrupted in the event of a power failure, incorrect placing of the secondary thermometer probe can be misinterpreted resulting in the unnecessary disposal of expensive and perfectly safe vaccines. Consideration must be given to the location of the secondary thermometer sensor and should be positioned in such a way to mimic actual product, wherever possible.
Jeff Klückers M.Inst.R. 14 February 2017
For further advice, please contact Shoreline (UK) Ltd, 01903 733 877 or firstname.lastname@example.org
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