How to Select Chart Recorders  Chart Recorder Selection Guide Chart Recorders Selection Guide

Circular Chart Recorder | Strip Chart Recorder | Paperless Chart Recorder

Image Credit: Newport Electronics, Inc. | Linseis Inc. | Brainchild Electronic Co., Ltd.

 

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 Format 

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.

 Chart Recorders Selection Guide

Image Credit: CAMERON Valves & Measurement

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.

 Chart Recorders Selection Guide

Image Credit: Cooper Instruments & Systems

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.

 Chart Recorder Selection Guide

Image Credit: Linseis Inc.

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.

 Chart Recorders Selection Guide

Image Credit: Yokogawa

 

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.  For a listing of suppliers who offer consumables for paper-based chart recorders please visit GlobalSpec's product area, Chart Recording Paper and Consumables.

 

 

Pros 

Cons

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

 

Monitoring

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.

 

Chart Recorders Selection Guide

Monitoring with a virtual chart recorder. Image Credit: Newport Electronics, Inc.

 

 

Input Signals

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.

o  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. 

o  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.

Chart Recorders Selection Guide

Image Credit: Being Ministry

·     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.

o  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.

o  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.

o  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.

 Chart Recorders Selection Guide

Image Credit: SHOWA MEASURING INSTRUMENTS CO.,LTD

 

Sensor Inputs 

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

 

Excitation 

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:

 

   Chart Recorders Selection Guide

Image Credit: Maxim

 

 

 

Resources:

 

Dickson - Chart Recorder Vs. Data Logger

 

Engineers Edge - About Chart Recorders

 

OMEGA Engineering, Inc. - Introduction to Chart Recorders

 

Veriteq - Switching From Chart Recorders to Data Loggers

  


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