Thursday, April 13, 2017

Digital Bar Graph Process Displays Still Have a Place in Your Panel

bargraph digital analog indicators for industrial control
Analog process value indicators are available in a wide
variety of form factors
Courtesy Ametek - Dixson
Analog indicators provide a graphic display of a process value. The value can be a setpoint for a particular operation, or the value returned from a sensor or transmitter. In the current digitally focused environment, we sometimes devalue analog displays. They do, however, have some attributes that set them apart from digital displays. Let's look at bar graph displays.

The ability of an analog display, such as a bar graph, to display useful information depends heavily on its graphical scale. The scale length and resolution should allow significant change in the process value to be displayed in a manner that is readily discernible to an operator. Bar graph displays are composed of illuminated segments, so any significant change in the process value should be sufficient to change the illuminated state of one or more segments.

The scale length and range of the display should extend across the whole of the process, and slightly beyond. An indicating range that far exceeds any possible process value can compromise the display resolution and fail to maximize the use of the instrument. For example, an operation with a maximum process value of 100 should not be paired with an indicating scale that extends to 2500. In this case, the entire range of possible process values indicated will only use four percent of the available indicating scale. A scale range extending to 150 would be more appropriate and deliver better performance.

Analog indicators, especially bar graphs, can provide rapid assessment of the state of a process value. As an illustration, it may not be necessary for an operator to know process temperature with resolution to a tenth of a degree. The key requirement may be to answer the question, "Is it too hot?". Analog displays excel at providing rapid answers to those types of decision-making questions. An onboard digital display of real time process value provides additional information about current process state.

Analog bar graph displays have a proven track record of accuracy and reliability over decades of field use. Modern units include programmable auxiliary functions and take advantage of their microprocessor based design to enable adaptation and setup for almost any application. More information is included in a data sheet below, or your can share your process indication requirements and challenges with instrumentation specialists. The combination of your own process knowledge and experience with their product application expertise will yield an effective solution.