What is a data logger, and what is it not?

The thermometer

A thermometer measures and displays. That is enough if someone is standing next to it and looking at it at the moment something goes wrong.

It is also what is built into most refrigerators and freezers. The display shows the unit's own measurement, and it is rarely stored.

The problem is not accuracy. The problem is that there is no history, and nobody is looking at two in the morning.

The data logger

A data logger measures at a fixed interval and stores the readings in its own memory. That means you can see afterwards what the temperature was yesterday, last week or in whatever period someone asks about.

It is a big step up from the thermometer, because the documentation now exists. But the logger still does not tell you while it is happening.

A data logger is therefore the right choice for shipments, where you read it on receipt, and for studies, where you analyse afterwards.

The monitoring system

A monitoring system is data loggers plus software plus alerting. The readings are sent on, collected in one place and continuously compared with the limits you have set.

When a limit is exceeded for longer than the agreed time, the system calls the person on call and keeps going down the on-call rota until someone acknowledges.

And everything is stored in a log that cannot be altered, along with who acknowledged and when.

When is a data logger enough?

For transport, where there is nobody to call along the way anyway. For mapping studies, where the purpose is to analyse afterwards. And for equipment where the consequence of a deviation is low enough that documentation is sufficient.

For everything else, the question is not whether you can document what happened, but whether you can do something about it in time.