Chart Recorders
Chart recorders and strip charts are data acquisition tools used to generate a plot, graph or other visualization of data versus time. Common applications include temperature and humidity measurements, chromatography, spectroscopy graphs, and plots of the output of a wide range of other lab and test equipment.
Chart recorders typically output data in one of four common formats. These include circular x-y plotter, strip chart, or paperless, video graphing. Circular charts rotate uniformly while variable is plotted radially. Advantages include relatively compact size and simplicity of operation. Revolution time may span from minutes to days. In an X-Y plotter two selectable variables are plotted against each other in rectilinear coordinates. The paper source is often a standard size sheet, but can also be a roll or drum. Plotted variables may include mathematical functions of data input. Strip charts plot variables with regard to time, as the paper strip advances uniformly through the plotter. Strip frequently comes off a roll for space conservation. In a paperless or video graphing system the plot is displayed on video screen; scaling and mathematical functions possible. Chart recorders and strip charts can produce multiple plots simultaneously.
Important data acquisition specifications for strip charts and chart recorders are differential analog input channels, digital I/O channels, sampling frequency, resolution, and accuracy. Differential channels use the difference between two signals as an input; common mode is filtered out. In some systems, differential inputs are combinations of two single-ended inputs; in this case, twice the number of differential channels would be available as single-ended inputs. Digital or discrete channels are used for low-level on-off signals used in applications such as communication, user interface, or control. The sampling frequency is the frequency of analog signal sampling and conversion to a digital value. Resolution refers to the degree of fineness of the digital word representing the analog value. A ten-bit number contains 210, or 1024, increments. A 0-10V signal could therefore be resolved into approximately 0.01V increments. A 12-bit representation would be in 212 (4096) increments, or divisions of 0.0024V for the same signal. Each additional bit doubles the resolution, and one bit is required for the polarity (sign) of a number. Accuracy depends on the signal conditioning linearity, hysteresis, temperature considerations, etc. It is represented here as percent full scale of measurement range. Some chart recorders and strip charts come equipped with recorder or datalogger, which has instrument functionality with specific capability for data storage. May be for general purpose or application-specific data acquisition.
Common signal inputs for strip charts and chart recorders are DC voltage, DC current, AC voltage, AC current, frequency and charge. Typical sensor inputs for chart recorders and strip charts are accelerometers, thermocouples, thermistors, RTDs, strain gauges or bridges, and LVDTs and RVDTs. Specialized inputs may include encoders, counters or tachometers, timers or clocks, and relays or switches.
Features common to chart recorders and strip charts include transducers and excitation supplies, auxiliary outputs, user interfaces, communications with computers, application software and memory and storage functions.