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AC RMS Digital Panel Meter Frequently Asked Technical Questions
In Laureate AC RMS meters, a total of five AC voltage ranges and four AC current ranges are user selectable and factory calibrated, with calibration factors stored in EEPROM on the signal conditioner board. AC RMS meters come factory set for a specific range, like RMV1. To change to a different range, move jumpers on the signal conditioner board and select the new range in software, as explained in the Laureate DPM user manual. If you change the range, also change the model number on the meter label.
Laureate AC RMS meters are modular with slots for an analog output board, a choice of relay boards, and a choice of communication boards, as illustrated in our Laureates Overview web page. These boards can be ordered installed with a new meter, but they can also be purchased separately later and be simply plugged in. The presence of a new board and the type of board are automatically sensed by the meter’s firmware or by  Instrument Setup (IS) software. If you change boards, also change the model number on the meter label.
True AC RMS meters are preferred because they can measure the energy content of sinusoidal or non-sinusoidal waveshapes, which can be chopped or distorted. They display the equivalent DC voltage or current that would create the same heating of resistive load. At any instant, the applied heat is V2/R or I2R, where V is voltage, I is curent and R is resistance. RMS stands for square Root of the Mean of the Squares. Lower cost AC meters use simple wave rectification and averaging to simulate RMS readings. Such similation is only accurate for perfectly sinusoidal waveforms.
A count is a unit of resolution. All ranges of Laureate AC meters go from 0 to 20,000 input counts. For example, in the Laureate 200.00 mV RMV1 range, an input count is 0.01 mV. The Laureate 600.0V RMV5 range and 300.0V RMV6 range are both based on a fictitious 2000.0V range, so an input count is 0.1V. The Laureate 5.000A RMV4 range is based on a fictitious 20.000A range, so an input count is 0.001A.
A display count is the resolution of the meter reading, or the step value of the least significant digits. For example, a meter reading of 465.7 is 4657 display counts. The decimal point is set separately as a decoration.
Input scaling is the process of converting the input counts to a reading in display counts. For example, to convert the 0-50.00 mV AC output of a current shunt to a 0-50.0A AC reading, use the meter’s 200.00 mV RMV1 range. 50.00 mV is then 5000 input counts. 50.0A is 500 display counts. To go from 5000 to 500, enter a scale factor (or multiplier) of 0.1 and an offset of 0.
Analog output scaling is the process of converting the meter reading to an analog output. That output can be selected as 4-20 mA, 0-20 mA, 0-10V or -10 to +10V. Simply enter the meter readings for the bottom and tops of the analog output range, and the output will be interpolated linearly between these two readings. Please see Section 17 of the Laureate DPM user manual.
Chances are that your meter is set for the factory default analog output, which is 4-20 mA, and is applying around 12V to force a current into an open circuit. To set your meter to a 0-10V or -10 to +10V analog output, set jumpers for unipolar 0-10V or bipolar -10 to +10V operation, select that range in software and scale it in software, and connect to the designated analog output pins, as explained in Section 17 of the Laureate DPM user manual.
Calibration is the process of setting the meter so that absolute readings in volts or amps are within specified tolerances of a recognized national standard. All ranges of Laureate AC input and DC analog output boards are factory calibrated to standards of the US National Institute of Standards and Technology (NIST), with calibration factors stored in EEPROM on the board itself. This allows boards to be swapped in the field with not need to recalibrate the meter as a system. Scaling relates the meter input to the dislayed reading. For example, a 0-5.000A AC input from a current transformer may be scaled for display of 0-100.0A AC.
Annual calibration to NIST standards can be performed by Laurel and by some of its distributors as a service. It can also be performed by outside calibration labs and by customers by using Laurel’s free Windows based  Instrument Setup (IS) software. Open the Main Menu of IS software, click on Calibration in the top menu bar, and follow the prompts. An external AC voltage or AC current standard is required. Also required in the meter is a communications board, like the Laureate USB interface board P/N LUSB. Following calibration, that board can be removed and be used in another meter.
The Laureate RMA4 current range accepts AC currents up to 5.000A AC. For higher currents, pass the current to be measured through the hole of an external doughnut-shaped current transformer (CT). CTs have a specified maximum current rating and a corresponding secondary output, which is typically 5A AC. Measure that output with the Laureate RMA4 5.000A AC current range. For example, to measure 0-50.0A with 0.1A resolution, use a CT rated 50:5 and scale the meter with a scale factor of 0.1. This will convert the 5000 input counts for 5.000A to 500 isplay counts. Move the decimal point one position to the left to display 50.0.
The Laureate RMA4 current range accepts currents up to 5.000A AC. For higher currents, you can use an external current shunt with the meter’s sensitive 200.00 mV RMV1 voltage range where each input count is 0.01 mV. A current shunt, available from multiple suppliers, is an accurate, very low resistance R resistor which is placed in line with the current I to generate a low voltage drop IR. Shunts are characterized by their maximum current, such as 50A, and the corresponding IR voltage drop, which is typically 50, 75 or 100 mV. Scale the meter to convert the measured input counts, which are units of 0.01 mV, to AC amps.
The highest AC RMS voltage that can be applied to a Laureate AC meter is 600.0V when the meter is set to the RMV5 range. For higher voltages, use an external step-down voltage transfomer (VT) to bring the voltage to be measured to less than 600.0V (RMV5 range). A transformer also provides electrical isolation and is an important safety tool. It can have a very low power rating. For example, to measure 1000V AC in even volts, use a 2:1 stepdown transformer for an output of 500.0V, use the meter's 600.0V RMV5 range, and scale the meter with a scale factor of 0.2. This will convert the 5000 input counts for 500.0V to 1000 display counts.
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