A data-logging system consists of four main parts:

1. sensors
2. interface
3. computer
4. software

Data-logging systems are used in all situations, such as: taking scientific measurements in space; monitoring life-support systems in a hospital; and measuring the mechanical systems in cars and other vehicles.

The sensors are the pieces of apparatus that are actually in contact with the thing being measured. For example, if you want to measure the heat energy contained within a cup of tea you would use a temperature sensor - which could be placed in the cup of tea.

There are many types of sensor - which are used to measure many different things. Some common sensors can measure: temperature, pressure, light level, radiation level, sound, movement, pulse-rate, voltage (potential difference), and current.

Usually, you use a different, specialised sensor to measure a different aspect of the environment.

The sensors measure different aspects of the environment and they, usually, produce an analogue, electrical signal. Most computers can't handle analogue electrical signals directly so the signal needs to be converted into a form that the computer 'understands'.

The interface takes in the output from the sensors and turns it into a digital signal. In other words, it takes the electrical signal from the sensor and turns it into numbers that the computer can then use.

The software is the part of the system that converts the raw data produced by the sensors and interface into information that can be used by a human. Often this information is in the form of a graph which you may see develop as the sensors continue to measure the environment.

Some software will let you control other devices depending on the data recorded by the sensors and interface, for example, an alarm could be switched on when the temperature of bath water reached a certain level.