A data logger (sometimes spelt "Datalogger") is an electronic instrument that records digital or analogue measurements over time. Some data loggers are small, battery-powered devices, equipped with a microprocessor, data storage and even a sensor. Others are general purpose devices that are designed to connect to a wide range of voltges and sensor types.

The terms data logging and data acquisition are often used interchangeably. However, in a historical context they are quite different. A data logger is a data acquisition system, but a data acquisition system is not necessarily a data logger.

• Data loggers typically have slower sample rates. A maximum sample rate of 5 Hz may be considered to be very fast for a data logger, yet very slow for a typical data acquisition system.

• Data loggers imply stand-alone operation, while the typical data acquisition system must remain tethered to a computer to acquire data. This stand-alone aspect of data loggers implies on-board memory that is used to store acquired data. Sometimes this memory is very large to accommodate many days, or even months of unattended recording. This memory may be battery backed Static_Random_Access_Memory, flash_memory or EEPROM. Older data loggers used physical forms of memory such as paper tape or paper disks - these were usually known as "chart_recorders".

• Given the extended recording times of data loggers, they typically feature a time- and date-stamping mechanism to ensure that each recorded data value is associated with a date and time of acquisition. As such, data loggers typically employ built-in real time clocks.

• The unattended nature of data logger applications implies the need to operate from a DC power source, such as a battery. Solar power may be used to supplement these power sources.

• Sometime dataloggers are connected to communication devices such as modems, cellular phones or satellite terminals to enable the remote collection of recorded data or alarming of certain parameters (such as an abnormal water level).

• Applications of data logging include;

- Unattended weather station recording (such was wind speed / direction, temperature, relative humidity, solar radiation).

- Unattended hyrographic recording (such as water level, water depth,water flow, water pH, water conductivity).

- Unattended Soil Moisture level recording.

- Unattended Gas pressure recording.

- Road traffic counting.

• Significant manufacturers of Data Loggers include Campbell Scientific, Stevens and Unidata.