Digital bargraphs measure process and electrical variables such as pressure, flow, temperature, speed, current, and voltage. There are two main types of products: light emitting diode (LED) and liquid crystal display (LCD). LED bargraphs consist of rows of tiny light bulbs that are illuminated by the movement of electrons in a semiconductor material. They present data digitally and use vertical or horizontal segments to indicate the degree of information displayed. Some digital bargraphs position the zero point at the end of the bargraph. Others place the zero point in the middle, displaying negative values on the left and positive values on the right. Most programmable, indoor-rated LED bargraphs use all red LEDs, or a combination of red and green LEDs to create 256 colors. Blue LEDs provide the highest degree of brightness and, when combined with red and green LEDs, can provide up to 1.7 million colors. Standard display colors for LED bargraphs also include amber, aqua, green and orange. High-intensity red LEDs with a common wavelength of 700 nm are also available.
Digital bargraphs differ in terms of inputs and outputs. There are several types of analog inputs. Examples include AC voltage, AC current, DC voltage and DC current. AC power inputs provide an adjustable range of AC current, voltage and/or frequency. Similarly, DC power inputs provide an adjustable range of DC current, voltage, and or/frequency. Other inputs for Digital bargraphs include strain gauges, piezoresistive sensors, thermocouples, resistance temperature detectors (RTDs), and thermistors. Most suppliers specify the number of analog input channels. In terms of outputs, devices with parallel communications and serial communications are commonly available. Relay outputs can be used with both AC and DC loads. Analog outputs are often used in applications that require digitally-controlled output voltages, usually within a range of ± 10 V or less. Digital-to-analog converters (DACs) convert digital codes into voltage levels that are processed or conditioned to produce the required output signal characteristics.
Selecting digital bargraphs requires an analysis of physical characteristics, performance specifications, panel face dimensions, and features. Physical characteristics include number of segments and bar scale length. Typically, suppliers specify the number of segments as the total number of bar graph segments, not the number of segments in the digital display. Similarly, bar scale length refers to length of all of the segments added together. Performance specifications for digital bargraphs include response time and accuracy. Response time is measured in milliseconds (ms). Accuracy is expressed as a percentage. Width, height, and depth are important panel face dimensions. In terms of features, many digital bargraphs provide scanners, set point options, adjustable displays, alarms, on/off controls, and signal conditioning. Intrinsically safe (IS) devices will not produce sparks, electrical energy or other thermal effects that would cause an explosion under normal or fault conditions. Digital bargraphs with totalizers count and record data for a set or indefinite period of time.