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Introduction
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 beprogrammable.
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.
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.
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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.
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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".
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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.
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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.
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