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Selecting A Recorder ( Printable Version ) | ||||||||||||||||||||||||||||||||||||||||
GETTING STARTED - A CHECKLIST
SIGNAL INPUTS Available input types Typical process recorders accept analog dc voltage inputs, thermocouple, or RTD temperature inputs or dry contact status input. Signal processing Linear scaling (conversion to engineering units) Thermocouple characterization Difference calculation Square root calculation HIGHER-LEVEL FUNCTIONS Intelligence Math functions: +, -, x, ÷, square root, absolute value, logarithm, exponential functions, max, min, time average, group average, summantion, (maxmin), standard deviation, and integration. Programming method Front panel Remote (downloaded) Communications RS-232C: serial point to point, 50 feet cable length maximum at 9600 baud; GPIB (IEEE-488): parallel (20 meter system cable length maximum, 2 meter distance between devices, up to 14 devices per controller); RS-422A/RS-485: Balanced/unbalanced, serial, up to 32 devices per system, cable length can extend to 1.2 km at 9600 baud. HARD COPY AND DISPLAY Recording method Galvanometer movement Servo Stepper-driven Fixed array Writing method Capillary ink Disposable felt-tip ink cartridges Dot printing: ink ribbon cassette or pressure-sensitive paper Thermal-moving head or stationary linear array Rotating ink wheel The most popular methods have become disposable ink cartridges for continuous (drag pen) recording, and multicolor ink ribbon cassette for dotprinting multipoint recording. These writing methods use an economical type of paper which is not sensitive to routine handling and does not require special storage considerations. Chart types For process recorders there are basically two types of charts, Z-fold or roll. Z-fold has become a predominant choice for process applications due to the ease of review of past traces without disrupting active recording. Chart speeds Fixed or programmable
Chart annotation Tag printing Digital printing List printing Alarm printing Prints in engineering units Message printing Scale printing Channel identifier (numeric or alphanumeric) Date and time Chart speed Snapshot digital measured values For continuous writing recorders, the annotation is accomplished by a separate writing pen so that trace information is not lost. For dot-printing recorders, the annotation is done by the dot-printer, with either single-dot or full character printing with each traverse of the printhead, depending on whether the instrument is performing analog trending or log reporting. Chart widths 100 mm 180 mm 250 mm Visual indicators Analog bargraph indication (% of full scale) Analog scale indication (% of full scale) Digital channel number and measured value Alarm status Engineering units Recorder setup Until the advent of the microprocessor, recorders were dedicated to measuring only the type of input signal and only the span specified at the time of order. To change input signal type and/or measuring span, hardware changes were required. Presently, recorders are available in which input signal type, measuring span, tag and unit designators can conveniently be set in by the user. The recorder setup is done by a keypad or, if the instrument has a communication interface, by means of a computer keyboard or downloading of a computer file. Modes Normal: Monitoring at set scan interval and trending at set chart speed, or logging at set intervals Print on alarm: Monitoring at set scan interval but not trending or logging until an alarm condition occurs Change on alarm: Trending or logging at a base chart speed or log internal when no alarm condition exists, automactically switching to alternate chart speed or log interval when an alarm condition exists RECORDER DEFINITIONS
Servo balancing: A means of positioning the pen of a drag pen recorder. Null-balance operation has no current flow at balance, nullifying the effect of lead resistance. Conventional servo balancing recorders use contact mechanisms in the feedback loop and brushes in the servo motor. New technology allows the use of a noncontact pen positioning transducer and a brushless dc servo motor. Scanning recorder: A multi-point recorder that scans all of its inputs to obtain new measured data every set time period (usually 2 to 6 seconds). Printing for all points is often performed during each cycle of the printing mechanism. Multi-color printing: A recorder that records trend traces in more than one color to make traces easier to differentiate. Drag pen recorders use a different color for each pen (usually four pens maximum). Mulit-point recorders typically record in six colors. Linear scaling: Recording of a voltage input in terms of the engineering variable, such as temperature, that the voltage represents. Transformation is Y (variable to be recorded) = mX (slope x input signal) + b (Y intercept). Pen offset compensation: In traditional multiple input drag pen recorders, each pen can travel the full width of the recording chart. In order to do so, the pens must be physically offset from one another. This puts the different pen traces on different time lines of the chart. By placing the measured data of the front-most pen(s) into a buffer and delaying their printing, the traces can be synchronized to the same time line, thereby compensating for their offset. Accuracy: The closeness to the actual signal that the measured value or trend position takes, stated as either a percentage of full scale or percent of reading. Separate accuracy statements are typically provided for measuring and recording functions. Tag ID: A means of designating a trace or digital measured value by an alphanumeric identifier instead of a numeric identifier. Typically available with up to seven characters. Digital printing: Printing of the precise measured numerical values for the various channels, along with their channel identifiers. Digital printing usually occurs in a margin of the chart so as not to interrupt trend recording. Log report: A printout of precise measured numerical values for the various channels, along with their channel identifiers. Typically prints in full character height per print cycle. During trending, prints on demand, resuming trending automatically. When trending is not being used, prints at a preselected time interval. May also include alarm status indication. Courtesy of Johnson Yokogawa Corporation. |
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