James Mimlitz — ‘Slim’ @ SCADAmetrics
Differential Pressure Meters are a class of no-moving-part flow meter whose principle of operation is based upon the Venturi Effect.
These meters are typically outfitted with a transmitter, which produces a 4-20mA signal, which is linearly proportional to rate-of-flow (gpm, lpm, etc…).
However, in many instances, it is also desirable to ascertain totalized flow (gallons, cubic feet, cubic meters, etc…). This can be accomplished by programming a connected PLC to perform a time-integral of the rate-of-flow signal (Remember calculus?!).
SCADAmetrics offers a hardware-based solution that does not require a PLC programmer, nor a PLC; and it is based upon two devices: The SCADAmetrics model MTP Milliamp-to-Pulse Converter, and the model APK.PLUS Analog/Pulse Kit.
The MTP Converter accepts a 4-20mA signal from the Flow Meter transmitter, and converts into a 0-20 Hz pulse output, which can then be totalized by the connected APK.PLUS.
As an example at the extremes, during periods of ZERO flow, no pulses are transmitted by the MTP. During periods of MAXIMUM flow, a 20 Hz pulse is transmitted by the MTP (20 pulses per second, or 1200 pulses per minute). Therefore, the pulse/gallon ratio can be calculated by dividing [MAX_GPM] by [1200]:
| Example: 4-20 mA : 0-20 Hz : 0-300 gpm … 300 gal/min / 1200 pulses/min 300 gal = 1200 pulses 1 gallon = 4 pulses 1 pulse = 0.25 gallon |
The APK.PLUS can be special-ordered to support a customized gallon/pulse ratio, and the customized ratio is activated by setting dip switches [9,10,11,12] = [on,on,on,on].
When ordering an APK.PLUS for this application, please make sure to specify the 20mA flow rate (gpm, lpm,…).
The following diagram illustrates the instrumentation hookup (click to zoom):
The APK.PLUS provides the user with a sequential display of the following:
- Master Total (Gallons, Cubic Feet, Cubic Meters, …) – Resettable
- Day Total – Resettable
- Instantaneous Rate-of-Flow (GPM, LPM,…)
- SCADA Output Status (4-20mA level)
The APK.PLUS also provides the user with the following extra SCADA outputs:
- 4-20mA Rate-of-Flow Signal (User-Scalable, via Dip Switches)
- Dry-Contact Pulse Output
- Dry-Contact Flow Switch Output (or 2nd Pulse Output)
Modbus and Allen-Bradley EtherNet/IP Solution
Instead of the APK.PLUS, a SCADAmetrics model EM.100 EtherMeter can be utilized to provide a fieldbus SCADA solution. The following diagram illustrates the connections (click to zoom):
Fine-Tuning
During no-flow conditions, it may be necessary to null out the 4-20mA signal so that the totalizing function does not integrate very-low-level flow signals. In other words, if the 4 milliamp signal is interpreted as 4.001 mA, then slowly creeping integration errors would accumulate. In such cases, it would be preferable to treat that as an absolute “no flow” condition, and a relay could be added to the input circuit of the MTP to enforce interpretation of “no flow” as 0.000 mA. For example, at a pumping station, the form-A or form-B relay could be controlled by an auxiliary contact of a pump motor starter or VFD.
Want to Learn More?…
Are you interested in learning more about how SCADAmetrics flow instrumentation, when paired with the Differential Pressure / Venturi Flow Meters, can provide a wealth of value-added SCADA, Telemetry, and Building Automation signaling options? Give us a call!… We’ll be glad to discuss the details!