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Data logger

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A data logger (sometimes spelt "Datalogger") is an electronic instrument (or specialised computing device in some cases) that records digital, analogue, frequency or smart protocol based 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 voltages and sensor types.

The programmability of data loggers varies widely. Some are completely static with no changing of parameters possible (commonly found in single temperature recorders) to others that are completely programmable with virtual machine, scripted programming languages and configurable parameters.

There is little standardisation amongst data logger manufacturers - however a number of data logging companies are starting to output data in XML format which is a good sign for the industry which has been traditional plagued with the problems of interchangeability of historical or archived data. As to reuse or interchangeability of instrumentation a smart protocol standard called '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. There are a number of datatlogger manufacturer who supply instruments supporting this standard such as Datataker and Campbell Scientific. 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. 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.

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 5 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 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). The new loggers will FTP and email information based upon a regular schedule of based upon alarms.

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

Instrumentation and Sensors

Instrumentation and Sensors for Data Loggers are typically low power, highly efficient designs optimised for long unattended periods of remote data operation. Instrumentation is typically not standard but specific to the Data Logger brand. Most instruments are designed to be voltage or frequency based output devices rather than the more common industrial current loop style. A number of smart instrument protocols exist the most prevalent in the environmental industry is called SDI-12 whilst CANBUS is also becoming more common.

Future Directions

Data Loggers are changing constantly. The original model of a stand alone data logger is changing to one of a device that collects data but also has access to 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

Manufacturers

Large manufacturers of Data Loggers include Campbell Scientific, Datataker , DeltaTRAK, Inc., Dickson, DATAQ Instruments, Grant Instruments, Greenspan, Logic Beach Inc., Scientific Technologies, Inc., Stevens, Unidata [1] and Veriteq Instruments.

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