A chart recorder is an electronic instrument that keeps track of various measurements required in industrial and laboratory environments. Broadly speaking, it is used to record:
- process measurements for such variables as temperature, pressure, flow, pH and humidity
Strip chart recorders consist of a roll or strip of paper that passes linearly beneath one or more pens. As the signal changes, each pen's deflection records the process being measured in the form of a chart. Well suited to recording of continuous processes, strip chart recorders are commonly used in both laboratory and process-measurement applications. For future reference, sections of the paper can be torn off and archived.
A circular chart recorder tracks data on a paper disc rotated beneath one or more pens, which, as in a strip chart recorder, deflect with fluctuating electrical signals. The difference is that the resulting chart is circular rather than linear. Circular chart recorders are ideal for batch processes that operate within a known time frame. They can be configured so that each rotation of the chart covers a standard time period -- one hour, 24 hours, seven days, etc. Some recorders will also accommodate non-standard periods. The advantage of a circular chart is that, at a glance, you get a complete history of one or several variables over the specified period.
For many users, the tried-and-true output of a circular chart recorder is so easy to handle, read and interpret that they wouldn't dream of switching to paperless technology. When the processes to be tracked involve limited variables that do not require a PC-based interface, paper and pen can still be the way to go. A data logger may accept a greater number of inputs, but only a circular recorder provides a truly continuous and highly intuitive trend display of a variable's change with time.
With advanced electronics such as microprocessors, improved ergonomics and new user-interface features, the latest models are anything but low tech. For example, some portable circular chart recorders can be configured to operate on a one-, seven- or 32-day chart. Some units offer dual-thermocouple input, dual-process input, and temperature and relative humidity models. The dual-thermocouple input model could use a Type J, K or T thermocouple input that can measure and record the temperature of virtually any process. By monitoring temperature over time, the user can determine whether process improvements are indicated. For example, the charted graphs could provide insight into the correlation between temperature and, say, the wear and tear of an engine.
A dual-process input model records any standard process voltage/current signal -- 0/5 Vdc, 4/20 mA, 0/10 Vdc, etc. -- providing a representative measure of flow, pressure, AC voltage/current and other processes. One application for this model would be to monitor the AC current of a machine vs. the temperature buildup in a component of the same machine. The charted graphs would show how the two parameters correlate, perhaps indicating that the component's temperature rise will lead to premature failure. Another application would be to monitor a machine's AC current and AC voltage over a selected period. The resulting graphs would let the user calculate total power consumed vs. time.
SIDEBARMonitoring air quality in a factory, laboratory, hospital, office, museum or other environment is a typical application of a temperature and relative humidity model of a circular chart recorder.
The charted data can indicate whether air quality needs improvement. A unit with a dual, backlit display would show temperature and humidity in real time, and the same data could be stored in the recorder's non-volatile memory. In some models, the stored data can be downloaded to a PC through an RS-232 serial port. Depending on the recorder model, outside processes could be controlled with two built-in relays and two voltage alarm outputs.
“These are just a few applications,” said Shahin Baghai, manager of product development at Omega Engineering in Stamford, Conn. “There are probably hundreds of ways in which circular chart recorders can help users who need to monitor and record two processes over time. The charted graphs give a tremendous amount of information on each process and how the two correlate, often providing insight into how improvements can be made.”