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Flow Meter Selection Guide Hall Effect Sensors and their use in Oval Gear / Positive Displacement Flow Meters

The "Hall Effect" is defined as "The electromotive force generated in a strip of metal longitudinally conducting an electric current and subjected to a magnetic field normal to its major surface."

For our purposes, this small sensor plays a big part in flow measurement applications as it converts the rotor movement in an oval gear and/or turbine flow meter into a square wave voltage signal. That signal is then used to directly calculate the volumetric flow.


The Hall Effect was discovered by Dr. Edwin Hall in 1879. While attending Johns Hopkins University in Baltimore, Maryland, Dr. Hall was attempting to confirm the theory of electron flow proposed by Kelvin approximately 30 years earlier and made an interesting discovery. When a magnet was placed so that its magnetic field was perpendicular to one face of a thin rectangle of gold through which current is flowing, a difference in potential appeared on the opposite edges. The voltage was also proportional to the current flowing through the conductor.

Although well received in academic circles, no real world applications for the Hall Effect were found for nearly 70 years!


When a conductor carrying current is placed into a magnetic field, a voltage will be generated which is perpendicular to both the current and the field. This is known as the Hall Effect Principle. Hall Effect sensors are used in many different types of measuring/sensing devices. As long as the quantity to be measured can be incorporated into a magnetic field, a Hall sensor can do the job.


The Hall Effect is an electronic switch, the basic magnetic field sensor. It requires signal conditioning to make output functional in the majority of applications. The signal conditioning electronics needed are an amplifier stage and temperature compensation. Voltage regulation is important if the Hall Sensor is being used with an unregulated power supply.

  • Uncertified Hall Sensors cannot be used in Intrinsically Safe Applications (FLOMEC Hall Sensors are not certified for Intrinsically Safe Applications).

  • Hall Sensors are sensitive to over voltage and reverse polarity. The FLOMEC Hall Sensor have over voltage and reverse polarity protection.

  • The Hall Effect Sensor is an electronic device, is both reliable and has a long life, unlike mechanical switches that can deteriorate with wear and tear.

  • Hall Effect maximum current draw is 20mA versus a maximum 10mA switching current, and although the maximum 20mA current draw is limited for some Hall Sensors, the maximum switching current is determined by the value of the Pull-Up resistor for NPN Hall Effects or Pull-Down resistor for PNP Hall Effects.

  • Ohms Law will determine the minimum resistor value for a given voltage to ensure maximum current (mA) is not exceeded.

The main factors to consider when electrically installing a pulse output flowmeter are; the type of output signal you require, the quality of the wiring connections, and any local regulatory requirements. Two types of output are available on every standard pulse output Oval Gear flowmeter; NPN Hall Effect and Reed Switch (contact closure). Each output type is linearly proportional to volumetric flow and each pulse is representative of an equal volume. Both outputs have their own advantages and disadvantages, and depending on the system requirements, your flowmeter may be installed using either output, or both outputs (wired to two separate instruments).

Hall Effect

The NPN Hall Effect is a high resolution solid state 3 wire device which provides an un-sourced, open collector, NPN transistor output. The term unsourced means that no voltage is applied to the output from within the flowmeter. The output of the Hall Effect must be pulled to a 'high' state by an external voltage between 5-24VDC, this is achieved by fitting a pull-up resistor between the signal output and the voltage supply. The pull up resistor ties the open collector output to the available DC voltage level, providing a square wave pulse output, which alternates between ground potential and the DC voltage available at the signal terminal.

The NPN Hall Effect output is a reliable output type, producing a consistent output irrespective of supply voltage variations, temperature variations, or mechanical shock. As the Hall-Effect output does not rely on a mechanical device, the service life of this output is theoretically infinite. The Hall Effect output should be preferentially chosen whenever a power source is available.

Many secondary flow instruments are fitted with an integral pull-up resistor, but if connecting the Hall Effect output to an electronic device that does not contain an integral pull-up resistor, one must be provided. The pull-up resistor should be sized to limit the Hall Effect switching current to less than 10mA; in order to achieve this, use Ohm's Law to calculate an appropriate pull-up resistance to suit your power supply voltage and current requirements. The recommended minimum pull-up resistor value is 2.4 kOhm, however much higher values of 10 kOhm are more commonly used.

Hall Effect

The total current draw of the Hall Effect output is nominally 20mA plus your switching current. If a 24V power supply is used with a 10K Ohm pull-up resistor, this equates to approximately 23mA.

When installing your Hall Effect output, the most critical consideration is the quality of the voltage regulation and the type of loads present on any common voltage sources. It is NOT recommended to combine any inductive loads on the same voltage supply as your flowmeter, as these components are commonly sources of high frequency interference that may affect the quality of the Hall Effect output signal. Another concern having to do with inductive loads on a common voltage source is the potential for voltage spikes well in excess of the 24VDC limit of the Hall Effect sensors.


By far the most common cause of a Hall Effect fault is electrical damage caused by incorrect wiring, voltage spikes, or short circuits. As Hall Effect sensors are solid-state devices their failure tends to be sudden and permanent. Unlike the Reed Switch, the Hall Effect sensor will completely cease to function once damaged, and tends not to produce intermittent or gradual failures.

Visual Inspection of the Hall Effect sensor will usually show if a failure has occurred (although not always). Over voltage, reverse polarity, or short circuits will usually create visual damage and a smell of burning. A failed Hall Effect can only be fixed by replacement of the Pulse Output Board.


The Oscilloscope is a useful tool that can be used for more advanced pulse output board troubleshooting; however it requires a greater understanding of electrical signals in order to be useful. An oscilloscope is most useful for detecting the presence of an output signal, and can give information about the quality of the signal that will allow diagnosis of some output problems without disassembly of the mechanical parts of the flowmeter.

PC oscilloscopes can be particularly useful when compared to regular oscilloscopes; as signal traces can be saved as image files (JPEG or similar) and emailed to a technical sales representative (at the flowmeter manufacturer or distributor) for help in troubleshooting your signal output issues. Economical single channel PC oscilloscopes can be purchased that will interface with any USB capable Windows PC. In combination with a notebook/netbook/laptop a low cost PC oscilloscope is a very useful and portable tool.

Testing with an Oscilloscope

When connecting an oscilloscope to your Pulse Output Board you must first set up the output as per the diagram above. If you intend to measure a Hall Effect output signal, the sensor must first be powered from a regulated voltage supply, and a pull-up resistor must be provided (see section 3.5.2 for selection of a pull-up resistor). The oscilloscope should be connected up to measure the voltage between the signal terminal and the -0V terminal; and will chart the voltage measurement against time allowing you to view the signal in real time, and measure features of your signal such as voltage, frequency, and duty cycle. When measuring the signal output from a Hall Effect signal, an electronic instrument may be connected in place of the pull-up resistor (be sure to turn on the internal pull-up in your instrument).

Shown Below there are various output signals that can be seen when inspecting signal outputs on an oscilloscope. The first signal trace shows a normal output from a Hall Effect sensor; there is some cyclic variation in pulse width in the signal, which is a function of magnetism tolerances in the magnet manufacture. Pulse widths are usually even within 10%, but the amount of variation shown below is normal and not a cause for concern.

Square Wave

We hope this article has been both informative and helpful to you in learning more about the origins and operating theory of the Hall Effect Sensor. And in particular, how they are used to enhance the communications on your Flomec/GPI Oval Gear Meter.

Flow Specialist - Bill Michie Written By: Bill Michie  
Flow Applications Specialist
Cross Company
Instrumentation Group
Phone: (866) 905-9790 (M-F, 8am-5pm Eastern)
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Special thanks to the Technical Team at GPI Austrialia for gathering, publishing and giving us permission to use the valuable information and images provided in this post.
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