Impeller Flowmeter Output Generation:
(a) Analogue Output (Litres per Minute):
- Impeller Rotation: As fluid flows through the meter, the impeller rotates at a speed proportional to the flow rate. The impeller blades are designed to minimize drag and ensure consistent rotation with flow changes.
- Magnetic Coupling: The impeller shaft is coupled to a magnet inside the meter housing. This magnet rotates with the impeller, creating a changing magnetic field around it.
- Pick-up Coil: A stationary pick-up coil positioned near the rotating magnet senses the changing magnetic field. This induces a voltage signal in the coil whose frequency is directly proportional to the impeller rotation speed (and hence, the flow rate).
- Signal Conditioning: The induced voltage signal from the pick-up coil is typically weak and noisy. Electronic circuitry amplifies and filters the signal to remove noise and ensure a clean, reliable representation of the flow rate.
- Scaling and Calibration: The conditioned signal is then scaled and calibrated based on the meter’s specific characteristics. This converts the signal voltage into an analogue voltage proportional to the flow rate in litres per minute (LPM). The scaling factor is determined during the meter’s calibration process using known flow rates.
- Analogue Output: The resulting voltage signal, representing the flow rate in LPM, is available as the analogue output of the flowmeter. This can be directly connected to an analogue-to-digital converter (ADC) for further processing or used directly in analogue control systems.
(b) Digital Output (Litres per Minute):
- Signal Conditioning: Similar to the analogue output, the induced voltage signal from the pick-up coil is amplified and filtered.
- Frequency-to-Digital Conversion: Instead of scaling the signal directly, a frequency-to-digital converter (FDC) is used. This chip measures the frequency of the signal, which is directly proportional to the impeller rotation speed and flow rate.
- Microprocessor and Calculations: The FDC’s digital output (representing frequency) is fed into a microprocessor within the flowmeter. The microprocessor uses the meter’s calibration data and internal algorithms to convert the frequency data into a flow rate value in litres per minute (LPM).
- Digital Output: The calculated flow rate in LPM is available as the digital output of the flowmeter. This can be displayed on the meter itself, transmitted through serial communication protocols, or used for digital control systems.
In both cases, the impeller rotation serves as the basis for flow measurement. However, the processing and interpretation of the signal differ, resulting in either an analogue voltage or a digital flow rate value as the output.