Tutorial: Panel mounted instrument displays
If a piece of instrumentation exists in isolation, or if you want a display in the field for local reading, a panel mounted readout can be a handy solution. When you start looking, you may be surprised at the wide variety available and potentially confusing specifications. Here are some questions to ask as you sort through the offerings.
What information is necessary? A readout can have a simple numerical display, or augment that with multiple data lines, bar graphs, alarm indicators, etc. Usually these additional features are helpful, but in some situations they may only confuse things. If you find your operators adding information on pieces of tape, you may want to upgrade the capabilities.
What accuracy is necessary? A display has an accuracy value just like an instrument, and in most cases the two should be similar. Accuracy is normally expressed in the same terms, such ascess variable is going to a DCS or other larger system, the panel display may only be for operator convenience and can be less precise.
How many digits (precision)? Can the reading be displayed with four digits and have sufficient precision? Will it take five? Six? The nature of the process variable will dictate precision.
What are the ambient light conditions? A readout installed where sunlight will shine directly on it should be different than one in perpetual shade or near darkness. LCD’s are good for bright light and can be backlit for dark conditions. LED’s are good where lighting is lower.
How close is the operator? If the operator has to read it at a distance, the digits will need to be large, which will influence how much you can display or require a larger package.
What data format will you be sending? There are readouts built for virtually every kind of data format:
How will it be powered? There are many options for powering displays. Sometimes the display powers the instrument, or vice versa. Some operate on batteries or loop power. The type of display (LCD vs. LED) may restrict your options.
Special approvals? If installed in a specialized environment (hazardous, sanitary, etc.) the display may need appropriate approvals. FM, CSA, UL, ATEX, CE, Mil-Std, nuclear, etc, are all available. Units can be certified for wash-down or sanitary applications in food or pharmaceutical manufacturing.
Other functions? Some displays will store data, trigger alarms, totalize flows, and other ancillary functions. These can be useful for isolated devices.
Most major instrumentation suppliers make panel displays, so that is one place to begin your search. There are also specialty display manufacturers, including:
Precision Digital Corp .
You can also search online at the Control Engineering Supplier Search .
—Peter Welander, process industries editor, PWelander@cfemedia.com ,
Control Engineering Weekly News