Chart Recorder

Typical Uses of Chart Recorders

Chart recorders are a familiar sight in manufacturing plants, where they track such variables as temperature, pressure, flow, pH, and humidity. Laboratories, meanwhile, use them to monitor scientific and engineering data generated in testing, diagnostics, statistical analysis, and other work requiring a graphic record.

When Should I Use a Chart Recorder?

When the process to be tracked involves limited variables that do not require a PC-based interface, paper and pen can still be the way to go. The advantage of a circular chart is its intuitiveness: at a glance, it gives a complete history of each selected variable over a specified period—a truly continuous display of a trend’s change with time.

Strip Chart Recorders
Strip chart recorders consist of a roll or strip of paper that is passed linearly beneath one or more pens. As the signal changes the pens deflect producing the resultant chart. Strip chart recorders are well suited for recording of continuous processes. Sections of the paper can be torn off and archived for future reference. Strip chart recorders are commonly used in laboratory as well as process measurement applications.

Circular Chart Recorders
A circular chart recorder records data in a circular format. The paper is spun beneath one or more pens. The pens are deflected in proportion to the varying signal resulting in a circular chart. Circular chart recorders are ideal for batch processes where a set process time is known. The charts are normally designed to rotate in standard time periods, such as 1 hour, 24 hours, 7 days, etc., although many recorders are flexible enough to accommodate non-standard time periods.

XY Recorders
Y recorders accept two inputs and create a chart or graph of one input versus the other. They are commonly used to determine the relationship between the two inputs. For example in a chemical process, an XY recorder might be used to monitor the effect temperature has on the pressure of the process.

Hybrid Recorders
A hybrid recorder can function as a recorder or data logger. Like a standard recorder the hybrid recorder can generate a chart of the inputs. However, it can also produce a digital stamp of the data similar to a data logger. They are commonly available in multichannel designs although one print head normally handles all channels. This makes the hybrid recorder a cost effective solution for multichannel systems although the response time is not as fast as recorders which have a unique pen for each channel.

Paperless Recorders
Paperless recorders are one of the latest types of recorders to emerge on the market. Paperless recorders display the chart on the recorders' graphic display rather than print the chart on paper. The data can normally be recorded in internal memory or to a memory card for later transfer to a computer. The major benefit of paperless recorders is conservation of paper and easy transfer to a computer.

How to Select a Chart Recorder?

Many applications are suitable and require the installation of a chart recorders. Before choosing one, make sure you consider the following questions:

• How many inputs need to be recorded?
• What types of inputs need to be recorded?
• Do different input types need to be recorded in the same unit?
• What type of recording is required?
• Multiplex scanning (what minimum scan cycle is required?)
• Is a communciation interface required?
• Is recorder to be bench style or panel mounting?
• What type of instrument power is available?
• Is log-type recording desirable instead of/in addition to trend recording?
• Is color differentiation available for trend lines?
• Is message printing required?
• Is the recorder to perform alarm functions?
• How many setpoints per channel?
• What types of alarms: threshold, rate delta?
• Are physical relay contacts available for external alarm output?

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.

Recorder Definitions

Hybrid Recorder: A recorder that combines analog trend representation and digital measured value printing on the same chart paper, without disruption of trend printing.

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.

Can I send my 4-20 mA control output to a chart recorder to monitor a process input?

No. A control output is designed to control a valve or some equivalent control device. If you need to send an analog signal to a recording device, then choose a controller that has a “retransmission or recorder output” option.