This article discusses the LCD resolution when measuring power with the Durant Ratemeter LCDA-P or LCDA-EP.

Display Resolution
The power resolution depends on the number of active digits shown on the display. In other words, if the least significant active digit displays 10’s of watts, then the display resolution is 10 watts.
Effective Resolution
The power resolution due to limitations of the LCD display, which cause steps or oscillations greater than one count.

LCD Calculator

When configuring an LCD, the LCD Calculator either selects the resolution automatically (if there is only one good choice), or provides the user with a list of two or more resolutions to choose from. This sets the basic display resolution (typically 10W or 100W). The selected value appears on the Power tab in the Display Resolution column. Note: the LCD can be configured to display a fixed zero in the rightmost position – since this is fixed at zero, this digit does not increase the resolution. In practice, the effective resolution is lower, because the LCD doesn’t always step by one display count and sometimes fluctuates between two values separated by more than one count.

We tested with the Agilent 33220A Function Waveform Generator to try to determine the LCD effective resolution. The actual results are a bit unclear: more testing could probably determine the pattern. Generally, at lower frequencies (below 100Hz), the LCD stays at a fixed value or varies by one count, indicating that the effective resolution is equal to the display resolution. At higher frequencies, the display count started jumping by more than one count.

ToDo: Measure more data for better accuracy. In particular, simulate a variety of CT sizes (and/or WattNode models). Test at frequencies from 1% to 130% of full-scale with fairly fine steps, say: 1%, 2%, 3%, 5%, 7%, 10%, 15%, 20%, 30%, 50%, 70%, 100%, 130%. Also do some testing for 4Hz output models and 100Hz output models (as may be common with the WNB Pulse).

Initial Test Data

LCDA-P, WNA-3Y-208-P (simulated), 150A

Frequency (Hz)Readings (W)Variation (W)Variation (%)
290.053990, 5402030W0.06%
290.254020, 5406040W0.07%
290.354020, 54050, 54060, 5409070W0.13%
29.085410, 542010W0.18%

LCDA-P, WNA-3Y-208-P (simulated), 400A

Frequency (Hz)Readings (kW)Variation (W)Variation (%)
400.0198.6, 198.7100W0.05%
290.3144.1, 144.2, 144.3200W0.14%
290.0144.0, 144.1100W0.07%
29.114.4, 14.5100W0.7%

LCDA-P, WNA-3Y-208-P (simulated), 600A

Frequency (Hz)Readings (kW)Variation (W)Variation (%)
400.0297.7, 297.8, 298.1400W0.1%
290.0215.9, 216.0, 216.1200W0.09%
150111.7, 111.8100W0.09%
10074.4, 74.6200W0.3%
5037.2, 37.3100W0.27%

LCDA-P, WNB-3Y-208-P (20Hz)

Tested with Fluke 6100A. Shows a variation of 0.2% (two counts) at full-scale.