No. 2-52, Higashi 2-chome, Nakanocho,
Tondabayashi, Osaka, 584-0022, Japan◆ TEL : + 81-(0)721-26-0511◆ FAX : + 81-(0)721-25-8210◆ E-mail : riko@riko.co.jp ◆ http://www.riko.co.jp

Notes regarding this catalog• Specifications and appearance of items depicted in this catalog are subject to change in the interests of product improvement.
• Colors reproduced in this catalog may differ slightly from those of the actual product due to variations in printing.

2014.7.5000(Z)Basics of Float-Type Level Switches

Basics of
Float-Type
Level Switches

Float-type level switches are indispensable
in a variety of industrial applications.
A sensor is a device that converts various physical phenomena or changes in such phenomena

Contents
Typical applications of
the float-type level switchespressure, temperature, humidity, and level of a liquid surface. In this article, we will restrict our focus
to the sensors that detect the level of a liquid and explain the float-type level switch.
This type of sensor, which precisely detects and measures the amount of liquid inside a container or
As a usage example, when the liquid level in the container rises and the liquid level exceeds
the standard (or vice versa), it causes the level switch to contact (detection), triggering a beep soundShuts off the combustion pump andtriggers a lamp and buzzer when
the kerosene is depleted.

1-7 Level switch handling precautions ・・・ 9Motor vehicles

1

Chapter 2. Reed Switches
2-2 Characteristics of the reed switch・・11

Illuminates a warning lamp on
the instrument panel
when the brake fluid runs low.

2-3 Explanation of codes and terminology・・12
2-4 Explanation of threshold value ・・・・・13
2-5 Explanation of contact capacity ・・・14
2-6 Structural classifications ・・・・・・・・・・・14

Commercial-duty
air conditioners
Turns on a pump to drain the water
when the drain pan ofa ceiling-recessed air conditioner is full.

• Heated toilet seats• Car wash machinery• Diesel equipment• Buses• Marine vessels• Agricultural machinery

2-7 Functional classifications ・・・・・・・・・・15

Chapter 3. Magnets
3-1 Characteristics of ferrite magnets・・・・17
3-2 Measurement of magnetic force・・・・・17
3-3 Examples of orientation of magnetic field・・17
3-4 Application examples oforientation of magnetic field ・・・・・・・18

3-5 Magnet types and features ・・・・・・・・・18
3-6 Changes in magnetic field ・・・・・・・・・・19
3-7 Primary magnet manufacturing process・・20
3-8 Design considerations ・・・・・・・・・・・・・・21

Chapter 4. Floats

Other applications• Machine tool lubricant• Fast photo developing machines• Trucks• Locomotives• Construction machinery

Electrophysical measurement
Auto parts industry

1-6 Features of different models ・・・・・・・ 8

2-1 Features of the reed switch ・・・・・・・・11

Lights a lamp and shuts down
the engine when the oil level is
low in a general-purpose engine
used for the generator of a foodstall or outdoor facility.

Industrial machinery

1-3 Operation principle ・・・・・・・・・・・・・・・・・・ 6
1-5 Model nomenclature ・・・・・・・・・・・・・・・・ 7General-purpose engine

Marine and aviation transport industry

1-2 Features of the level switch・・・・・・・・・ 5

Kerosene heaters

and activating a control unit. In other words, it has an important control function.
toibutes andrtnoIt c dustries
int rolemany n importan y.
aplays our economin

1-1 Appearance and structure・・・・・・・・・・ 5

1-4 Structure of the reed switch・・・・・・・・ 6

into signals and data for output. The many types of sensors available are used to monitor position,

the like, is generally referred to in short as a “float switch.”

Chapter 1. Level Switches

4-1 About floats ・・・・・・・・・・・・・・・・・・・・・・・・・23
4-2 Float materials and application examples・・23
4-3 Relationship between float andspecific gravity

・・・・・・・・・・・・・・・・・・・・・24

4-4 Float specific gravity and performance table・・25
4-5 Float specific gravity and liquid level・・25
4-6 NBR float manufacturing process・・26
4-7 Internal structure of float ・・・・・・・・・・・27

Household appliance industry

4-8 Multiple float design・・・・・・・・・・・・・・・・・28
4-9 Design considerations ・・・・・・・・・・・・・・29

2

Riko’s float-type level switch flexibly accommodates
the movement of specific liquids.
As liquid flows, the float rises and falls, enabling the float-type level switch to detect
the liquid level (as a liquid surface) as the reed switch inside the main unit cycles between

Basics of Float-Type Level Switches

ON and OFF status from the magnetic force of the magnet contained in the float.

Chapter1

Level Switches

General Applications
Detection of a liquid quantity
and control of an electric system
Generally, an alarm triggers a lamp or buzzer fromthe electrical signal generated when the float switch detectsthat the liquid in a tank is depleted or replenished.
This approach is used in many control systems that actuate orturn off a motor or heater by closing or opening a power supply.

Widely used in liquid level control
The float-type level switches are available with one,
two or three contact points, allowing you to selectthe most suitable for a particular application.
All models are used for control of a liquid level and can beused in various industrial plants, unmanned flexible
manufacturing systems, space heating and cooling units,
washing devices, and various medical applications forthe health-care profession.

1 1 Appearance and structure ・・・・・・・・ 5
1 2 Features of the level switch・・・・・・・ 5
1 3 Operation principle ・・・・・・・・・・・・・・・・・ 6
1 4 Structure of the reed switch ・・・・・・ 6
1 5 Model nomenclature ・・・・・・・・・・・・・・・・ 7
1 6 Features of different models・・・・・・ 8
1 7 Level switch handling precautions・・ 9

Let’s read a basic introduction to float-type level switches.

Liquid level control

3

Detection of liquid quantity

4Chapter1

Level Switches

1 1 Appearance and structure

1 3 Operation principle

ナット

Here, using the example of a Type MFS 17 Level Switch, we explain the name and structure of each part.
As outlined in the figure below, as the float falls, it has the status of “switched ON.” Likewise, as the float rises,
it has the status of “switched OFF.”■ Outline view and nomenclature■ Basic structure

Stem

A reed switch is mounted inside the stem of the float.
The magnetic field of the N pole and S pole generated
by the magnet acts on the reed section of the float, which
enters the ON state as a result of magnetic attraction.
When the magnet is moved away, the magnetic field isremoved and the reeds’ elasticity opens the contacts,
restoring the OFF state.

In operation

N poleReed switch

Lead wire

MagnetProtective
tube

Plating

Float
Inert gasNut

S poleN poleMagnetS poleN poleStem(fixed nut)
Switched

ON

Float

Switched

OFF

UP

DOWN

S poleStopper■ Typical models

1 4 Structure of the reed switch
As shown in the figure, a reed switch comprises twoferromagnetic reeds enclosed within a glass tube.
To prevent oxidation (rust) of the points of contact, thetube is filled with an inert gas (nitrogen) and the surfacesof the points of contact are plated with a noble metalsuch as rhodium. (In the case of a single reed switch,
the contacts are spread apart.)

MFS17-A

12

MFS17-B

MFS17-C

Glass tube

Contact section

Features of the level switch
The simple mechanism is relatively trouble-free.
2 The compact design requires little space.
3 The highly reliable reed switch contacts have a high repeatedaccuracy and long service life.
4 No adjustment after installation is required,and little maintenance is needed.
5 The float-type configuration accommodates non-conductive liquids.
6 The stable design exhibits little irregularity of movement.
7 It is lower-priced than other types.
1

5

MFS17-D

Not in operation

ng
tandi
s
t
u
o
An esigndInert gas

Contact section
Reed end

6

Chapter1

Level Switches

1 5 Model nomenclature

MFS

1 6 Features of different models

R FS

Model Name

(RFS models contain stainless steel parts; MFS models contain plastic parts)

Features

RFS-2

Compact

RFS-3

Direct mounting from top

RFS-4

Horizontal, L-shaped type

RFS-5

Flange type

RFS-6

Heat-resistant type (200°C max)

RFS-8

Pressure-resistant type (4 MPa max)

RFS-9

Pressure-resistant/L-shaped type

RFS-11A

Horizontal, inner mounting

RFS-11H

RFS-11A heat-resistant specification

RFS-12

Horizontal, outer mounting

RFS-12P

RFS-12 with connector

RFS-12H

RFS-12 heat-resistant specification

RFS (base material: stainless steel), 2 (model, development no.)With the RFS series, a model without a stainless steel float is indicated as RFS-2;
a model provided with a “B float” is indicated as RFS-2B.

RFS-13Ultra-compact horizontal/inner mounting

RFS-14Ultra-compact horizontal/outer mounting

RFS-16Heat-resistant type (200°C max)

PVC-2C

Essentially the same as the RFS Series but designed with PVC (polyvinyl chloride) as the base material.

TFS-8

Titanium, heat-and-pressure-resistantspecification

TFS-8

Essentially the same as the RFS Series but with some titanium parts (those in contact with liquid,
including the main body, float, and stopper). The “T” indicates titanium.

Model Name

Features

Miniature

Float

SeriesSwitchBase material of product(such as stem and float) is plastic.

Series

Riko

Float

SwitchBase material of product(such as stem and float) is stainless steel.

■ Examples of model names

MFS17-A

RFS-2

HFS-2

RFS-12H

MFS (base material: plastic), 17 (model, development no.), A (float number).
The MFS series includes a float number because in many cases only the float material
differs from that of the stem of the same model. In some cases, different materials
can have the same shape.

The HFS Series is unique in indicating “H” for hysteresis.
Note: “Hysteresis” refers to models with a level switch requiring a large measurement difference to cause
switching between the ON and OFF states (large distance required to switch “ON → OFF” or “OFF → ON”).

The meaning of the “H” suffix in models RFS-11H and RFS-12H is not the same as the “H” prefix
in HFS-2, which indicates a heat-shock-resistant type.

Note: Previously, a “-1” or “-2” suffix was appended to model numbers, such as “MFS17-A-1.”
The suffix “-1” indicated a model with a maximum switching voltage of 100 V; “-2”indicated a model with a maximum switching voltage of 200 V.
In 2007, maximum switching voltage was unified at 200 V for all such standard models,eliminating the need for the “-1” and “-2” suffixes.

MFS17-ACompact design, oil-compatible

MFS17-B

Compatible with oil andlow specific gravity liquids

MFS17-C

Compatible with water and chemicals

MFS17-D

Compact design, compatible withwater and chemicals

MFS21-E

Horizontal, oil-compatible

MFS21-K

Horizontal, compatible withwater and chemicals

MFS25-J

Compact design, compatible
with water and chemicals

MFS9-N1

Horizontal, inner mounting

MFS9-N2

Oil-compatible version of MFS9-N1

MFS10-N1

Horizontal, outer mounting

MFS10-N2

Oil-compatible version of MFS10-N1

Product Examples

RFS-2

RFS-4

RFS-6

RFS-8

RFS-11A

RFS-9
RFS-12

RFS-3

RFS-16

RFS-13

RFS-5

Product Examples

MFS9-N1

MFS17-A

MFS17-B

MFS9-N2

MFS10-N1

MFS17-C

MFS17-D
MFS10-N2

MFS17-A (Union)
MFS17-B (Union) Union specification
MFS17-C (Union)
MFS17-D (Union)

7(Accepts extension rod.)
MFS21-E

MFS21-K

8

Chapter1

Level Switches

1 7 Level switch handling precautions
Avoid external shocks.

External shock from a fall or the like can change
the behavior of the reed switch,causing it to be permanently “ON” or “OFF.”2

Keep away frommagnetic fields.

The reed switch is activated by the magnetic field
generated by a magnet.
Any nearby magnetic field can cause a malfunction
by triggering a false “ON” or “OFF” state.

3

Never allow any liquid
to penetrate the stopper.

Water or another liquid bypassing the stopper and penetrating
the unit can result in a permanent “ON” state.
In addition, even a small amount of liquid penetrating poor insulation
can result in a short circuit.

4

Avoid installing inlocations subject tostrong vibration.

The gap between the contacts of the reed switch is too small
to be seen by the naked eye. If the level switch is located in
an area subject to vibration, a malfunction can result.

Never pullon the lead line.

In most models, the upper part of the unit is filled only witha molding agent. As a result, subjecting the lead line toexcessive pulling force can result in the reed switch becoming
dislodged. In some cases, this can result in a short-circuit frombreakage of the lead line or tearing of the film.

2 2 Characteristics of the reed switch・・・11

Install circuit protection to safeguard the contacts if using
an electric motor or solenoid valve capable of imparting
an inductive load or surge load (inrush current) on the level switchthat exceeds the load capacity of the contacts.
(Also be cautious about backflow currents in the “OFF” state.)

2 5 Explanation of contact capacity ・・・・14

5

6

7

9

Provide circuit
protection forthe contacts.

Other precautions

Using the device under conditions outside
the specified values indicated in the catalog,
or installing the device in an unstable diagonal
orientation, can result in a malfunction
or erroneous operation.

Basics of Float-Type Level Switches
Chapter2

Iron board

1

Reed Switches

2 1 Features of the reed switch ・・・・・・・・・・11
2 3 Explanation of codes and terminology ・・12
2 4 Explanation of threshold value・・・・・・13
2 6 Structural classifications・・・・・・・・・・・・・・14
2 7 Functional classifications ・・・・・・・・・・・・・15

10Chapter2

Reed Switches

2 1 Features of the reed switch

The contacts are unaffected by the outside atmosphere because they are sealed ina glass tube filled with an inert gas.
2 The small mass of the operating components results in rapid response.
3 The switch is compatible with products incorporating high-frequency transmission
because the device circuitry and operation system is constructed with coax cable.
4 The switch features a lightweight and compact design.
5 The contacts offer a longer service life thanks to their superior resistance to wear
and corrosion as well as their stable switching operation.
6 It can be configured as an economical proximity switch simply by adding a permanent magnet.

2 3 Explanation of codes and terminology

12 2 Characteristics of the reed switch
When constructing a reed switch that operates with a permanent magnet, the characteristics (ON/OFF states)vary with the kinds of reed switch, the threshold value, activation threshold, processing state, and thecomposition of the permanent magnet as well as its shape and magnetization state. The representative
characteristics of the reed switch are shown below.

Vertically oriented magnetic field

Internal-external magnetic field

X (mm)

HOLD

ON

※

OFF

HOLD

ON

※

0

-Y(mm)HOLD

Y (mm)ON-Y(mm)0X

X
N poleS pole-Y

Y (mm)N poleS poleOriginY

OFF

OriginY-Y

Notes • When a strong magnet is used, check operation at two or three points.
• Vertically oriented magnetic field and internal-external magnetic field are presented separately on p. 17.

Other examples of activation

OFF

N

S

OFF

ON

ON OFFN

ActivationthresholdＤＯ

ＡＴ

This is a product of the current value and number of coil turns that causes the contacts toenter an activated state through coil excitation. It is correlated with threshold value andrepresents a secondary value.

Contact resistancethresholdＣＲ

ｍΩThis value is the resistance between the terminals when the contacts are closed andincludes the conductor resistance.

Breakdown voltage
between contacts

Ｖ

This is the breakdown voltage between the contacts. A reed switch can be safely operated at thismaximum switching voltage; this is also the prescribed transient overvoltage that can be temporarily
endured due to a surge on the circuit in use caused by an external factor or other similar phenomena.

Insulation
resistanceＶ

Determined by the insulation resistance between the terminals, this is the resistance value forleakage current of the glass tube of the reed switch and its surface.

Ｗ

Contact
capacity

ＶＡSON OFF

This is the product of the usable voltage and current of the contacts and is an importantvalue in establishing switching performance. This value must not be exceeded during opening
and closing of contacts to ensure their longevity and reliability. The value will be less than the“maximum switching voltage” × “maximum switching current”. When designing, do not
exceed the value of the product of the voltage and the electric current applied to the contacts.
It is also referred to as the allowable contact power or contact rating.

Ｖ

This is the maximum voltage capable of activating the contacts and the reference voltage fordetermining performance of contact switching. This is the value that must not be exceeded
during opening and closing of contacts to ensure their longevity and reliability. It is also
referred to as the rated contact voltage or maximum voltage.

Maximumswitchingcurrent

A

This is the maximum current capable of activating the contacts and the reference current for
determining performance of contact switching. This is the value that must not be exceeded
during opening and closing of contacts to ensure their longevity and reliability. It is also
referred to as the rated contact current or rated disconnection current.

Maximum
activation
current

A

As the maximum current value for activating the contacts, this is the current value foractivating the contacts in succession without switching the contacts. Exceeding this current
value when activating the contacts can compromise their longevity and reliability. It is also
referred to as the rated activation current or the maximum contact activation current.

Resonance
frequency

Hz

This is a vibration frequency specific to the particular reed switch. Exposure of the reed switch
to a nearby vibration of this frequency can cause a malfunction.

Maximum driving
frequency

Hz

This is the maximum driving frequency that permits normal opening and closing of a reed switch.
Frequencies above this value can interfere with normal opening and closing operation signals in
relation to operation time and bounce time.

Operating time

Top

ms

This is the time from application of a voltage to the activation coil until the contacts operate.
Unless specified, bounce time is not included in operating time.

Bounce time

Tb

ms

This is the time from application of a voltage to the activation coil to close the contacts until
the contacts are completely closed.

NumberThis is a dedicated coil used to measure the characteristics of the reed switch.
Various types of coils are used.

■ Typical applications of reed switches• Engine control• Auto door locks• Cruise control• Power steering

N
S
Ring magnet

ON

ON
ON

S
OFF
2-pole ring magnet

Ring magnet

OFF

N

ON

Burnt lamp sensor

Speed Sensors

S

OFF
N

11

ＡＴ

OFF ON

OFF

S
OFF
Rod magnet

ＰＩ

Reed switch

OFF

ON

ThresholdvalueThis is the most important characteristic of a reed switch. It is the product of the current value andnumber of coil turns required to excite the coils and activate the contacts. [This represents thesensitivity of the reed switch; the lower the (numerical) threshold value, the higher the sensitivity.]Standard coilN

Explanation

Code Unit

Maximumswitchingvoltage

X (mm)

OFF

Term

ON

ON

N
S

OFF

Brake fluid level float

S
N

ON

OFF
4-pole ring magnet

OFF• Monitor
Reed switch

Engine oil float• Monitor

Engine temperature sensor• Electric fan • Monitor

Reed switch
Float magnet

Reed switch
Sensing part

Airbag sensor
Power seat sensor

Thermoferrite

12Chapter2

Reed Switches

2 4 Explanation of threshold value

Threshold value (AT)2 5 Explanation of contact capacity

This is the most important characteristic of a reed switch. It is the product of current value and the number ofcoil turns necessary to operate the contacts with a coil magnet. It indicates the sensitivity of the reed switch,and the lower the numerical value, the better the sensitivity (ON state achieved with a small magnetic field).

Battery

Contact capacity (W, VA)

This is the product of the usable voltage and current of the contacts and is an important value in establishingswitching performance. This value must not be exceeded during opening and closing of contacts to ensure
their longevity and reliability. The value will be less than the “maximum switching voltage” × “maximum
switching current.” When designing, do not exceed the value of the product of the voltage and the electric
current applied to the contacts.
■ Example with Model MFS17-A

Buzzer• Maximum switching voltage: 300 V (DC/AC) • Maximum switching current: 0.5 A (DC/AC)• Contact capacity: 50 VA (DC/AC)

Maximum switching voltage 300 (V) ×
Maximum switching current 0.5 (A) = 150 (VA)

Coil magnet (1,000 coil turns)
Reed switch

The numerical value for contact capacity listed in the catalog is “50 VA.”

In other words, 150 VA exceeds the contact capacity, so you must lower the voltage (to 100 V max.) or30

add resistance to control the electric current (to 0.16 A max.).
Power supply

mA

Variable resistance

Current meter2 6 Structural classifications

In the above illustration, if the buzzer sounds when the state is 30 mA, “30 mA” is “0.03 A”, andChapter 1 explains the basic structure of the reed switch. The different types include those in which each of
the individual reeds are of different lengths as well as those with different structures and functions such as thetypes that enclose a set of three reeds.

current 0.03 (A) × no. of coil turns 1,000 (T) = 30 (AT),the threshold value of this reed switch is 30 (AT).
(amperes)(turns)(ampere turns)

In this type, the contacts are positioned near the center of the sealed glass.

1

Center type

Generally, the threshold value increases as the gap between the contacts increases, and the threshold value decreases
as the gap decreases. (However, in cases of strong reed elasticity, the threshold value can be high even with a small gap;
the opposite is also true.)Contacts are located in the center.

Also known as the offset type, this type positions the contacts offset fromthe center of the sealed glass.

■ Variation in threshold value (example)

8• The threshold value changes when the reedsof the reed switch are trimmed.

2

6
AT

• As the reed lengths increases, the thresholdvalue increases.

4Off-center typeContacts are offset.

2
L1

L2
2

4

6

8

10

Length of cut terminals: L1 + L2 (mm)1314

Chapter2

Reed Switches

2 7 Functional classifications
The classifications of Types A, B, C introduced here, also commonly used for general switches,
are used to classify reed switches by structure.

1

2

Type A reed switch

The A type reed switch is activated when subjected to a magnetic field,
and the normally open circuit is then closed, causing current to flow.
It is referred to as a normally open (“make”) contact, with two reeds enclosed
in a glass tube serving as the open contacts. This is the most common typeof reed switch currently being manufactured.

Type B reed switch

In contrast to the Type A reed switch, normally the contacts are closed;
the contacts open when a magnetic field is applied. This type is also known asa normally closed (“break”) contact. Currently, prototypes have been made to
demonstrate the feasibility of the theory, but some of the characteristics are
inadequate. Therefore, the Type B reed switch has not yet been adopted forpractical use.

Basics of Float-Type Level Switches
Chapter3

Magnets

This is also known as the transfer contact type (switchover type).
When this reed switch is subject to a magnetic field, the contact in the closed
state switches to the contact that is open (the circuit is switched over).

3

Type C reed switch■ Configuration of Type C reed switch contacts

3 1 Characteristics of ferrite magnets・・・17
3 2 Measurement of magnetic force・・・・・17
3 3 Examples of orientation ofmagnetic field ・・・・・・・・・・・・・・・・・・・・・・・・・・・・・17
3 4 Application examples oforientation of magnetic field ・・・・・・・・・・18
3 5 Magnet types and features ・・・・・・・・・・・・18
3 6 Changes in magnetic field ・・・・・・・・・・・・・19
3 7 Primary magnet
manufacturing process ・・・・・・・・・・・・・・・・・20
3 8 Design considerations ・・・・・・・・・・・・・・・・・・21

15

16

Chapter3

Magnets

3 1 Characteristics of ferrite magnets

3 4 Application examples of orientation of magnetic field

Ferrite magnets are permanent magnets with stable magnetic characteristics. They are used in applications
that do not require a strong magnetic field. They are made primarily from iron oxide (Fe 2 O 3 ), barium
carbonate (BaCO3), and strontium carbonate (SrCO3) sintered with powder metallurgy techniques.
These magnets are economical because the so-called iron oxide (also seen as rust occurring on iron) is
fire-hardened during manufacture.

3 2 Measurement of magnetic force
Measurement of the magnetization state (magnetic force) is performed with a gauss meter or flux meter asshown below.

Vertically oriented
magnetic field

This, the most common orientation of a magnetic field, is easy to use when designing
and assembling a level switch. Many in-house products use this type.

Internal-external
magnetic field

Used especially in level switches, this commonly used type is referred to as a“hysteresis” type.

Note: As shown in section 3-3, various magnetization orientations are available; however, any types other than the above arerarely used for level switches.

3 5 Magnet types and features■ Measurement with a gauss meter [measured in gauss (G) units]
A gauss meter is used to measure the density of the magnetic force lines per unit of area.

The maximum value is measured by placing the measurement area (Hall element) on the surface ofthe magnet’s N pole.

Magnetic flux density

Measurementofmagnetic flux
density

Measurement
methodMagnetic flux

Ferrite magnet

In wide use, this magnet offers stable magnetic properties
at low cost. The “homolytic” type is magnetized after
molding and sintering; the “anisotropic” type is moldedand sintered after being magnetized.

Alnico magnet

This type is often used when stable precision is required.
It has superior characteristics regarding temperature, but
has the disadvantage of being easily demagnetized.

Plastic magnet

This type can be freely molded into even complex thin-walledshapes. It is made by mixing powdered ferrite magnet
and rare earth magnet with plastic for molding.

Rubber magnet

Flexible enough to be bent, this magnet is more resistant tobreakage and chipping than sintered magnets. It has a wider
range of application and is easy to cut, punch and print.

NeodymiummagnetNeodymium magnets provide the highest magnetic energy
among existing magnets. It is a molded sintered productmade mainly of neodymium, iron and boron. Because it
easily oxidizes, it is usually nickel-plated.

Samarium-cobalt
magnetAfter neodymium magnets, this type offers the best
characteristics. It is a molded sintered product made mainlyof samarium and cobalt. It is resistant to high temperaturesand corrosion. As for disadvantages, it breaks easily.

Gauss meter

N
S

Magnetic lines of force

Placement ofHall element tomeasuremaximum value

MagnetHall element■ Measurement with a flux meter [Measured in Weber (Wb) units]
A flux meter is used to measure changes in magnetic flux when a magnet is interlinked with a closed circuit (coil).
It measures the electromotive force required to raise the magnet.

Measurementofmagnetic flux

Visualization of magnetic flux

Measurement method

Flux lines visualized

Closed circuit (search coil)
MagnetN
S

Magnetic flux is measured
by raising magnet

3 3 Examples of orientation of magnetic field
Vertically oriented
magnetic field

Internal-external
magnetic field
S

N
S

17

N

Examples of other orientations of magnetic field

N

S
N

S

NS
SN

18

Chapter3

Magnets

3 6 Changes in magnetic field

3 7 Primary magnet manufacturing process

Ferrite magnets lose 0.18% of their magnetic force with each degree Celsius increase in temperature.
(Alnico magnets lose 0.02%/°C; neodymium magnets lose 0.1%/°C.)

Homolytic ferrite magnet

Example : A 600 gauss magnet at a room temperature of 25°C is raised toa room temperature of 30°C

5°C change (difference between 30°C and 25°C) × 0.18(%) = 0.9, therefore
600 – ( 600 × 0.9(%) ) = 594.6 (gauss)Magnetic force decreases with temperature rise, but the magnetic force isrestored when the room temperature returns to 25°C. Be aware that if the magnet
reaches its “Curie point” (temperature at which the magnet loses all magnetic
force*), the magnetic force does not return even if temperature falls afterwards. The
magnetic becomes demagnetized and the force does not return when it reaches“service temperature” (temperature at which the magnetic force of the magnet isrendered unrestorable**).

Composition: barium carbonate (BaO3), strontium carbonate (SrCO3), iron oxide (Fe2O3)

Raw materials &compounding

Calcination

Grinding

Molding

Magnetization

Cleaning

Processing

Sintering

Inspection

Finishedproduct

Alnico magnet* The “Curie point” of a ferrite magnet is about 450°C to 460°C; for an alnico magnet, it is about 850°C.
**The “service temperature” of a ferrite magnet is about 300°C; for an alnico magnet, it is about 450°C.
As the temperature rises, magnetic force falls, and as the temperature falls, the magnetic force is restored.
It is advisable to remember “service temperature,” the temperature at which the magnet is demagnetized andloses its ability to recover its magnetization.

Composition: aluminum (Al), nickel (Ni), cobalt (Co), iron (Fe), copper (Cu)Raw materials &compounding

Dissolution

Casting/molding

Heat treatment

Inspection

Magnetization

Processing

Aging process■ Irreversible low-temperature demagnetization of a homolytic magnet (TF)
If the operating environment is less than the normal temperatures shown in the graph below,
the magnet will demagnetize as the temperature falls. This condition is called “irreversible
low-temperature demagnetization,” and the magnet will not return to its original state evenafter the temperature returns to normal.
1• Measuring temperature:
26.5°C• Coercivity (HcJ):
3.1 kOe• Permeance coefficient:
0.6

Initial demagnetizing factor (%)• Magnet shape:
20 mm dia. × 4 mm

0

Neodymium magnet-1

Composition: Neodymium (Nd), iron (Fe), boron (B), dysprosium (Dy)-2

Raw materials &compounding

Dissolution

Grinding

Press molding

Magnetization

Surface treatment

Processing

Sintering andheat treatment

Inspection

Finishedproduct

-3
-4
-5
-50

19

Finishedproduct

-40

-30

-20

-10
0
10
Temperature (°C)

20

30

40

20

Chapter3

Magnets

3 8 Design considerations
1

Confirm that proper clearance exists between the inner diameter of the magnet (or inner diameterof the float) and the outer diameter of the stem. In some cases, if the clearance does not exceed
1 mm, the float might not move because of the presence of dirt and the like.

Basics of Float-Type Level Switches
Chapter42

Confirm that the magnetic force of the magnet is sufficient to move the reed switch. If not,complaints may arise such as “sometimes the switch doesn’t work.”

3

Check the need for hysteresis. Since hysteresis can be as small as 1 mm if the magnetization isvertically oriented, it is necessary to orient the magnetic field to “internal-external magnetization”
when hysteresis is required.

4

Check whether the reed switch is operating at three points. If it operates at three points, change
to a weaker magnetic force (smaller size) or change the threshold value of the reed switch.

5

Confirm that magnet material is the appropriate type. Determine the magnet material after
considering cost and quality.

6

Confirm that the magnet is firmly attached to the float. Confirm that a ring-shaped or rod-shaped
magnet has not detached from the float.

7

A thicker magnetic field results in a longer “ON” region. With a larger radial orientation, the “ON”
state begins at a position distant from the reed switch contacts.

8

Confirm that the magnet maintains sufficient force to activate the reed switch even at maximum
operating temperature.

4 4 Float specific gravity andperformance table ・・・・・・・・・・・・・・・・・・ 25

Note: Normally, depending on the service environment, malfunctions will not occur even if the magnetization is 30% lower thanits normal level at room temperature. Final confirmation must be performed on-site. However, when designing with a magnet
strong enough to overcome predicted demagnetization, confirm with 3-point operation.

4 5 Float specific gravity andliquid level ・・・・・・・・・・・・・・・・・・・・・・・・・・・・・ 25

Floats

4 1 About floats ・・・・・・・・・・・・・・・・・・・・・・・・・・・ 23
4 2 Float materials andapplication examples ・・・・・・・・・・・・・・ 23
4 3 Relationship between float andspecific gravity ・・・・・・・・・・・・・・・・・・・・・・・ 24

4 6 NBR float manufacturing process ・・ 26
4 7 Internal structure of float ・・・・・・・・・・ 27
4 8 Multiple float design ・・・・・・・・・・・・・・・・ 28
4 9 Design considerations ・・・・・・・・・・・・・ 29

21

22

Chapter4

Floats

4 1 About floats

4 3 Relationship between float and specific gravity

A float is one of the most important components of a sensor that detects liquid level.
It floats in a fluid such as water or oil, rising and falling with the changing level of liquid.

The specific gravity of a substance is the ratio of the weight of a given volume of the substanceto the weight of the same volume of water.

Specific gravity = weight/volume (SG = W/V)

4 2 Float materials and application examples(

BUNA-N
RF-2
Fully independent
foam)

SUS304
SUS316
Internal
cavity )(Example: Specific gravity of a float with a weight of 10 g andfloat volume of 20 (cm3)

10 ÷ 20 = 0.5, so the specific gravity is 0.5

This is foamed by the addition of phenolic resin to
NBR (nitrile rubber).
Typical applications : Oil, various liquid chemicals
Made of NBR

Made of press-molded stainless steel with plasma welding.
Typical applications : Oil, various liquid chemicals,
high-temperature and high-pressure equipment
Made of stainless steel(

Plastic
mixture ofcontiguous andself-supporting
plastic)

Example 1
Weight : 10 g
Volume : 20 ㎤

Made of PP (polypropylene) and nylon foamed in a resin mold.
Typical applications: Water, various liquid chemicals

Speciﬁcgravity : 0.5
Made of PP (polypropylene)

Previously, wood and cork were used for floats, but this made it difficult to maintain
uniform quality and obtain a stable supply of material. They are not currently usedbecause of quality and cost problems and issues of insertion of sensor magnets.

Wood, cork

In the above case, the calculation is that half of the float is submerged in water. In addition,
at least one-half of the float will be submerged if a float with a specific gravity of 0.5 isplaced in a liquid with a specific gravity of 0.8. In other words, the lighter the specific gravityof the liquid, the more the float sinks into the liquid; and the heavier the specific gravity of
the liquid, the greater the amount of the float appearing above the surface of the liquid(floating margin). On the other hand, the lighter the specific gravity of the float, the larger theamount of float appearing above the surface of the liquid; and the heavier the specificgravity of the float, the larger the amount of float that is submerged in the liquid.

()

Other types

Weight : 8 g

Volume : 20 ㎤Speciﬁcgravity : 0.5

Liquidlevel

BSpecific gravity of liquid 0.8 (oil)
B is larger than A.
(More than half of the float is in liquid.)

Weight : 12 g

A

A

Volume : 20 ㎤
Liquidlevel

BSpecific gravity of liquid 1.0 (water)
Specific gravity of the float is light.
(A is larger than B.)

Speciﬁcgravity : 0.6

Liquidlevel

BSpecific gravity of liquid 1.0 (water)
Specific gravity of the float is heavy.
(B is larger than A.)

Example : “Float is in floating condition” when float is on water.

Products of titanium and fluorine exhibit very goodcharacteristics (including good resistance to hightemperature and high pressure). Currently, because ofhigh cost, marketing is difficult and demand is low.
Made of titanium

Made of fluorine

Specific gravity 0.2

23

B

A

Example 2Speciﬁcgravity : 0.4

PVC (polyvinyl chloride) pipe is processed with PVC glue or PVC weld.
Typical applications: Water, various liquid chemicals

Liquidlevel

Specific gravity of liquid 1.0 (water)
A and B are the same length(About half the float is in liquid.)

Volume : 20 ㎤

PVC
SUS and similar
with internal cavity

Weight : 10 g

A

Specific gravity 0.5

Specific gravity 0.8

24Chapter4

Floats

4 4 Float specific gravity and performance table
A

B

C

D

E

K

F

J

R1

RFS-2

RFS-6

RFS-8

φ51×60L

TFS-8

Volume

4.40

11.60

10.68

5.03

14.96

15.02

4.87

6.15

8.35

11.14

63.64

30.60

87.38

55.60

Weight

3.23

5.42

7.42

3.52

7.77

10.51

3.80

4.00

6.70

7.80

35.00

18.36

39.32

35.55

Specific
gravity

0.73

0.47

0.70

0.70

0.52

0.70

0.78

0.65

0.80

0.70

0.55

0.60

0.45

0.64

Pressure (Pa)

0.2M

0.2M

0.2M

0.2M

0.2M

0.2M

0.2M

0.2M

0.2M

1M

1M

4M

1M

5M

Heat resistance
(°C)

80

80

80

80

80

80

80

80

80

120

200

120

120

200

Liquidcompatibility

Oil

Oil

Water,
chemicalsolutions

Oil

Outer
diameter

Water,
chemicalsolutions

Water,
chemicalsolutions

Water,
chemicalsolutions

Water,
chemicalsolutions

Water,
chemicalsolutions

Water,
chemicalsolutions

Water,
chemicalsolutions

Water,
chemicalsolutions

Water,
chemicalsolutions

Water,
chemicalsolutions

φ20.0

φ26.5

φ25.0

φ18.0

φ31.0

φ31.0

φ23.5

φ24.0

φ25.0

φ28.0

φ50.0

φ43.0

φ51.0

φ51.0

Inner
diameter

φ8.5

φ9.0

φ9.0

φ8.0

17×9

17×9

φ8.0

φ8.0

φ9.0

φ9.5

φ15.5

φ10.5

φ15.5

φ15.0

Height

20.0

25.0

25.0

25.0

25.0

25.0

14.0

16.5

20.0

27.0

48.0

42.0

61.0

48.0

Composition

N.B.R

N.B.R

P.P

P.P

N.B.R

P.P

P.P

P.P

P.P

SUS

SUS

SUS

SUS

TP340

Dimensions

Float Name

(Unit of pressure resistance: 1 MPa = 10.2 kgf/cm2 ≈ 10 kgf/cm2)

Note: Floats under columns E and K above are compatible only with horizontal applications; they are not interchangeable.
• Because the above values are the respective maximums, the floats might not be resistant if both temperature and pressure apply concurrently.
(Absorbed water can increase the weight, or dents and other deformations can change the volume and specific gravity, etc.)
Example: When the level switches are fabricated with an RFS-6 float, and a dented float is at 200°C and 1 MPa, it is usable in performance but the appearance is poor.
If it is at atmospheric pressure, no problem will occur even at 200°C. There is no problem at 1 MPa at normal temperature.

4 5 Float specific gravity and liquid level

4 6 NBR float manufacturing process

In level switch design, the product of the float height multiplied by the float specific gravity determines
how much of the float is below the water surface (for the purpose of calculation).

15 mm height × 0.6 specific gravity = 99 mm of the float is below the liquid surface. (Water)

15

If the specific gravity of the liquid is not the same as water (1),calculate how much of the float will be below surface bymultiplying the float height by its specific gravity and dividing
by the specific gravity of the liquid.

Speciﬁc3 Mixed compound 4 Extruding & cutting

gravity: 0.6

9

3.75
15

Speciﬁcgravity: 0.6

5 First curing
Press

6

Liquidlevel

With water (specific gravity: 1.0),
9 mm of float is below water, 6 mm is above.

15 mm height × 0.6 specific gravity ÷
0.8 specific gravity of liquid = 11.25
11.25 mm of the float is below the liquid surface.

25

2 Basic mixing

C ut

Example 1 : with float height of 15 mm and float specificgravity of 0.60

Example 2 : When using oil of specific gravity of 0.80 withthe above float

1 Composition

11.25

With water (specific gravity: 0.8),
11.25 mm of float is below water,
3.75 mm is above.

Liquidlevel• Test results• Any other data

Mooney viscosity

6 Burr finishing

7 Second curingCu• Rolling process
standard chart• Fully blended state• Compounding daily• Work standard chart
Press• Surface finishingconditions• Confirming temperature• Dimensions,and time
weight and shape• Temperature control ofpress machine

8 Cooling

9 Finishing

10 Inspection• Foaming condition• Post-mortem
visual inspection• Weight, appearance
and dimensions• Performance testingt

Checking weight
for excessive burrs• Confirming temperatureand time• Foaming condition• Temperature control
of press machine

Water temperature
control26

Floats

4 7 Internal structure of float

4 8 Multiple float design

To create a level switch, insert a magnet inside a float by some means.
A float not fitted with a magnet obviously has no magnetic force to function as a level switch to turn areed switch in the stem ON or OFF.
The magnet is most often mounted not in the center, but on one side, to enable the float to be “reversible”(meaning the direction of operation can be changed). This type is becoming more popular.
(However, not all floats with magnets mounted on one side are reversible, as this is dependent upon
the distance between the stoppers.)

Refer to the following examples when designing your system.

)

φ25.0

A float

B floatC float

B float

40 mm minimum

4

40

RFS-2

Sg=0.70

Sg=0.70
ON

C float

40 mm minimum

ON

RFS-2

40 mm minimum

12
27

42

Sg=0.62
ON

ON

φ10.4

φ28

5

φ9.5

RFS-6
71

Sg=0.55
RFS-8

4

RFS-8 float

Sg=0.62

Sg=0.55
RFS-8

ON
30

ON

57 mm minimum

Note

RFS-6
45

■ Structure of RF-2 (NBR float)

Because the individual air cells
are totally enclosed, very little
water penetrates. Even ifpunctured with a through-hole,buoyancy is unaffected and it
will not sink.

57

φ43

RFS-2 float

27

ON

28

φ26.5

Sg=0.47
ON

C float

4

40

4
φ9

Sg=0.70

Sg=0.70

20.5

φ20

φ9

ON

B float

21

φ8.5

ON

25

20

25

9

14

12

Sg=0.47

40■ Examples of internal structure of float (the magnet is indicated as

In the design of a level switch, a single float presents no problem. But when using two or more floats
requiring more than two contact points, allow sufficient operating distance; otherwise, malfunctions
will result or the design will be impossible to manufacture.

27

Chapter4

71 mm minimum

All these examples show the same float in the same orientation of operation.
Because the operating distance differs with the orientation of operation, be sure
to prepare a sample for testing.

28

Chapter4

Floats

4 9 Design considerations
1

Confirm that the clearance (gap) between the internal diameter of the float and the outer diameter ofthe stem is appropriate. At least 1 mm of clearance is required, as a small amount of dirt or the likecan prevent the float from moving.

Postscript
As indicated in the title, this book is presented as a general reference containing only basic information. For more information about each component,

2

Confirm that the internal diameter of the float can accommodate the stopper. If the internal diameterof the float and external diameter of the stopper are the same, the stopper will bind on the internal
diameter of the float as the liquid level rises and falls, and the switch will not operate properly.

please refer to the catalogs and other relevant documents.
Note that the contents of this publication are subject to change in the interestsof technical progress.

3

4

Confirm whether the distance between the float and the stopper’s ON area and OFF area are appropriate.
Normally allow a distance of 3–4 mm for the ON side and 4–5 mm for the OFF side.
When the operation distance of the float is long, confirm the operation at three points.
If the float operates over a long distance (distance from one stopper to the other), use a thresholdvalue without three-point operation when selecting the reed switch.

Please utilize this publication as an introductory reference as needed, as it
contains limited information about float-type level switches.
Note: All model names, specific gravity values, and information related to operational orientationof level switches and floats in this publication refer to products of Riko Float Technology
Co., Ltd. In addition, information on reed switches and magnets has been extracted fromthe catalogs and websites of the respective manufacturers.

5

Confirm whether the specific gravity of the liquid is lighter than the specific gravity of the float,
and use a float with a specific gravity at least 0.1 less than that of the liquid. For example,
assuming a float with a specific gravity of 0.7, the usable liquid must have a specific gravity of atleast 0.8 to avoid a malfunction of float movement.

6

Confirm that the materials used for the float material and the stem are compatible.
Determine the composition of each component, including the float, in consideration of the service
environment. Be sure to avoid situations leading to comments such as “the float is adapted toservice, but the stem does not last” or “the stoppers don’t last.”(Consider not only resistance to liquid but also to temperature and pressure.)

7

Confirm whether the liquid surface is still. In locations where the service environment for the levelswitch is characterized by an unstable liquid surface, use the hysteresis type (for example, instead of
the usual 1 mm distance for operation switchover from OFF to ON or ON to OFF, use around 5 mm);otherwise, the switch will start chattering (rapidly and repeatedly switching between ON and OFF
states), leading to breakdown or malfunctions by stressing the reed switch.

Basics of Float-Type Level SwitchesFirst edition published April 1, 1990.
Seventh edition published May 28, 2014.

2930