The new standard in vaccine storage safety

What is an "slush test" or "slurry test"?

A slush or slurry test is where a thermometer is placed in water that is almost 0°C to test its accuracy.

The theory runs like this:

  • water freezes at 0°C
  • if you have a cup with a mix of ice and water then it must be close to 0°
  • if you place a thermometer in this, it must be close to 0°C, and so the reading shown is the accuracy of the thermometer. It should be less than 0.5°C to be compliant.

How to do an "slush test" or "slurry test"?

There are basically two techniques for making the slush.

Method 1: Partially freezing water

The method described in Strive For Five involves taking a cup of water and placing it in a freezer until a fine layer of ice forms on the top, and partially within the water.

Method 2: Shaving ice

Another common method is to take ice and shave it (ideally) or crush it, and then mix this with a small amount of water.

In both cases, the result is something that is mostly water with a small amount of small ice particles in it.

The user then places the thermometer into this mixture, waits two minutes and then takes a reading.

Problems with a "slush test" or "slurry test"

The biggest problem with a slush test is that the water is not 0°C. If it was, it would be ice. Water is ice at 0° by definition.

To make this worse, pure water is ice at 0° but these tests are not done with pure water, but tap water. That also changes the temperature at which the water freezes.

If after two minutes there is no ice left in the cup then the water is substantially warmer than 0°. The slurry condition needs to exist at the end of the test, not just the start.

The thermometer can’t be touching either the sides of the cup, or pieces of ice. If there is too much ice in the cup then the sensor may register a temperature significantly below 0°. Ice forms at 0°, but could be significantly colder. A typical freezer is -15° to -20°C.

At best, a slush test can indicate that a thermometer is working (yes, it changed to a cold temperature, or no, it didn’t do much at all), but is not a reliable means of determining the accuracy of a temperature sensor.