The infamous "ice-slurry test"
The ice-slurry test is an often (ab)used test for thermometers and temperature loggers to see how accurate they are.
The two big benefits of the ice-slurry test are:
- it provides a “reasonably” accurate and predictable temperature of 0°C
- it holds if for an extended period, giving the device time to settle down to a constant reading
Many places will take a NATA certified thermometer and then expect to be able to walk up to any other thermometer or temperature logger, and immediately compare the readings. Another common variation on this problem is when a user looks at a vaccine fridge display and it says one thing while the temperature logger says a different temperature. It is very unusual for the two readings to agree.
The second benefit of the ice-slurry test was holding the temperature constant for an extended period of time. Not having a constant temperature introduces two very significant problems when it comes to measuring the temperature.
Timing / sychronisation
The thermometer is showing you the temperature now, but the logger is showing you the temperature when it last did a sample.
With a 5 minute sample rate, this information could be up to 5 minutes out of date. There is also have the added complexity that the gateway will hold onto the temperature for a short time before passing it onto the gateway.
For normal monitoring this is not a problem, but it means you can not assume that you are looking at the same instant in time when comparing results.
It is fairly typical to see a fridge cycle of about 20 minutes (10 minutes cooling, 10 minutes warming up again). That means that over the space of 5 minutes, the fridge could actually go from it’s highest temperature half way to its coolest.
When we compare the results of a logger to another logger, or to a thermometer, we tend to ignore the specific time of the reading and focus on the high and low limits. That, however, doesn’t help us for this exercise.
Thermal latency variations
The second issue is that different devices respond to changes in temperature differently. A 300mL bottle will heat up or cool down faster than a 2L bottle. The type of material makes a huge difference. There are thermal insulators and thermal conductors.
And temperature loggers and thermometers are no different. Some will respond very quickly to temperature changes while others respond slowly.
We did one experiment where we took 3 different devices and moved them from room temperature into a fridge. After 10 minutes the readings on each were 10°, 8° and 4°. Which one was “correct”? The answer is “all of them” (or “none of them”). If it had been a 2L bottle then it would likely still be above 10°. If it had been an egg than it would probably be closer to 4°.
As a general rule of thumb, thermometers tend to have a faster response than loggers. With loggers, external probes tend to respond faster than internal ones, but are then used for glycol vials which are slower.
Practically, we see this has two impacts when looking at a log:
- The temperature range (difference between the minimum and maximum temperatures) that a fridge will operate in
- The time it takes to measure a step change (e.g. taking the logger from room temperature and putting it in a fridge)
The ice-slurry test removed these impacts by having an infinite amount of time for the devices to stabilise to a fixed temperature.
The goal is to provide other fixed temperatures to test.
A simple solution
The solution is to have something that has a relatively large thermal mass that is then thermally insulated, like a…
Alternatively you can also try something like a larger bottle (750mL or larger) or ice-cream container, and use Styrofoam or material as an insulator.
If you are using a thermos, Styrofoam is a great way of sealing the lid while allowing you to insert the probe and thermometer.
You need to wait for the temperature logger to reach a steady temperature. With Clever Logger that is relatively easy because you can just check log to see when you have a couple of constant readings.
With the thermometer, you also need to wait for it to reach a constant temperature. This is a bit of a nuisance if you have to open the fridge to see what is going on.
What if it is the built-in thermometer?
Forget about it.
You are measuring a different location then the logger, with a temperature that is continually changing.
Instead of focusing on the instantaneous readings agreeing, compare the min/max readings. Reset the fridge min/max and let it run for an hour.
BUT if the fridge goes through a defrost cycle it will stop measuring the temperature while it is happening. The logger will see it, but the fridge will deliberately ignore it. You will need to ignore it in the log when doing the comparison.
So were we, for a long time.