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Glossary

AC
Alternating current or voltage waveforms, normally with equal positive and negative values.

AC Ammeter
A digital panel meter that measures alternating current in amperes or milliamperes.

AC Frequency Meter
A digital panel meter that measures ac mains frequency in Hertz.

AC Power Meter
A digital panel meter used to measure true ac power in Watts or kilowatts.

AC Voltmeter
A digital panel meter that measures alternating voltage.

Average Responding Meter
A method of measurement ac signals using a rectifier circuit. Average responding meters are suitable for measuring only sinusoidal waveforms. Significant errors can result when they are used to measure irregular waveforms.

Crest Factor
The ratio of an ac waveform’s positive (or negative) peak voltage to its rms voltage.

Current Meter
An instrument designed to measure and display (in either digital or analog format) ac or dc current (amperes); also referred to as simply “Ammeters”.

Daylight Readable Display
An alphanumeric display with sufficient contrast and/or brightness that enables it to be read in full sunlight.

DC
A voltage or current which only has one polarity (i.e., it’s always either positive or negative).

DC Ammeter
An instrument that measures direct current.

DC Voltmeter
An instrument that measures direct voltage.

DC Shunt
A low-value resistor typically employed in dc ammeters. This low-value resistor “shunts” high currents around the ammeter’s sensitive input circuit.

Digital Panel Meter
An instrument used for measuring a wide variety of voltage or current waveforms and converting the measured value to a numeric or alphanumeric display. Most digital panel meters employ either LCD (liquid crystal) or LED (light emitting diodes) as the display element.

Digits
For LED and LCD based panel meters, the number of numeric characters that can be displayed. For example, a four-digit panel meter can display inputs with a maximum resolution of 9999.

Donut CT
A term commonly used to describe a type of instrument transformer used to step-down very large ac currents to a lower value (5A is the most common rating). Their shape resembles a donut.

Full-Scale Input
The maximum value of an input voltage or current that can be safely applied to a digital panel meter.

Hertz
Internationally recognized unit of measure for frequency. A one Hertz waveform repeats itself every second. AC line frequency meters usually display Hertz in 0.1 or 0.01Hz resolutions.

Input Impedance
The resistance, usually specified in millions of Ohms or Meg Ohms, presented to a signal by a panel meter’s input circuit.

LCD Display
An alphanumeric digital display employing a ‘liquid crystal’ material, sandwiched between two layers of glass. LCD displays typically require a minimum level of ambient light in order to be visible. However, backlit LCD displays can be viewed in total darkness.

LED Display
An alphanumeric digital display employing semiconductor-based, ‘light emitting diodes’ as the active element. LED displays are visible in most ambient lighting conditions. New to the market are super-bright LEDs that are readable in outdoor applications.

Loop Burden
A term that describes the resistance seen by a 4-20mA transmitter’s output drive circuit. ‘Loop-drop’ (expressed in volts) is sometimes used to describe loop burden.

Loop Powered
A term used to describe any electrical instrument or transducer that derives all of its operating power directly from a 4-20mA current loop.

Multi-function Meter
A panel meter which measures two or more parameters on one or multiple displays.

NEMA-4
A ‘National Electrical Manufacturers Association’ standard that defines the requirements for instrument enclosures designed for use in harsh environments.

Panel Meter
An electrical measuring instrument designed to measure and display applied dc or ac voltages. Its physical design allows it to be easily mounted in instrumentation panels. Panel meters can be digital (with alphanumeric displays) or analog (with moving pointers).

Power Factor
The ratio of the real power (in Watts) flowing to an AC load to the apparent power (in volt amps). Power Factor is usually expressed as a number between 0 and 1.

Process Monitors
A panel meter specifically designed to convert industrial signals such as 4-20mA current loops or 0-5Vdc into physical parameters (weight, pressure, temperature, etc.). Process monitors typically include potentiometers for adjusting input signal offsets and span.

PLC
Programmable Logic Controllers, or PLCs, find widespread uses in industrial control applications. Digital panel meters can be used to visually display a PLC’s analog outputs (4-20mA, 0-5V, etc.).

Self-Powered Meter
A category of panel meters that derive all of their operating power directly from the signal they are measuring, providing simple, 2-wire operation.

True rms
‘rms’ stands for “root-mean-square”. The true rms value of a voltage or current waveform is a measure of its potential power or heating effect. Unlike their average responding counterparts, true rms reading panel meters can accurately measure the value of non-sinusoidal waveforms (step, triangle, square, etc.).

UL
Well known abbreviation for ‘Underwriters Laboratories’. Underwriters Laboratories is an internationally recognized safety testing organization. AC-powered panel meters are typically required to pass stringent UL testing before they are allowed to carry the UL mark.

Volt
Unit of measurement of electrical potential. One volt applied across a one ohm resistor will produce a current of one ampere.

Volt Ampere
An AC unit of measure. Volt amperes (abbreviated as ‘VA’) are the product a circuit’s rms voltage and its rms current. The volt-ampere is also referred to as ‘apparent power’.

Voltmeter
An instrument designed to measure and display, in either digital or analog format, ac or dc volts.

Watt
Unit of measurement of electrical power. In a purely resistive circuit, one ampere flowing through a potential of one volt produces one watt.

4-20mA current Loop
A method of transmitting signals to a remote location by way of a varying dc current. For example, a 4mA level typically denotes a signal’s zero value, while a 20mA level denotes a maximum reading for the signal of interest. 4-20mA current loops are widely used in industrial applications.

2-Wire Meters
A category of digital panel meters designed to be powered directly from the signal they are measuring and, as their name implies, only require two wires.

0-5V and 0-10V Process meter
A digital meter designed to measure varying analog voltages within the fixed ranges of either 0 to 5Vdc, or 0 to 10Vdc. These types of process meters (or monitors) are used in industrial applications.