1. Field of the Invention
This invention generally relates to data loggers. More specifically, the invention concerns data logging devices which adapt, control, or adjust the recording or logging parameters in response to changes in the data being logged. The invention is applied, in a preferred embodiment, to a multiple channel data logger having extremely high data resolution and accuracy.
2. Description of the Prior Art
A data logger may generally be described as a recording device for creating a time-sequence history of events. Such events might include, as illustrations only, variations in pollutant levels in lake water, sewage treatment plant outfall, or ambient air. Data loggers are used in the field of weather observation to obtain time records of wind speed and direction, relative humidity, barometric pressure, or rainfall, and in the field of geological surveying for detecting earth movement.
Probably the most common use of data loggers is temperature recording, with the aid of thermistors, thermocouples (such as Type J or Type K), or other temperature measuring devices. In temperature recording and the other abovementioned applications, it is usually desirable to obtain a long-term record. The particularly desired information being recorded, however, may occur over only a very short segment of the total long-term recording period. This is generally the case in failure detection applications, where the data values logged are roughly constant over substantially the entire term, until and unless some component or process being monitored fails, at which time a great range of value change or fluctuation may occur.
Furthermore, the data values during these "critical" periods may differ from the normally encountered values by one or more orders of magnitude. Of course, there are applications in which the logged data values are expected to vary over several orders of magnitude and at frequency intervals which may also greatly fluctuate.
These aforementioned substantial problems are handled in only the most basic of manners in the prior art data loggers. The first problem, i.e., the fact that there are scores of different types of sensors which may be used for logging data, each of these probes having a different scale or protocol for its provided sensed values (usually provided as a voltage), has been solved in the past by custom building each data logger to the specification and for the particular sensor or sensors desired by the customer.
The second serious problem lies in the inability of prior art data loggers to provide high degrees of data accuracy and resolution. Here, too, the solution has been to custom design into each separate data logger the particular range over which meaningful data values may be encountered. In this extremely expensive fashion, a user can obtain a device for a desired purpose, and the device may even provide somewhat satisfactory data accuracy and resolution, so long as the encountered data values do not exceed the design envelope.
In another prior art attempt to solve this second particular problem, a metering band having generally satisfactory accuracy and resolution characteristics is established about the first logged data value. The predetermined bandwidth may be five percent of the data value, plus or minus, for the purpose of illustration. So long as the encountered data values do not exceed the limits of the metering band, sufficiently accurate values are recorded. Once a data value traverses either of the band limits, however, the band is shifted in a predetermined fashion, for example, by establishing the limit-crossing value as the new band center.
A third and equally substantial problem briefly noted above is that data loggers are generally used to obtain long-term records, even though the relevant events of interest may occur very rapidly and over only brief intervals during the much longer time record. Prior art data loggers merely provide sufficient data storage media, e.g., continuous strip chart paper, magnetic storage medium, or the like, for data recording at preselected regular intervals throughout the entire period. In the common situation where much of the relevant data is generated in a relatively short period within the long-term logging period, the density of information at that critical time period precludes meaningful data capture, the frequency of data value recording being unvaryingly slow with respect to the frequency of new and meaningful data values.
Still another drawback of known data loggers is the unwieldy format of the time record. In the case where a continuous strip of paper or other "hard copy" medium is used, the entire strip must be visually examined in order to locate the useful or relevant portions. The search is complicated by the fact that a time record strip for a period covering a number of months may actually be hundreds of feet long. Even after the relevant portions of the record are located, little more than visual inspection is possible. Any additional analysis, e.g., statistical, to be performed first requires that the dat values be scanned, digitized, or otherwise input into calculating means. This may be cumbersome in many instances, and may be so onerous as to be impracticable.