Part of the discussion at the IFSQN web site recently was on measuring the temperature of frozen items that may have been compromised.
Following is my reply:
I will run through some assumptions and my reasoning on this issue. I sell temperature loggers so most of my “research” is listening to others.
You are talking about a frozen item which is thawing. This means that the inside will be colder than the outside. So the external temperature is more critical than the internal.
Especially on larger items, the risk of part of the item perishing due to partial thawing is highest on the outside.
In the case of a broken freezer, walking in with a thermometer will just confirm the equipment is dead. Measuring the outside will just confirm what you already feel – it’s getting warmer. Measuring the inside will confirm that it used to be colder.
But cooling or freezing an item is the exact opposite. The outside will drop in temperature before the middle. So the need to measure the internal temperature is critical but usually neglected.
If your concern is about the item’s temperature but the equipment has been repaired and you are investigating hours or days later, then any type of thermometer won’t help you. It’s the realms of a temperautre logger. Most people will just have a single logger for the freezer which will give a reasonable indicator of the chance of damage.
But I know some people who will place the loggers inside their product. In some case this means piercing or cutting the product (which can make that item unusable) but the compromise of between packed items (as mentioned by FSB) is totally legitimate.
Practically, I would recommend a logger with a probe (and most of the manufacturers offer one) but if you want an incredibly small logger that fits anywhere then I think the Thermocron logger is currently the smallest on the market (about the size of a coin and looks like a coin-cell battery).
So to me it’s obvious. You are talking about a process of thawing and potentially refreezing which means a one-off reading isn’t of great value. It’s the job best done by a logger.