Once data has been acquired, there is often a need to store it for current and future reference. Today, alternative methods of data storage embrace both digital computer memory and that old traditional standby-paper. Covering in any detail the scores of instruments on the market today for recording, logging, printing, and storage of data is far beyond the scope of this section. The aim here is to identify and define major categories and offer the prospective user some practical guidelines to a selection best suited for the application.
Broadly speaking, there are two principal areas where recorders or data loggers are used:
Process measurements for such variables as temperature, pressure, flow, pH, humidity; and,
Scientific and engineering applications such as high-speed testing (e.g., stress/strain), statistical analyses, and other laboratory or off-line uses where a graphic or digital record of selected variables is desired.
Instrumentation for each of these fields can have many operating principles and features in common. Each area, however, may place more emphasis on the importance of such factors as cost, method of use (e.g., benchtop vs. panel mounting), portability, degree of accuracy, speed of response, and number of points (channels) to be measured.
For some time now, digital computer systems have had the ability to provide useful trend curves on CRT displays-displays that could be analyzed and printed out. Today, a similar approach permits digital inputs to recorders and data loggers to be stored electronically on-board or sent to a remote PC, distributed control system (DCS) workstation, or full-color printer.
There is now a plethora of data acquisition hardware and software that can work with PCs-making it quite plausible to classify them as recorders or, alternatively, data loggers. These are referred to as PC-based solutions.
There remains a continuing role for conventional standalone recorders that still use paper and pen or printer, but here, too, such products have gained greatly in capabilities through advances in electronics and new technologies such as inkjet and laser printing.
New technology has also made paperless, or videographic, recorders a practical reality, with attendant savings in materials and maintenance. This mushrooming class of instruments mimics conventional trend recorder displays; it also has the capabilities of a data logger and can export data to a PC or DCS workstation, in real-time or periodically for later analysis.
The traditional data logger, whose original basic function was to print out digital values of a measured variable versus time, also has benefited from new technology that has added disk or card memory storage and faster logging capabilities. Some can provide real-time recording via network connection to a host system.
In recent years, conventional analog recorders have had the functions of a data logger added to their capabilities. Called "hybrid recorders," they combine analog trend representations with digital information on the same chart paper.
Scores of specialized software packages are offered not only to implement the PC-based functions mentioned above but also to perform complex mathematical operations or signal conditioning on input and output data.
For high-speed recording, the traditional analog oscilloscope has also been enhanced by a digital storage version (DSO), which finds an important application in test and analysis of today's many electronic products-both consumer and industrial.
The moral of this overview? There are no longer sharp distinctions between many of the products that serve the "front end" of a data acquisition system. In fact, the breakneck evolution of computer and networking technologies will continue to meld alternative offerings for process monitoring, data acquisition, and test and measurement applications.
Definitions & Classifications
The first terminology distinction to draw is that between a recorder and a data logger. Acknowledging that sharp lines no longer exist, consider the fundamental ways in which each type of recording instrument functions to store, analyze, and record data. A traditional recorder accepts an input and compares it to a full-scale value. The recording pen is then moved to a point on the scale that represents a useful value of the input-say, 500°F with a chart range of 0 to 1000°F-at a time that is given by the calibrations on the recorder's moving chart (Figure 7-1).
On the other hand, a data logger accepts an input which is fed into an analog-to-digital converter and stored digitally, or printed out as a series of time-stamped values. Today, many such instruments have some form of on-board intelligence which provides the user with diverse capabilities for signal conditioning as well as data storage and analysis.
As an alternative to a recorder, a data logger can have the ability to accept a greater number of inputs (referred to as channels), with better resolution and accuracy. Only a recorder, however, can provide a truly continuous trend display of the variable's change with time.
The term graphic recorder more aptly describes the majority of recorders used in data acquisition systems. It is defined as an instrument which, in some automatic fashion, draws a graph. Such a graph may relate two or more variables to time, or to each other (the latter, typified by an X-Y recorder). "Graphic" is also used widely today in reference to CRT displays, many of which do not qualify as graphs. Graphic coordinates hearken back to basic geometry, where curves are plotted on a rectangular coordinate matrix. The strip-chart recorder, so common in a great variety of forms today, had its origin in this basic concept (Figure 7-1). Note that the vertical coordinate (time) is the X axis of a graph and the horizontal coordinate (measured variable) is the Y axis. The heavy vertical line in the center of the chart is the curve plotted by the recorder (often called the chart record or trace).
Recorders can have adjustable chart speeds-fixed, once configured-to suit the application. Thus, at 4 (o'clock) in Figure 7-1, an operator can see that the temperature was 625°F. He or she can also note that the temperature has remained rather steady. The trend is often as important, or more important, than the precise value. Thus, this type of instrument is often called a trend recorder.
With an X-Y recorder, the time coordinate X is replaced by some other variable, such as stress, plotted versus strain. Other versions of this type may have two different variables, Y and Y' plotted vs. X or have time (t) also as one of the variables.
Circular-chart coordinates (Figure 7-2) were developed early in recorder history to utilize the convenience of a spring-wound clock and synchronous motor movements. Depending on application needs, the circular chart speed can be one rotation in 24 hours, one rotation in a week, or another arbitrary time interval. Note the advantage of a circular chart in giving at a glance a complete history of one or several variables over a period of time-albeit not with a high-accuracy readings.
Chart sizes are referred to by total diameter, such as 10 inch or 12 inch, although full-scale pen movement is often much less. Because many plant operators are used to reading circular chart recorders, suppliers still find a strong market for them.
From a commercial cataloging standpoint, graphic recorders are generally classified by their common usages or physical appearance, such as strip chart, circular chart, or special purpose (X-Y, event, or oscillographic). A further broad distinction is whether the instrument is flatbed for benchtop mounting or vertical, for panel or rack mounting.
Data loggers, too, can be classified as benchtop units; others are described as panel-mount printers and still others are "vertical" benchtop units. Smaller, handheld units also are available.
As with any method of classification, such names leave gray areas. For example, some flatbed types utilize strip charts and some are vertical. Many data loggers are bench-top, but some are vertical and panel-mounted. And videographic, or paperless recorders can provide screen displays that look like a strip chart or a circular chart.