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Flow Meter Selection Guide All about Reed Switches

W. B. Ellwood invented the Reed Switch at Bell Telephone Laboratories in 1936. In the 1940's it began to find itself in demand as a sensor and Reed relay. In this application, it was used in a variety of stepping/switch applications, as well as electronic and test equipment.


Today, Reed Switch technology has branched out from its humble beginnings in telecommunications and is now a major player in other markets such as automotive, commercial and residential security components, as well as in many types of measurement technologies, flow measurement in particular. Given that it is hermetically sealed, the Reed Switch is ideally suited for almost any environment.

Although structurally simple in its design, a typical reed switch is dependent on many different technologies for such a seemingly straight forward product. Most critical to the reliability of a reed switch is the quality of the glass to metal hermetic seal. To maintain that seal, both the glass and the metal used need to have a very exact linear coefficient of expansion. This refers to the relative change in length of the materials used per degree of temperature change.

Also key to the rising popularity across many industries is the physical size of modern Reed Switches. Originally measuring roughly 2 inches in length, most reed switches in use today are just under a quarter of an inch long.


Features
  • Contact resistance (on resistance) typical 50 milliohms (mO)
  • In its off state (normally open), it requires no power or circuitry
  • Ability to offer a latching feature
  • Operating time in the 0.01 to 0.03 ms range
  • Ability to operate over extreme temperature ranges from -40°F to 176°F
  • Ability to operate in all types of environments including air, water, vacuum, oil, fuels, and dust laden atmospheres
  • Long life. With just a few moving parts, load switching under 5 Volts at 10 mA and careful selection/sizing of the flow meter for any given application will allow for an extended lifespan of the Reed Switch Sensor.

When Should Reed Switches Be Used

With their low cost and high versatility, Reed Switches are a great option for use with remote counters, rate indicators and especially PLC's and other logic devices. Large current surges, which are common in inductive loads, can severely limit the life span of a Reed Switch. PLC's typically include a built in current limiting circuit which in turn provides the necessary surge protection to the reed switch.


Common Misconceptions
  1. Reed switches require a hazardous area certificate from ATEX, CSA, or IECEX:
    They are classified as simple apparatus that can be used in Zone 1 or higher provided they are switching less than 1.2V, 0.1A, 25 mW

  2. Reed switches will work indefinitely; Reed Switches are mechanical devices with a finite number of cycles: Their life will depend on the frequency required, the voltage and current being switched, as well as the level of vibration experienced.

  3. The reed switch in a meter board cannot be externally powered by a DC source: The reed switch can switch higher voltages but it will shorten the switch life.

Reed Switch Basics
Reed Switch Basics

The basic hermetically sealed Form 1A (normally open) Reed Switch and its component makeup.

A Reed Switch consists of two ferromagnetic blades, usually made of iron and nickel. These blades, which overlap each other leaving a gap between them, are contained within a hermetically sealed glass capsule. The parts of the blades that make contact with each other are plated or sputtered with a hard metal, typically Rhodium or Ruthenium. The hardness of these two metals directly contributes to the longevity of the Switch. The glass capsule itself is filled with Nitrogen or an equivalent inert gas. Some Reed Switches are manufactured with an internal vacuum which can increase their ability to operate and more importantly, to withstand high voltages.

In operation, these blades act as magnetic flux conductors and make contact with each other when exposed to a magnetic field from a permanent magnet or electromagnetic coil. Poles of opposite polarity are created and the contacts close when the magnetic force exceeds the spring force of the reed blades. As the external magnetic field is reduced, the force between the reed blades lessens allowing the contacts to open up again.


Closing Reed Switch Contacts
Generally speaking, there are two methods used to close Reed Switch contacts. A permanent magnet and a copper wound coil.
Permanent Magnet

A) Permanent Magnet

The basic operation of a Reed Switch under the influence of the magnetic field of a permanent magnet. The polarization of the reed blades occurs in such a manner to offer an attractive force at the reed contacts.

Coil Field

B) Coil

A Reed Switch sitting in a solenoid where the magnetic field is strongest in its center. Here the reed blades become polarized and an attractive force exists across the contacts.

When you bring a permanent magnet into the proximity of a Reed Switch the individual reeds become magnetized with the attractive magnetic polarity. When the external magnetic field becomes strong enough, the magnetic force of attraction closes the blades.

When the magnetic field is withdrawn, the magnetic field on the reed blades also dissipates. If any residual magnetism existed on the reed blades, the Reed Switch characteristics would be altered. But, the reed blades are annealed and processed to remove any magnetic retentivity. Proper processing and proper annealing clearly are important steps in their manufacturing. Annealing is a heat process whereby a metal is heated to a specific temperature /color and then allowed to cool slowly.


Application Notes:
  • Current and voltage demands of the load must NOT exceed the current and voltage ratings of the switch. For DC voltage always observe polarity. Voltage must be 5 - 120 VAC/VDC and current must be 4 watts maximum.

  • Reed switches cannot be connected directly across the power supply without a series load. Doing this will damage the switch.

  • Never test a switch with a filament light bulb. Severe inrush currents will cause damage or early failure.

  • There are three types of loads: Resistive (PC or PLC), Capacitive (long wire runs), and Inductive (solenoids).

  • Always keep the area around the switch clean and dust free.
  • Reed switches do not have built in surge suppression. When using the switch to actuate a solenoid or any inductive load, always use a surge suppression version of solenoid and/or surge suppression connectors. Without these precautions large inductive spikes can severely limit switch life expectancy.

  • Use the switch to signal end of stroke only. Do not rely on the switch alone to stop the piston in mid stroke.

  • Reed switches are equipped with indicator lights. The light always depicts an output voltage from the switch.

Extending the Life of your Reed Switch

There are several ways to extend the life of a Reed Switch. As mentioned, Reed switches are mechanical devices with two normally open (NO) ferromagnetic blades enclosed in a hermetically sealed, gas filled glass chamber. In an Oval Gear meter, a magnet embedded in the rotor creates a magnetic field that momentarily closes blades with each full revolution of the rotor. With the blades closed, contact allows a current to pass through the switch.

To maximize the life of the reed switch, this switching voltage should be limited to under 5 volts at 10mA. Fast changes in temperature and excessive vibration should also be avoided as either condition can potentially crack the glass enclosure.

Pulse Frequency will also limit the life of a Reed Switch sensor. Under ideal conditions, you can expect roughly two billion pulses from it. To put that into practical terms, at 67 Hz (a Hz or Hertz is defined as one cycle per second) the Flomec OM025 has the highest frequency of any of the OM Series Oval Gear meters. This means that if you were to run this meter at its maximum flow rate of 40 gpm continuously, the reed switch would have an expected life span of one year for two billion pulses. To extend this, you could a) specify a larger meter with a lower frequency or b) consider changing from a Reed Switch to a Hall Effect Sensor where longevity is not theoretically limited by frequency.

It's important to note that we do not recommend continuously running at 100% of a flow meters published range. Doing so will lead to early failure of the rotor bearings in most instances and as mentioned, will shorten the life of the reed switch sensor.


Flow Specialist - Bill Michie Written By: Bill Michie  
Flow Applications Specialist
Cross Company
Instrumentation Group
Phone: (866) 905-9790 (M-F, 8am-5pm Eastern)
Contact the Experts

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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