Reduce Adverse EMI Effects Upon Flow Meter Signals.

Jim ‘Slim’ Mimlitz, SCADAmetrics

The purpose of this article is to illustrate practical techniques that will minimize the unwanted effects of EMI (Electromagnetic Interference) upon flow meter instrumentation.

Background…

Personal and customer-reported experience has shown that, in the absence of proximal EMI noise sources, trouble-free flow metering instrumentation is generally achieved easily. However, occasional unique challenges still exist.

The industry-standard, encoded AMI transmission signal types — Sensus, Neptune, and Honeywell (Elster) protocols — are based upon a voltage level-based communication method that was originally designed exclusively for battery-powered interrogation instruments (AMI radios). Such instruments are thus electrically-isolated, and therefore differences in ground potential are negligible.

However, when water meters are connected to mains-powered instrumentation with longer cable distances, effects from stray capacitance and common-mode interference can creep in — leading to measurement errors and/or communication breakdown.

The likelihood of meter signal degradation increases even further when the equipment is installed in the presence of EMI (Electro-Magnetic Interference) generating equipment, such as Variable Frequency Drives, Electric Motors, Welders, and SCADA radios.

Adverse Effects of Common-Mode Interference Upon Flow Metering Instrumentation

The following diagram illustrates how undesirable common-mode currents can be generated by a proximal noise source (e.g. VFD) and superimposed upon the (desired) differential-mode 3-wire AMI signal. Common-mode currents can interfere with the operation of the flow meter, the connected instrumentation, or both.

Elimination of Common-Mode Interference with a Rejection Filter

The following illustration demonstrates how a Common-Mode Rejection Filter can be utilized to suppress the undesirable common-mode currents, while preserving the desirable differential-mode currents.

SCADAmetrics Common-Mode Rejection Filter

**Model CMR.100 EMI Filter for AMI Water Meters** – For Meter Applications with Signalizer or EtherMeter. When Particularly Strong EMI is Present, Two (or More) CMR.100 Filters May Be Installed in Series.

Common-Mode Noise Interference should be suspected if the flow meter operates correctly in isolated, standalone mode — but behaves erratically when connected to a mains-powered instrument. Mains-powered instruments generally consist of any non-battery-powered reading devices, such as the model EMP Signalizer or EM.100 EtherMeter.

Customers have reported that the installation of one (or more) SCADAmetrics Model CMR.100 EMI Filters for AMI Water Meters has provided effective suppression of VFD-induced flow metering anomalies.

CMR.100 EMI Filter Installed in a Motor Control Center at a Booster Pump Station.

Cable and Filter Recommendations

In order to achieve optimal performance of a meter-reading device, we recommend the use of shielded instrumentation cable, such as Belden 8771 or Carol (General) Cable C2526A. Both are jacketed cables with three (3) 22AWG conductors, a shield, and a drain wire.
For metering applications that include a mains-powered EMP Signalizer or EM.100 EtherMeter, install one (or more) SCADAmetrics model CMR.100 EMI Filter for AMI Water Meters on the 3-wire encoder cable. Multiple CMR.100 Filters may be installed in SERIES to achieve extra attenuation. Ideally, the CMR.100 should be installed in close wire proximity to the water meter register. However please note that the CMR.100 is not rated for submersion.
The Belden 8771 and Carol C2526A cables feature three (3) color-coded conductors: RED, WHITE, BLACK; and the water meter cables usually feature conductors color-coded: RED, GREEN, BLACK. For simplicity, splice as follows: RED-RED, BLACK-BLACK, WHITE-GREEN.
Special Case: Zenner Stealth Ultrasonic Flow Meter, Encoder Cable (Mains-Powered Version). Recommended Extension Cable: Alpha Wire 1295C SL002 Multiconductor Cable, 5C, 22 AWG, Foil Shield, Gray PVC, 500, 1295 Series (Colors = Red, Green, Black, Orange, White). Available HERE.
Bond the drain wire to ground at one end of the cable only (generally at the meter-reader end).
Minimize overall cable distance by cutting off unnecessary, extra cable.
Bond the water meter body to control panel ground with a dedicated 14AWG wire.
Power the meter reading device (EM.100 EtherMeter or EMP Signalizer) with a dedicated DC Power Adapter (no other devices powered by the adapter).
For mains-powered water meter reading devices (EM.100 EtherMeter and EMP Signalizer), use an isolated AC-DC power adapter, such as the SCADAmetrics MDR.20.24. For especially-challenging wire distance applications, a higher-capacity HDR.100.24 is recommended (not because of its higher-amperage (4A/100W) capacity, but because of its increased class 2 isolation and increased noise-filtering capability).
Minimize the distance between the DC Power Adapter and the Meter Reading Instrument.
For stations that feature VFDs (Variable Frequency Drives), installation of Line Reactors will reduce the amount of harmonic noise fed back into the station’s AC power bus.

Of the above recommendations, #1-6 are considered necessities, whereas #7-11 may be reserved for tackling particularly problematic situations.

Important! — Shielded 3-Wire Encoder Extension Cables

Carol Cable (General Cable) C2526A or Belden 8771 – Shielded Cable for Encoder-Type Water Meters

Comprehensive EMI Prevention Techniques Illustrated…

Enhanced Power Supply Isolation Techniques

If one (or more) SCADAmetrics model CMR.100 EMI Filter for AMI Meters are installed, and satisfactory noise reduction is not achieved, then another available method to significantly reduce the level of EMI noise effects upon a Signalizer and Flow Meter circuit is to install a medical-grade isolation transformer. Medical-grade transformers are ideal for protecting sensitive electronic equipment in hospital (and industrial) environments. Faraday shielding reduces the cumulative leakage current of the Isolator and connected equipment to levels less than 100 microamps. These transformers remove EMI/RFI noise, utility switching transients, and harmonics generated by other on-site loads. Additionally, medical-grade transformers reduce the impact of utility-related surge conditions. Their inherent filtering provides continuous common-mode noise rejection with no wearable moving parts. Example models include the Model MD-250-U (Triad Magnetics) and the Model IS-500 (Eaton/Tripp-Lite).

Questions?…

Are you interested in learning more about how SCADAmetrics flow instrumentation, when implemented with good signal filtering and wiring practices, can provide reliable and accurate signaling to your SCADA, Telemetry, or Building Automation System? Give us a call!… We’ll be glad to discuss the details!