Image Credit: Newport Electronics, Inc. | KNS Associates, Inc. | Knick Elektronische Messgeraete GmbH & Co. KG
Digital panel meters are digital displays intended to be mounted in a panel, such as a control panel. They typically read some type of input, be it a voltage, current, or resistance, and display its value digitally. Often this input is a measurement from a sensor which is converted and displayed as a temperature, pressure, or other system variable. These meters may also act as simple controllers which trip a switch or perform a simple function when the reading reaches a preset measurement.
Functions of Digital Panel Meters
To select a digital panel meter, industrial buyers must first decide what they'd like it to do.
Basic Display Output
The basic function that all digital panel meters perform is taking an input signal and converting it to a displayed value. This input comes initially from some sensor in the process or system, and is sent to the device through the panel the meter is connected to. The panel provides the power for the device and inputs for the display.
Flow diagram - basic digital meter function.
In addition to simple display, some digital panel meters have optional control or interface functions available.
Analog functionality means the device can provide an analog output (such as a variable voltage or current) proportional to the input signal, for use in analog devices.
Data interface functionality allows the user to interact with the information that is displayed for data analysis.
Set point functionality allows the user to preset input values which will activate alarms or simple control functions. These on/off functions correspond to either solid state or electromechanical relay outputs depending on the design.
Flow diagram - Dual channel function with additional optional outputs.
Image Credit: Laurel Electronics, Inc.
Once a buyer determines what functions they require, the actual type of meter needs to be considered as well. Digital panel meters are distinguished by type based on both their display and their lighting technology.
The display of digital panel meters is either numeric or alpha-numeric.
Numeric display means the meter displays only numbers. Most digital panel meters are numeric, because the main purpose of meters is to display numeric values.
Alpha-numeric display means the meter displays both numbers and letters. This is useful for providing further information to the user, such as a reading's corresponding channel or units.
The lighting used in the digital display is either LED or LCD.
LED (light emitting diode) displays are made up of diodes which light up the segments. LED displays can be seen in the dark.
Image Credit: Autonics USA, Inc.
LCD (liquid crystal display) technology displays dark/black digits across a blank gray or green screen. LCDs consume less power than LEDs, but require backlighting to be seen in the dark.
Positive LCD display - Image Credit: Hoyt Electrical Instrument Works, Inc.
Signal Input Type
In order to function correctly, the device must be designed to accept the analog input sent to it. Types of inputs include:
AC Voltage - input is received as a measurable form of AC voltage in volts AC (VAC).
AC Current - input is received as a measurable form of AC current in milliamps (mA AC) or amps (A AC).
AC Power - input is received as an adjustable range of AC current, voltage, and/or frequency.
DC Voltage - input is received as a measurable form of DC voltage in volts DC (VDC).
DC Current - input is received as a measurable form of DC current in milliamps (mA DC) or amps (A DC).
DC Power - input is received as an adjustable range of DC current, voltage, and/or frequency.
There are a few important specifications to consider when selecting digital panel meters, all relating to the display itself. These include the number of digits, the size of the panel face, and the display color.
Number of Digits
The number of digits determines how many digits are present in the display output. This number should be less than or equal to the number of significant figures in the signal input, depending on the accuracy desired. In meters rated to display half digits (e.g. 4.5 digits, 2.5 digits), the first or the last digit can only be '1' or off, or '5' or off. This "half" digit allows for the upper limits of display to be extended (1000 vs. _999) or allows for the display of intermediate values (100.5 vs. 100_).
4-digit and 6-digit meters - Image Credit: Omegadyne, Inc.
The size of the panel face determines the device's physical compatibility with the panel it is mounted to. Typically, face sizes are rated in inches (" or in) or millimeters (mm) and are specified by length and width. Sometimes, the depth of the meter and the dimensions of the cutout need to be considered, though most panels will accept standard depths associated with the face size. Additionally, the display should be large enough for the user to read the digits/letters comfortably when mounted.
Digital panel meters are designed to display certain colors. In LCD meters, color is determined by the backlight behind the black digits. In LED meters, color is determined by the type of diodes built in to the device. Color choices include:
Image Credit: OMEGA Engineering, Inc.
Red is the default LED color and often LCDs have no backlight (no color). Multicolor options exist, which are particularly useful when multiple displays or channels are used. Setting one color to each display can help users quickly identify each one.
The connections for digital display meters include both the electrical inputs and outputs and the physical connections (e.g. screw connectors) essential for secure mounting to the panel. These connections must be compatible with the design and provisions of the associated panel.
Digital panel meters may incorporate a number of features that provide the device with additional functionality. These include set point options, alarms, on/off control, and adjustable display.
Set point options provide 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.
Alarms (either audible or visual) may be built in to the meter to indicate when limits or set points are reached.
On/off control allows the meter 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.
Adjustable display allows the meter to be adjusted to show different displays.
Additional channels allow digital panel meters to receive and interpret multiple signals from different sources.
USB/serial ports allow the meter to be connected to other devices such as data-analysis tools or computers via USB or serial connectors.
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