Chart Recorders Information
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 charts, x-y plotter charts, strip chart, and paperless or video graphing.
Circular charts rotate uniformly while variables are plotted radially. Advantages include relatively compact size and simplicity of operation. Revolution time may span from minutes to days.
|X-Y plotter charts graph data points over rectilinear coordinates using two selectable variables. The paper source is often a standard size sheet, but may be a roll or drum. Plotted variables may include mathematical functions of data input.||
|Strip charts plot variables with regard to time. A paper strip advances uniformly through the plotter producing a record of measured parameters. The strip frequently comes off a roll to conserve space.||
|Paperless or video graphing recorders display and record data in a digital format. They may be offered with several display options or may be configured and programmed to display several types of charts. Scaling and mathematical functions are possible.||
Traditionally, chart recorders generated a hard copy. The function of the device is to generate a graph, plot, chart, or series of data points versus time for record keeping and analysis. In some applications a hard copy output is still desired. However, paperless or video graphing recorders and data loggers have taken a competitive advantage, as they do not require ink, pens, paper stock, or other consumables. To learn more about consumables for paper-based chart recorders please visit Engineering360's, Chart Recording Paper and Consumables specification guide.
|Paper Based Chart Recorders||
Produces a Physical Hardcopy; Real-time Output; Standardized Output
Consumable Needed; Costly to Maintain and Operate; Lack of Flexibility; Space Requirements
|Paperless or Video Graphing Recorders||
Customizable Output; Programming Options, Graphing Options; No Consumables Needed
Larger Up-front Cost
Chart recorders monitor processes and systems by recording data points over a timed interval. They are customized for a particular application and may require specific modules in order to accommodate the type or number of inputs used. They may also be equipped with on-board memory, removable storage, or may interface with a computer used to process, store, and visualize the data. Some chart recorders are equipped with a switched output used to trigger an alarm or activate a switch when a pre-set condition is met or exceeded.
Monitoring with a virtual chart recorder. Image Credit: Newport Electronics, Inc.
Chart recorders may receive a digital signal, analog signal or sensor input.
Analog signals are a wave signal which means that the value changes steadily over time and can have any value in a range. Products with analog inputs can have multiple channels. Channels are either single-end or differential.
Single-end inputs have only one low wire shared by all inputs. For example a board could have 2 single-end inputs; there will be two input lines and one ground line. Single-end inputs are less expensive and allow for twice the number of inputs in the same size wiring connector since they only require one analog input and one ground input which is shared by all the inputs. They save space and are easier to install. When single-ended outputs are available, suppliers often specify the maximum number of analog channel outputs as twice the number of differential outputs.
Differential channels have two inputs. The voltage is the signal processed between the two inputs. The board will have one signal and one ground pin for each input to allow for measurement voltage difference between two signals tied to the same ground. Differential channels provide excellent common-mode noise rejection. This type of input should be used when EMI, RFI or noise is present.
Digital signals do not have 'in between' values. They are an on or off signal producing a square wave. Digital signals break down the information into binary code, which is a series of 1s and 0s. The data receiver reassembles the code into useful information. Digital signal allows users to send more information in a smaller space.
Digital vs. analog signal shapes. Image credit: Wikimedia Commons
Accuracy is defined as the difference (error) between the true value and the indication expressed as percent of the span. Accuracy, which is represented as a percentage of a full-scale measurement range, depends on signal conditioning linearity, hysteresis, and temperature considerations. It includes the combined effects of method, observer, apparatus and environment.
Static accuracy is the combined effects of linearity, hysteresis, and repeatability. It is expressed as +/- percentage of full scale output. The static error band is a good measure of the accuracy that can be expected at constant temperature.
Linearity is the deviation of a calibration curve from a specified straight line. One way to measure linearity is to use the least squares method, which gives a best fit straight line. The best straight line (BSL) is a line between two parallel lines that enclose all output vs. pressure values on the calibration curve.
Repeatability is the ability of a transducer to reproduce output readings when the same pressure is applied to the transducer repeatedly, under the same conditions and in the same direction.
Hysteresis is the maximum difference in output at any pressure within the specified range, when the value is first approached with increasing and then with decreasing pressure. Temperatures hysteresis is the sensor's ability to give the same output at a given temperature before and after a temperature cycle.
Hysteresis. Image credit: Showa Measuring Instruments
Sensor inputs can be accelerometer, thermocouple, thermistor, RTD, strain gauge or bridge, and LVDT or RVDT. Specialized inputs include encoder, counter or tachometer, timer or clock, relay or switch, and others.
Sensors can be classified as either active or passive devices. Passive devices like thermocouples can generate a signal without a power supply. Active sensors need a power supply in order to control the flow of electrons and make a measurement. In some cases active sensors are powered by the signal conditioner. The output from the signal conditioner that powers the device is referred to as an excitation source. Signal excitation can either be a voltage or current output. The following schematic showcases how an excitation voltage powers a Wheatstone bridge:
Excitation voltage powering a Wheatstone bridge. Image Credit: Maxim
Newport Electronics, Inc. | Linseis Inc. | Brainchild Electronic Co., Ltd. | CAMERON Valves & Measurement | Cooper Instruments & Systems | Yokogawa | SHOWA MEASURING INSTRUMENTS CO.,LTD. | Wikimedia Commons