Last revised: October 28, 2024
Reviewed by: Scott Orlosky, consulting engineer

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

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.                             

Figure 1: Basic Digital Panel Meter Signal Flow

Additional Outputs

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.

Types

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.

Display

The display of digital panel meters is either numeric, alpha-numeric, or graphic.

• 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.
• Graphic Display uses LCD technology to create a visual representation, often a bar graph or a “speedometer” type display with a moving needle.

Technology                                                                  

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 a seven-segment numeric display. LED displays can be seen in the dark.

• LCD (liquid crystal display) technology displays dark/black digits across a blank gray or green screen. LCDs consume less power than LEDs, and have more versatile display options, but require backlighting to be seen in the dark.

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.
• Resistance — input is received from elements which measure a change in resistance, such as piezoresistive sensors or strain gauges.
• Temperature — input is received from a thermocouple, RTD, or thermistor based on a change in temperature. See the Temperature Sensors area for more information.

Specifications

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, the display color and, if needed, any alarm functions.

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_).

Size

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.

Color

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:                            

• • Red
  • Green
  • Amber
  • Blue
  • Yellow
  • Orange
  • Aqua

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.

Connections

The connections for digital display meters include both the electrical inputs and outputs and the physical connections essential to secure mounting to the panel, i.e. screws. These connections must be compatible with the design and provisions of the associated panel.

Features

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. This allows for data capture and troubleshooting.

Digital Panel Meters FAQs

What are the key differences between analog and digital panel meters in terms of accuracy and usability?

Analog Panel Meters

Environmental sensitivity: Analog voltmeter readings can be affected by environmental factors such as an unlevel surface or nearby magnetic fields, which can cause errors in measurement.

Parallax error: Analog meters are prone to parallax error, which occurs when the meter is not viewed at the correct angle. This can compromise the accuracy of the reading by several degrees. Often a thin mirrored section of the “sweep” range is built in to provide alignment clues to help avoid the parallax error.

Lack of auto-polarity: Analog meters do not have auto-polarity technology, meaning that incorrectly connected test leads can result in needle deflection and potential damage to the device.

Trend visualization: The needle movement in analog meters provides a better visual indication of trends and order of magnitude, which can be useful for certain applications.

No power supply needed: Analog meters do not require an external power supply beyond the test current source, making them simpler to use in some scenarios.

Multiple scales: Analog meters often have multiple scales, which can cause confusion and make them harder to read accurately.

Digital Panel Meters

Superior accuracy: Digital voltmeters are generally more accurate and precise compared to analog meters. They are less susceptible to environmental factors that can affect analog meters.

Static accuracy: The accuracy of digital meters is influenced by factors such as signal conditioning, linearity, hysteresis, and temperature considerations. Static accuracy is expressed as a +/- percentage of full-scale output.

Digital meters provide a clear numerical readout, which eliminates the possibility of parallax error and makes them easier to read accurately.

Digital meters often come with additional features such as auto-polarity, which prevents damage from incorrect connections, and other functionalities that enhance usability.

How does signal conditioning affect the accuracy of digital panel meters?

Signal conditioning plays a crucial role in determining the accuracy of digital panel meters.

Linearity refers to how well the output signal corresponds to the input signal over the entire measurement range. Any non-linearity can introduce errors in the measurement, affecting the overall accuracy of the digital panel meter.

Hysteresis is the offset in the output signal when the input signal is increasing versus when it is decreasing. This can cause discrepancies in readings, especially in applications where the input signal fluctuates frequently.

Temperature variations can affect the performance of signal conditioning components, leading to changes in the output signal. This is particularly important in environments with significant temperature fluctuations.

Static accuracy is a combined measure of linearity, hysteresis, and repeatability. It is expressed as a +/- percentage of the full-scale output. This metric provides a good indication of the accuracy that can be expected under constant temperature conditions.

What are some common applications where digital panel meters are preferred over analog meters?

Digital panel meters are preferred over analog meters in several common applications due to their superior accuracy, ease of reading, and additional features. Here are some typical scenarios where digital panel meters are favored:

Industrial automation

Industrial automation systems are popular for monitoring and controlling various parameters such as voltage, current, temperature, and pressure. Their high accuracy and clear numerical readouts make them ideal for precise measurements and real-time monitoring.

Laboratory and Research

In laboratory settings, they are preferred for their precision and ability to provide exact numerical values. This is crucial for experiments and research where accurate data collection is essential.

Medical Equipment

Medical devices often require precise and reliable measurements. Medical equipment to monitor vital signs and other critical parameters, ensuring accurate and consistent readings are a common use.

Telecommunications

In telecommunications, they are used to monitor signal strength, voltage levels, and other parameters. Their ability to provide precise and stable readings is important for maintaining the quality and reliability of communication systems.

Power Distribution and Management

They are also commonly used in power distribution systems to monitor electrical parameters such as voltage, current, and frequency. Their high accuracy and additional features like alarm settings and data logging make them suitable for managing and optimizing power usage.

Consumer Electronics

In consumer electronics, these meters are used in devices like battery chargers, power supplies, and audio equipment. Their ease of use and clear display make them user-friendly for consumers.

How do temperature considerations affect the accuracy of digital panel meters?

These components are responsible for processing the input signal and converting it into a readable output. Any temperature-induced changes in their behavior can lead to inaccuracies in the final measurement.

Linearity refers to the consistency of the output signal relative to the input signal across the entire measurement range. Temperature fluctuations can cause deviations in linearity, leading to errors in the readings.

Hysteresis is the difference in the output signal when the input signal is increasing versus when it is decreasing. Temperature changes can exacerbate hysteresis effects, causing discrepancies in measurements, especially in applications with frequent signal fluctuations.

Static accuracy is a comprehensive measure that includes the effects of linearity, hysteresis, and repeatability. It is expressed as a +/- percentage of the full-scale output. Temperature variations can affect all these factors, thereby impacting the overall static accuracy of the digital panel meter.

What are some methods to mitigate temperature effects on digital panel meters?

To mitigate the effects of temperature on digital panel meters, several methods can be employed.

Implementing temperature compensation techniques in the design of digital panel meters can help counteract the effects of temperature variations. This involves using components and algorithms that adjust the readings based on the detected temperature changes.

Using high-quality signal conditioning components that are less sensitive to temperature variations can improve the accuracy of digital panel meters. These components are designed to maintain their performance across a wide range of temperatures.

Placing the digital panel meter in a controlled environment where temperature fluctuations are minimized can help maintain accuracy. This could involve using climate-controlled enclosures or installing the meters in temperature-stable locations.

Regular calibration of digital panel meters can help ensure that they remain accurate despite temperature changes. Calibration should be performed at different temperatures to account for any variations in readings.

Designing the meter with thermal isolation features can help reduce the impact of external temperature changes. This can include using insulating materials or designing the enclosure to minimize heat transfer.

Keeping the installation as “clean” as possible from a design standpoint. Make sure that sensors are as close as practical to the meter and don’t place high voltage circuits near low voltage signals.

What are some high-quality signal conditioning components used in digital panel meters?

Precision resistors are used to ensure accurate voltage division and current sensing. They are designed to have minimal temperature coefficient and high stability over time.

High-quality operational amplifiers are used for signal amplification and filtering. They are chosen for their low offset voltage, low drift, and high linearity to maintain signal integrity.

High-resolution analog-to-digital converters (ADCs) are crucial for converting the conditioned analog signal into a digital format. They are selected based on their resolution, sampling rate, and accuracy.

Integrated temperature sensors are used for temperature compensation. They help adjust the readings based on the detected temperature changes to mitigate the effects of temperature variations.

Stable voltage references are essential for maintaining consistent ADC performance. Low-drift voltage references ensure that the reference voltage remains stable over a wide range of temperatures.

Isolation amplifiers are used to electrically isolate different parts of the circuit, which helps in reducing noise and interference, thereby improving the accuracy of the measurements.

Digital Panel Meters Media Gallery

References

GlobalSpec—Understanding an Analog Voltmeter

GlobalSpec—Analog Voltmeter

GlobalSpec—Signal Conditioners