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PADDLE-WHEEL HALL-EFFECT FLOW SENSOR
A compact flow-transducer component that converts fluid flow-rates into a pulsed output signal.
Similar to RS product: 508-2704 [£84.15 + vat]
Basic principle: The fluid passes across a 'paddle wheel' impeller containing three ceramic magnets. The rate of rotation of the paddle wheel is proportional to the rate of flow of fluid through the chamber [within limits]. A 'hall effect' sensor sealed in the rear portion of the assembly switches a transistor output as each magnet passes.
Suitable for most fluids: Water, Petroleum Fuels, Light Oils, Alcohols etc
Can be used with gases and air - BUT it must be remembered when measuring fast flow that the impeller will continue to spin for a moment after the gas flow has stopped and this can compromise the accuracy of flow-rate measurement.
Translucent case with visible rotary impeller - gives visual indication of flow.
Ports are very slightly under 10mm external diameter. Can be used with hose or most 10mm push-fit or compression fittings. Internal diameter is 6mm. The ports could possibly be given a 1/8" bsp thread.
DIMENSIONS - End to end: 75mm
PULSE RATE: Linear over a wide range [I have tested a sensor over 0.4~3LPM range]. Over this range the sensor gave one pulse per 0.88 cubic centimeter. Or 1140 pulses per litre. Accuracy in this range was definitely within a 5% margin - but in practice you will find that accuracy will be dependent on the range of flow-rates and other factors such as sudden flow-rate changes, entrained bubbles, fluid viscosity etc.
I expect that the exact rates will differ slightly for different sensors. For greater accuracy you will need to calibrate your system using measured quantities of fluid at the flow-rates anticipicated.
The sensor can give outputs in excess of 1.5kHz. Possibly much more.
Can be mounted in any orientation. And will give an output with flow in both directions.
The following link gives data for a similar hall effect sensor [precise ratings may not be identical to those shown]:
The output is transistor based. In order to give a useful output the user must connect a single external 'pull-up' resistor between the red and blue wires. Please see the pdf link below for a simple wiring diagram and the required resistor value for your application. I can probably supply the required resistor - just ask.
The diagram below shows a typical serving suggestion showing a similar type of sensor and the output run to the input of a PLC. I've included this diagram because it helps to see the principle involved. The transistor must not sink more than 20mA. And for many applications where a low current voltage source is all that is required to trigger external circuitry it is OK to use a 6~10K pull-up resistor.
Applications:
FLUID TOTALISER: requires connection to a pulse COUNTER. User should ensure that the counter can count at the maximum required pulse rate.
FLOW-RATE meter: requires connection to a 'frequency meter' even the Hz setting on a multimeter or 'frequency to voltage converter' and 'volt meter' OR: you can make your own circuit to generate a timed pulse triggered by the leading or falling edge of the sensor output pulse. The timed pulse length would have to be shorter than the minimum expected output pulse cycle. The timed pulses can be fed through to a simple capacitor and discharge resistor arrangement and this will give a voltage on the cap. which will be proportional to the sensor pulse rate. A '555' IC monostable timer circuit would probably be the cheapest and easiest way of generating regular pulses.
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