Digital panel meters measure and display all types of processes and electrical variables, such as pressure, flow, temperature, speed, current and voltage in an alphanumeric or numeric format, often as a bar graph. Many of these displays have totalizing, recording, conditioning, and other functionalities.
Display types for digital panel meters can be either alphanumeric or numeric only. Alphanumeric displays can display both numbers and letters. They are either a sixteen-segment display or a 5x7 dot matrix display. Numeric displays can display numbers only. They are either a seven-segment display or a 4x7 dot matrix display. When specifying a digital panel meter it is important to specify the number of digits displayed. Common character sizes for digits on digital panel meters include 0.37 inches to 0.56 inches, larger than 0.56 inches and less than 0.37 inches.
The most important parameters to consider when specifying digital panel meters are the dimensions. Most digital panel meters are configured to be mounted into a rack or panel, so the face height and face width are important considerations.
Features common to digital panel meters include set point options, alarms, on/off control, bargraph display and adjustable display. A digital panel meter that has set point options has the ability to set control limits or set points for process parameters such as speed, temperature, pressure or humidity. In a heating application, the unit sends an off control signal when the set point temperature is approached or exceeded and an on control signal when the temperature drops below the set point. Limit (on-off), linear (proportional, PID) or other non-linear control technologies can be used to generate the control signal. A panel meter that has on/off control has the ability to send an on-off output signal to activate or deactivate a process unit (oven, motor, fan, etc.) utilizing relays, open collector transistors or other technologies. On-off, bang-bang or limit control is one of the simplest techniques for process control.
Inputs to digital panel meters can be one of many types. These include but are not limited to, AC voltage, AC current, AC power, DC voltage, DC current, DC power, strain gauge, resistance and temperature. Many digital panel meters are configured to accept multiple inputs and have adjustable displays to switch between these inputs.
Output types available for digital panel meters include serial communications such as RS232, relay outputs and analog outputs. Analog outputs are used in applications that require digitally controlled output voltages usually within the range of 10 V or less, and at current levels below 10 mA. The heart of an analog output channel is a digital-to-analog converter (DAC), which converts a digital code into a voltage level. The output of the DAC is then processed or conditioned to produce the required output signal characteristics.