Why do vaccine temperature loggers disagree with each other?

Why is it possible to have different brands of vaccine temperature loggers sitting next to each other and give very different results? We recently received this email (slightly paraphrased and brand name dropped out):

“Please find attached a plot from an ETemp unit we recently purchased from OnSolution , as well as a plot from one of our BrandX.

You will note that there is a discrepancy between the two units that were in the same fridge at the same time and location

  • BrandX shows fridge temperatures varying between two and eight degrees Celsius.
  • the Etemp shows the temperatures varying between one and 8.4 degrees Celsius

A Max Min monitor also used in the fridge to provide data for daily records was in the same location and more closely aligned to the BrandX – at no stage was a temperature of one degree Celsius recorded.“

Following is slightly modified copy of my response:

Chances are the Thermocron and the other units are all correct. I will explain why.

Before I do, I will just mention that I had the same conversation with the Pharmacy Guild 10 years ago when they converted from using BrandY to Thermocrons. The BrandY loggers they were using were giving a virtually constant temperature and the Thermocrons were showing a larger fluctuation. So fridges that were passing with perfect results were failing with the Thermocron.

I then had the same conversation with fridge manufacturers including Rollex (who now distribute the Thermocrons), and other manufacturers (who all bought Thermocrons to ensure the fridges pass).

So here we go…

“Accuracy” of a logger is an indication of how close to the true temperature the logger will measure provided it is held at a constant temperature. Practically, when a logger is sent to calibration lab (we use ECE Fast) it is held at a constant temperature for long enough for the logger to reach a stable temperature. The difference between the measured temperature and the chamber temperature is then the reported accuracy of the logger.

Thermocrons get there faster. So if there is a temperature change, Thermocrons will respond sooner.

Some loggers are really bad. They are thick plastic with the sensor inside the plastic. As I mentioned, the pharmacy guild were using one that virtually flat lined because it was so unresponsive.

The other brand we sell is the Logtag because it has the sensor on the outside of the plastic case. It has the benefits of the display or indicator, but without burying the sensor.

The outcome of the fridge manufacturers using the Thermocrons directly was a simple change to the settings on the fridge. They had to control the temperature within a tighter spec. Given that these were computer controlled fridges it was not consuming extra power, it was just shortening the time the compressor was on but turning it on more frequently.

What we do know is that a Thermocron will respond a bit faster than the smallest vaccine vial. If the Thermocron says it’s OK, then everything should be OK.

What we also know is that other loggers (not naming brands at this stage) responded slower than vaccine vials. If the logger says it’s OK then there is no guarantee that the vaccine is OK.

Vaccine fridges (I know the LEC range do this, not sure about others) have a “vial size” setting which basically averages out the temperature readings to slow the change down. The larger the “vial size” setting, the slower it will change temperature. This is to try and mimic the temperature of the vaccine. The only issue is that you have to make sure that the fridge is set to the smallest vaccine vial that is stored.

For the Pharmacy Guild, they liked the fact that they had a stronger test that made full use of a vaccine fridge’s potential. The manufacturers initially hated it, but once they made the necessary setting adjustments, all their fridges started to pass again.

One consideration, however, is how the logger is placed. If you place the Thermocron on a surface it will be a very good heat (or cold) conductor and will respond rapidly to the changes. If you leave it dangling in the air you won’t get the same speed of response. For other loggers, putting a piece of plastic on a metal surface partially compensates for its thermal insulation.

Long answer, but hopefully starts to explain why.