Panel meters are instruments that display an input signal in an analog or digital form. Many panel meters also include alarm options as well as the ability to transfer data to a computer. Panel meters take a sample of the voltage or current to create a visual representation of the measured value. Voltage measurements are read across the line, while current measurements are taken in series with the load, sometimes utilizing shunts or current transformers when the load current exceeds 10 Amperes overload protection device. When used in such applications, they are often referred to as motor circuit protectors.
Analog Panel Meters
In analog meters, the interface is application specific and can display any unit of measure, with nearly any input signal that is representative of the process. An analog meter can display speed, frequency, Voltage, current, temperature, strokes per minute, or feet per second. The actual input signals are analog voltage or current, in either AC or DC. An example is 0 to 10 volts DC, four to 20 milliamps DC or 0 to 100 volts DC. Digital meters operate in much the same way; however digital meters will often-times also include digital scaling of the display relative to the input signal.
Analog meters have two methods for displaying readings. The first is a taut band that has the needle suspended between two ribbons of metal and is ideal for environments prone to high shocks. The other method is a pivot and jewel, where the pointer has more stability for higher vibration environments.
Digital Panel Meters
Amongst the different types of panel meters, the digital panel meters offer the greatest resolution. This is often confused with accuracy, but analog meters can be just as accurate as a digital meter. Digital Meters offer an easier to read display, especially in low light, also providing greater resolution. The readings can be the same between an analog and digital meter, but in the cases where the input signal constantly changes, the analog meter may be preferable.
Using a motor as an example, the panel meter could be wired with a shunt or CT to see how many amps a motor is drawing at any given time. To be sure that the motor is drawing close to 20 amps, an analog meter can give a good visual indication for how many amps are being drawn. With an increase or decrease in torque, the needle will fluctuate within the range on the display. As for the digital meter, the constant changing of numbers makes it more difficult to read. This of course is not true for all applications. With the digital meter, the display is much easier to see at a glance than an analog meter. Digital panel meters can also offer greater functionality.
A Digital panel meter can be reconfigured to display volts, amps, temperature or any other engineering unit. Relay outputs are sometimes available, allowing the meter to control a process based on the measured input signal or display value.