Data logger

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A data logger (or datalogger) is an electronic instrument that records data over time or in relation to location. Increasingly, but not necessarily, they are based on a digital processor (or computer). They may be small, battery powered and portable and vary between general purpose types for a range of measurement applications to very specific devices for measuring in one environment only.

It is common for general purpose types to be programmable.

Standardisation of protocols and data formats is growing in the industry and XML is increasingly being adopted for data exchange. The development of the Semantic Web is likely to accelerate this trend. A smart protocol, SDI-12, exists that allows some instrumentation to be connected to a variety of data loggers. The use of this standard has not gained much acceptance outside the environmental industry. SDI-12 also supports multi drop instruments. Some datalogging companies are also now supporting the MODBUS standard, this has been used traditionally in the industrial control area there are many industrial instruments which support this communication standard. Some data loggers utilize a flexible scripting environment to adapt themselves to various non-standard protocols.

Another multi drop protocol which is now stating to become more widely used is based upon CANBUS (ISO 11898) this bus system was originally developed by Robert Bosch for the automotive industry. This protocol is ideally suited to higher speed logging, the data is divided into small individually addressed 64 bit packets of information with a very strict priority. This standard from the automotive/machine area is now seeping into more traditional data logging areas, a number of newer players and some of the more traditional players have loggers supporting sensors with this communications bus.

[edit] Data logging versus data acquisition

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 1 Hz may be considered to be very fast for a data logger, yet very slow for a typical data acquisition system.

• Data loggers are implicitly stand-alone devices, while 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. Earlier data loggers used magnetic tape, punched paper tape, or directly viewable records such as "strip 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 whose published drift can be an important consideration when choosing between data loggers.

• Data loggers range from simple single-channel input to complex multi-channel instruments. Typically, the simpler the device the less programming flexibility. Some more sophisticated instruments allow for cross-channel computations and alarms based on predetermined conditions. The newest of data loggers can serve web pages, allowing numerous people to monitor a system remotely.

• The unattended and remote nature of many data logger applications implies the need in some applications to operate from a DC power source, such as a battery. Solar power may be used to supplement these power sources. These constraints have generally led the data logger industry to ensure that the devices they market are extremely power efficient relative to computers. In many cases they are required to operate in harsh environmental conditions where computers will not function reliably.

• This unattended nature also dictates that data loggers must be extremely reliable. Since they may operate for long periods nonstop with little or no human supervision, and may be installed in harsh or remote locations, it is imperative that so long as they have power, they will not fail to log data for any reason. Manufacturers go to great length to ensure that the devices can be depended on in these applications. As such dataloggers are almost completely immune to the problems that might affect a general-purpose computer in the same application, such as program crashes and the instability of some operating systems.

[edit] Applications

Applications of data logging include:

• Unattended weather station recording (such was wind speed / direction, temperature, relative humidity, solar radiation).
• Unattended hydrographic 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.
• Process monitoring for maintenance and troubleshooting applications.
• Wildlife research.
• Tank level monitoring.
• Environmental monitoring.
• Vehicle Testing
• Monitoring of relay status in railway signalling.
• For science education enabling 'measurement', 'scientific investigation' and an appreciation of 'change'

[edit] Future Directions

Data Loggers are changing more rapidly now than ever before. The original model of a stand alone data logger is changing to one of a device that collects data but also has access to wireless communications for alarming of events and automatic reporting of data. Dataloggers are beginning to serve web pages for current readings, email their alarms and FTP their daily results into databases or direct to the users.