Data recording's greatest hits.
If you go just by what you see in the high-technology press, you'll get the idea that researchers are hot to save data to disk and think of strip-chart recorders as outdated instruments. In reality, the heavy press that paperless recording systems receive reflects their novelty and rapid development, rather than their popularity or utility.
That's not to say that paperless recording systems aren't extremely useful and worth the technological development they get or the ink being spilled in their honor. They, in many respects, are truly the "wave of the future." It's just that chart recorders are in no hurry to go away, either. In fact, it's quite possible that they may never go away. The reason is that there are many situations where an immediate paper record is exactly what you want and there is no reason to set up, integrate, and program a data acquisition system just to get it.
In the final analysis, there are many ways to capture and permanently record your data. Each of them has advantages and disadvantages. What is important is to find the method whose advantages help you the most in your application and whose disadvantages cause you the least trouble.
Data recording options fall into five broad categories: chart recorders, XY recorders, data loggers, tape recorders, and data acquisition systems. The most obvious difference between them is the nature of the primary permanent record they lay down.
Strip chart recorders scroll a single, long sheet of paper past a pen (or other marking device, such as a thermal-printing array) continuously at a fixed speed. The roll is long enough so that at typical chart speeds you can have a continuous record covering a time span up to several days long.
Often, however, much of that data is inconsequential. In a typical chart-recorder application, the chart streams by while nothing goes on. The most valuable thing about that long, boring record is that it provides absolute proof that nothing of interest happened. You roll it up, slap a date sticker on the side, box it up, and shove it into a storage bin in case some day, somewhere along the line, someone says, "Are you sure nothing happened?" Then you can trot out to the warehouse, dig out the roll and say, "Yep, absolutely nothing happened."
This is opposed to the times when something does happen. When you know that something will happen, but can't predict exactly when is the time to use a chart recorder.
Seismology is a classic chart-recorder [TABULAR DATA OMITTED] application. In certain geographic locations, you can be pretty sure that earth tremors will happen, but you have little exact knowledge of when they will happen. When they do occur, you want to collect data without delay because, by the time it's happened, it's too late to turn on your system. You have to take data continuously - forever.
When the Big One finally hits, you want to be able to go back and look at the data after the fact. You want to compare recordings from different locations made independently at the same time as well. The fact that a seismometer in Hollywood bounced up and down, while one in Long Beach went side-to-side is important, as is the fact that one in Peoria jiggled just a little.
Another important chart-recorder application occurs when short-term trends are extremely significant and when you have to make real-time decisions based on those trends.
Suppose you are performing an experiment that includes watching for a liquid-to-solid phase transition during a cooling process. Furthermore, you want to perform an anneal at a specific fraction of the phase-transition temperature.
The simplest way to do this is to run a strip-chart recording of the temperature vs. time during the cool down phase. The phase transition will appear as a sudden plateauing of the temperature while the melt solidifies, followed by resumption of the downward trend at a different cooling rate. A few taps of a calculator keypad will tell you exactly what to use for an anneal temperature.
To set this experiment up using a chart recorder is a simple matter. All the information appears instantly on the chart as the experiment unfolds, and there is no other way to get the same information as easily or simply.
XY recorders also provide squiggly lines on paper. But, they typically show the evolution of one variable with respect to another, rather than to time. Instead of moving the paper steadily under the pen, they move the pen over the paper in a pattern where vertical motion represents the change in one variable, and horizontal motion represents the change in another.
Most XY recorders provide a timebase for those situations when you need one, but that is not what they are best suited for. Time really runs continuously, like a paper chart. The variables in a good XY-recorder application, on the other hand, vary over a limited range, just as a single sheet of paper provides a limited "writing" space.
Both chart and XY recorders work best for relatively slowly varying signals. Chart speeds and pen accelerations are limited by mechanical movements. They cannot trace signals that vary on time scales more rapid than their mechanical systems can follow. Dealing with signals whose bandwidths are greater than a few hertz forces you to abandon paper as the primary permanent record.
Paperless electronic recordings cover most of the useful frequency spectrum. And, data acquisition systems, specifically, cover the most important range of all - from a few hertz to a few hundred kilohertz.
So, one advantage of paperless recording systems is speed. Many experimental processes occur faster than any paper recording system could possibly follow. That is why, for example, high-energy physics laboratories always push the state-of-the-data acquisition art. Any increase in data acquisition speed means a better understanding of the time evolution of their experiments.
The second advantage of electronic recording systems is the amenability of the data to analysis. Analyzing data is what computers do best, and there is no better way to pull useful information out of raw data than through computer analyses. It follows, therefore, that the best record to analyze is a digital one.
Data acquisition systems, however, are more complex and, usually, take more effort to set up than stand-alone recorders. At a minimum, a data acquisition system consists of a set of sensors, a transmission line to carry signals from those sensors to a central location for processing, a means to convert the analog signals to digital signals and feed them to a computer as a data stream, a computer to receive the data stream, and, finally, a drive that lays down the permanent record. You need all of these elements whether your experiment includes one or 500 data channels.
Clearly, for simple experiments with only one or two data channels, the system overhead of setting up a data acquisition system can be a burden. For highly complex situations, however, a data acquisition system can be easier to organize and connect.
The other main disadvantage of paperless recording is one that most researchers don't think about: electronic records are not as permanent as paper records. The problem is not physical deterioration of the media, but obsolescence of the recording format.
We can still read Galileo's notebooks after 400 years. Data recorded on the floppy disks popular 15 years ago are unreadable now because disk drives that read them have disappeared. A lot of data has already been lost because it is simply uneconomical to salvage it by copying to modern formats.
Data loss through format obsolescence is not an intrinsic property of electronic records. But researchers need to consider how long their data needs to be preserved and think about how to make it happen.
Of course, there are various forms of electronic records. Since many records have archival applications, short-term media such as battery-backed semiconductor memories are disregarded in this discussion, as are conventional CD-ROMs, which are more of a publishing medium.
That leaves magnetic media as the current paperless recording technology of choice. Depending on the required data-set size and the transcription speed needed, one can use either magnetic disks or tape.
Most researchers use magnetic disks in the form of "floppy" disks. Floppies are particularly useful because they are so universal. They are inexpensive, readily available and make data sharing easy.
The high-speed and high-capacity end of the electronic recording range is the province of video tape data recorders. Video tapes can be used to record analog signals up to 24 MHz. At lower bandwidths, they can be used to record over 100 channels simultaneously or to record 30 hr of data.
These recorders can use videotape cassettes, but their resemblance to your home VCR ends there. They are full-function, stand-alone measurement systems that accept direct sensor inputs and even display readings on LCD front panels.
Extremely slow signals also benefit from paperless recording, this time in the form of data loggers. Data loggers were originally digital voltmeters that automatically printed readings every second or so on adding-machine tape. They now more often record their readings electronically.
A good data logger application will have low bandwidth - on the order of fractions of a hertz. Fast transients may be present, but they will not be the most important feature. There may be few data channels, but data loggers exist that can simultaneously multiplex sixteen channels. Finally, data loggers can collect and record data unattended for very long periods of time.
While paper-tape output is still available, most data loggers upload their data to a host computer. Typically, they collect data in internal memory or on a floppy disk, then send that data to the host.
Combining technologies can often combine advantages while canceling disadvantages. Data loggers that lay down electronic records on floppy disks are one example. Another is chart recorders that link to computers. Like most benchtop instruments, chart and XY recorders often provide parallel or serial ports, or GPIB ports through which they can upload data to a computer in real time.
Thus, for many applications, hybrid recorders that lay down both paper and paperless records, may just be the real wave of the future.
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|Title Annotation:||selecting data recording systems|
|Publication:||R & D|
|Date:||Aug 1, 1997|
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