Nokhen Sensors - Frequently asked Questions

Level sensors are devices that measure a material level directly using a physical property of the measured material. This can be said for both continuous level monitoring (level measurement of a range where the level changes) and point level detection (level detection at certain points such as a high limit and a low limit within the range where the level changes). For example, float sensors and pressure transmitters use the specific gravity of the measured liquid, capacitive sensors capacitance, and ultrasonic sensors the speed at which the ultrasound travels in the medium, to report a level. In solid applications, solid shapes, bulk density, angle of repose and moisture content are important factors. These properties may change significantly depending on the application conditions, posing challenges to level measurement.

Liquid level sensors are the sensors that measure the level of the following:
- liquid surface ? the interface between the gas phase and the liquid phase
- interface between two liquids, such as water and oil
- interface between liquid phase and solid phase, such as the bottom of septic tanks and crude oil tanks.
Measured liquids vary greatly. Some contain single and others multiple components, and have different density, viscosity, or dielectric constant. They may also be highly viscous, sticky, or muddy, have foam on the surface or tend to crystalize. These properties require special attention in selecting a level sensor.
Process conditions such as the material behavior, pressures and temperatures in a vessel are important factors that affect measurement accuracy. Other conditions that need to be checked include:
- rate of level change when filling/discharge vs. sensor response time,
- surface turbulence, vortex or bubbles resulting from agitation,
- vibrations.
For applications where high accuracy measurement is required, precautions have to be taken such as:
- not mounting close to inlet or outlet,
- providing a guide pipe or stillpipe so that high flow does not carry away the sensor.

When selecting a level sensor, the physical properties of the material to be measured have to be carefully checked.
Sensors that utilize the specific gravity include float sensors and pressure transmitters. Change in the specific gravity leads to measurement error. Level sensors, however, are typically used under control conditions where specific gravity does not change so greatly. In such cases level sensors programmed for the specified specific gravity will work flawlessly.
Under conditions different from normal operating conditions, such as commissioning of the plant, however, cares must be taken as the sensors may produce measurement errors.

Capacitive level sensors use dielectric constant to measure the level. Dielectric constant of water is 88 at 0℃, and 48 at 100℃. Dielectric constant is sensitive to temperature changes; when the temperature rises, the dielectric constant falls. When using capacitive level sensors, therefore temperature conditions are a key to successful measurement. Generally speaking, once the process conditions are determined, the temperature change is small enough to pose no issues to capacitive sensors. Even minor temperature change, however, requires attention if the material is sticky or with low dielectric constant, or high accuracy measurement is required.

Solid level sensors measure the solid level. Solids have many properties that are totally different from liquids, and some of them are not known well to this day. Solid level sensors have been, and still are being developed through experiments and experience.

Angle of repose is one of the properties of solids. When solids are poured onto a horizontal surface, a conical pile will form. The internal angle between the surface of the pile and the horizontal surface is the angle of repose. The smallest angle at which solids slide is called the slip angle, and the slip angle is always smaller than the angle of repose.
The angle of repose varies with particle sizes, moisture contents, and particle shapes. Generally speaking, the more moisture the solids contains, the smaller the angle of repose will be. Also, the smaller the particle is, the larger the angle of repose will be.

In water, pressure exerted on a point is same in all directions. In solids, when a pressure is exerted, the smaller pressure than the original pressure is exerted on all directions, and the smallest is in the vertical direction. Pressures exceeding a certain level cause the solid to slide. The pressure applied to the vessel bottom becomes the largest when the material depth equals to 3 to 5 times the inner diameter. At lower levels, most of the solid mass is exerted on the wall and the pressure applied to the vessel bottom does not increase.

Solids tend to clump.
Solids are generally discharged by gravity flow. Even low viscosity solids can clump above the outlet forming a bridge, and discharge flow will stop if the outlet is small. Sticky solids can adhere on the wall close to the tank bottom, leaving an opening only right above the outlet. These phenomena are called “bridging” and “rat holing”. High fluidity solids of particle sizes smaller than 1 mm will not form a bridge if the outlet is at least 6 times larger than the particle size.

To select the sensor best suited for your application, you need to make clear:
- what you want to measure and why;
- which information you are going to collect;
- what you are going to do with the collected information and how you want it to be processed.
Measured material can vary from liquids to solids to granular materials, and the purposes of measurement include measurement of accepted or delivered amount, process monitoring or control, or production or storage management.
Output capability of the sensor is also important to gain necessary information from the level sensor. Level sensors may:
1) only have local display;
2) operate with a transmitter;
3) calculate the output signals and convert them for other types of signals for recording, triggering alarms or controlling other devices.
Using the best matching sensor to your application is the key to successful measurement.

Explosion protection is to prevent electrical equipment from causing explosion. Sensors that have explosion protection approvals are designed not to ignite explosive gases, vapors or dusts in the environment. Explosion protection designs are regulated by the Industrial Safety and Health Law.

Explosive gases, vapors or dusts explode under a certain conditions.

Because electric equipment generates sparks and high temperatures, which can ignite surrounding explosive atmosphere.

Explosion protection is divided into two categories: gas explosion protection and dust explosion protection.

To prevent electric equipment installed in industrial areas where explosive gases or vapors exit, or may exist, from igniting the atmosphere. Japan developed two standards for gas explosion protection. One is the conventional system in Japan, and mainly applied to products made in Japan. The other takes into account international standards, and mainly applied to products manufactured overseas.

To prevent electric equipment installed in industrial areas where explosive dusts exit, or may exists, from igniting the atmosphere.

There are 6 types: flameproof enclosure (d), oil immersion (o), pressurized apparatus (p), increased safety (e), intrinsic safety (i), and special protection (s). Intrinsic safety is divided into two types, “ia” and “ib”, depending on the protection level. “ia” is higher in protection level than “ib”. Indication of “i” only generally means “ia”.

Flameproof enclosure encloses the parts that could ignite an explosive atmosphere. The enclosure is designed to withstand the pressure applied to the housing when the explosive gases or vapors inside are ignited, preventing ignition to an explosive atmosphere outside the housing.

With this type of protection, the parts that could ignite an explosive atmosphere are immersed in oil, thus preventing ignition to an explosive atmosphere.

Pressurized apparatus is designed such that penetration of an explosive atmosphere into the housing of electrical equipment is prevented by an ignition preventing gas (clean air or inert gas).

With this type of protection, equipment is designed such that sparks or high temperatures are not generated on parts or components under normal operating conditions, thus increasing safety of the equipment.

With this type of protection, equipment is designed such that ignition of an explosive atmosphere is prevented by limiting the current, voltage, power and temperature under normal operating conditions or in the event of errors. The equipment has to be tested by a notified body.

With this type of protection, equipment is certified by a notified body that the ignition of an explosive atmosphere is prevented in a way different from the flameproof enclosure, oil immersion, pressurized apparatus, increased safety or intrinsic safety.

The explosive gases are categorized by the explosion groups and the temperature classes.

The explosion groups are related to the maximum experimental safe gap. The maximum experimental safe gap is the gap at which a flame flashover just no longer takes place in a test vessel with a gap length of 25 mm. The maximum safe gap and the corresponding explosion group are shown below.
Maximum safe gap Explosion group
> 0.6 1
0.4 to 0.6 2
< 0.4 3

The explosion group 3 is further divided into 4 groups. Below are the groups and the gases that belong to each group. 3a: Water gas (made by passing steam over a red-hot carbon fuel heated to higher temperatures than 1000℃) and hydrogen 3b: Carbon disulfide 3c: acethylene 3n: all gasses in the explosion group 3

The temperature class is related to the maximum allowed surface temperature of equipment. The temperatures are divided into 6 groups based on the lowest temperature of a hot surface at which ignition of the gas/air or vapor/air mixture takes place.

The hazardous areas are classified based on how long and how often the explosive atmosphere is present in the area.

Zone 0
Areas in which dangerous concentrations of flammable gases/vapors are present continuously or long-term under normal operation conditions.
Zone 1
Areas in which dangerous concentrations of flammable gases/vapors may be present under normal operating conditions.
Zone 2
Areas in which dangerous concentrations of flammable gases/vapors may be present under faulty operating conditions.

Following are the hazardous area class and the required types of protection.
Zone 0: intrinsic safety (ia)
Zone 1: intrinsic safety, flameproof enclosure, pressurized apparatusv
Zone 2: intrinsic safety, flameproof enclosure, pressurized apparatus, increased safety, oil immersion

There are 6 types : flameproof enclosure (d), oil immersion (o), pressurized apparatus (p), increased safety (e), intrinsic safety (i), special protection (s).
They are the same as those in the conventional Japanese system.

The gases are classified based on the maximum safe gap.

The gases are classified based on the minimum ignition current ratio.

The temperature are calcified based on the maximum permissible surface temperature.

See the chart below.

See the chart below.

The hazardous areas are classified based on how long and how often the explosive atmosphere is present in the area, in the same manner as the conventional system.
Zone 0
Areas in which dangerous concentrations of flammable gases/vapors are present continuously or long-term under normal operation conditions.
Zone 1
Areas in which dangerous concentrations of flammable gases/vapors may be present under normal operating conditions.
Zone 2
Areas in which dangerous concentrations of flammable gases/vapors may be present under faulty operating conditions.

Following are the hazardous area class and the required types of protection, which is same as the conventional system.
Zone 0: intrinsic safety (ia)
Zone 1: intrinsic safety, flameproof enclosure, pressurized apparatus
Zone 2: intrinsic safety, flameproof enclosure, pressurized apparatus, increased safety, oil immersion

There are 3 types: SDP, DP, and XDP.

With this type of protection, the enclosure is sealed and the connections of the enclosure are equal to or deeper than a specified value, or have a packing that is equal to or deeper than the specified value, to ensure no dust enters into the enclosure.

With this type of protection, the enclosure is sealed and the connections of the enclosure are equal to or deeper than a specified value, or have a packing, to ensure no dust enterss into the enclosure.

Products with this type of protection prevents ignition of dust in a way different from SDP or DP, and certified by a notified body.

Explosive dusts can cause deflagration or dust explosion.

Dusts such as metal particles that can explode when suspended in atmosphere without much oxygen or in carbon dioxide tend to cause deflagration.

Explosive dusts that cause exothermic reaction with oxygen in the air tend to cause dust explosion. These dusts include nonconductive dusts such as flour, starch, sugar, synthetic resin and chemicals, as well as conductive dusts such as carbon black, corks, iron, and copper.

The temperature classes are determined based on the ignition temperature.

Dust hazardous areas include industrial areas where explosion capable amount of dust particles are suspended in the air, or explosive dusts are accumulated and may become suspended.

Dust hazardous areas divided into two groups based on the type of explosive dusts: deflagration hazardous areas and dust explosion hazardous areas. Areas where explosive dusts are accumulated and may become suspended include where:
1) explosive dusts are continuously, periodically, or intermittently suspended or accumulated under normal operating condition;
2) explosive dusts are or may be suspended or accumulated under faulty operating conditions such as device failure or erratic operation.

SDP products are required in the deflagration hazardous areas, and XDP or DP products are required in the dust explosion hazardous areas.

Tilted float switches are used to detect the presence of water or wastewater at a specified level, and give alarm or control pumps. They are mostly installed in sewage water or wastewater detecting applications. They are suitable also for slurries and high viscosity liquids, offering flexible adaptation to a wide range of applications.
Main features include easy installation and on-site adjustment capability, and also reliable operation without being affected by bouncing due to turbulence or high flow.
(NOHKEN products: Cable Suspended Float Sensor FQ)

Ball float switches are a level switch mounted horizontally on the tank, whose floats are almost in the shape of a ball. They use a microswitch or a reed switch to detect liquid presence.
(NOHKEN products: Magnetic Level Switch FM, HM)

The guided float sensors have a float with a magnet and a pipe with a magnetic sensing element. The sensing element in the pipe detects the magnetic field from the magnet inside the float. Guided float sensors and resistive level sensors use a reed switch as the sensing element, and magnetostrictive sensors use a magnetostrictive wire.
(NOHKEN products: Resistive level measurement LR, Magnetostrictive level measurement MS)

Resistive level sensors have a pipe and a float that rises and falls on the pipe. The pipe incorporates resistors and reed switches, and the float a magnet. The reed switch closes its contacts when the float approaches, indicating a level detection.
(NOHKEN product:s: Resistive level measurement LR)

Magnetostrictive level sensors are a level sensor that has relatively recently been put into commercial operation. The sensor operates based on the Wiedemann effect, and accurately detects the magnet position in the float.

When a current pulse is passed through a magnetostrictive wire, a magnetic field encompasses the wire. If this wire is placed in an external magnetic field, the interaction generates a strain pulse that travels down the wire. This is called the Wiedemann effect.
The speed of strain pulse is proportional to the distance the pulse travels. Magnetostrictive sensors use this phenomena, and measures the elapsed time over and over again, thus offering highly accurate level measurement.

Guided float switches are typical float stiches that use a float to detect liquid presence at specified levels, and give alarms or control solenoid valves and pumps. They are suitable for chemicals as well as water and oil.
(NOHKEN products: Magnetic Float Sensor FR, Miniature Float Sensor OL, LS, SH)

Pressure transmitters measure water pressure exerted on the tank bottom to know the liquid level. If the specific gravity of the liquid is constant, the pressure exerted on the tank bottom (reference point) is proportional to the liquid level.
If the tank is pressurized from inside, the differential pressure needs to be measured to correct the affection of inner pressure. Pressure transmitters include air bubbler systems, bellows pressure detectors, differential pressure transmitters and hydrostatic sensors.

Air bubbler sensors have an air purge pipe that is inserted into the tank. The sensor continuously supplies air (or inert gas) via the tube until the back pressure equals to the liquid pressure exerted on the end of the tube. The sensor then transfers the pressure to the differential or pressure transmitter to report a level.
Air bubbler sensors are simple and easy to install. The differential (pressure) sensor is not affected by process conditions.
(NOHKEN products: Air Bubbler System LA)

Bellow pressure detectors use a bellows and an indicator near the bottom of the vessel to report the level. The sensor is of very simple design, but can also report volume or mass by using a scale that have scales for such values.

Differential pressure sensors measure the difference between two pressures.
When the specific gravity of liquid in a vessel is constant, pressure exerted on a point is proportional to the liquid level. Differential pressure sensors compare the pressure above and below the liquid to report the level.

Force balance pressure sensors convert pressure into force by using a sensing element such as a diaphragm and a bellows. A balancing force is generated to exactly cancel the process pressure’s force.
Many of the differential pressure sensors are force balance sensors because of their performance and easy maintenance. Most pneumatic transmitters are also force balance sensors.

Motion balance pressure sensors are mechanical sensors such as Bourdon tubes and bellows. Since they do not need power or air supply, they are used for a local level display or a differential switch.

Development in displacement sensor, electric circuit and mechanical engineering made it possible to produce displacement differential pressure sensors. Currently 3 types of theses sensors are put into commercial operation: capacitance type, semiconductor strain gauge type, and silicone resonant type.
Features include:
- compact,
- remote installation poss
ible,
- high performance, - high accuracy measurement resulting from simple design.

Capacitance pressure sensors have diaphragms at high and low pressure sides, and stationary and moving electrodes. Pressure exerted on the diaphragms cause the moving electrode to move. The sensor measures change in capacitance between the two electrodes, and convert the capacitance to electric signals.
(NOHKEN products: Air Bubbler System LA)

Semiconductor strain gauge pressure sensors convert pressure into resistance (piezoresistive effect), and send electric signals. These sensors generally have a differential pressure sensor, a static pressure sensor and a temperature sensor mounted on the same silicone chip.
(NOHKEN product: Air Bubbler System LA)

Silicone resonant pressure sensors were put into commercial operation in early 1990’s. Pressure exerted on the seal diaphragm is transferred via encapsulated silicone oil to the sensor assembly. The sensor assembly incorporates a diaphragm chip made of single-crystal silicon. The single-crystal silicone has a vacuum chamber, where an H-type resonator vibrates at its natural frequency. The measured pressure is exerted on the diaphragm chip, and strains the diaphragm. The degree of strain depends on the amount of pressure. The strain causes change in H-type resonator’s tension, changing vibration frequency. The incorporated microprocessor processes the change in vibration frequency, and a current output is given.
(NOHKEN product: Air Bubbler System LA)

Hydrostatic pressure sensors have a detector and cable or chain. The detector is suspended or placed under water in the tank, and measure liquid pressure to report the level. The sensors are used in a wide range of applications including distribution reservoirs, dams, tanks, effluent streams, sewers, sumps, rivers and oceans.
Installation is easy without any special construction.
(NOHKEN products: Hydrostatic Level Measurement PL, DR)

Capacitive level switches are comprised of a measurement electrode, an earth electrode and an insulator. They detect capacitance between the two electrodes to determine material presence. Capacitive switches can measure virtually any liquids, solids and bulk solids, so they are used in a wide range of applications.
(NOHKEN product: Capacitive Level Sensor K)

Capacitive proximity sensors have a detector comprised of measurement electrode only. They detect change in capacitance between the electrode and the measured material to determine material presence.

Capacitive level sensors are comprised of two mutually insulated electrodes, a measurement electrode and an earth electrode. The earth electrode is electrically continued to the metallic tank. The sensor measures change in capacitance between the measurement electrode and the tank wall to determine material presence.
(NOHKEN product: Capacitive Level Sensor CM)

Detector-type capacitive level sensors can measure level of liquids and solids, and the interface level of two different liquids.
The sensors are comprised of a plastic pipe, a detector assembly, and a transmitter. The pipe, in which the detector assembly is suspended with a coaxial cable, is inserted in the tank. The detector assembly incorporates a measurement electrode, and moves up or down as the level or the interface rises and falls. The transmitter controls the detector assembly, and also gives an output to report the level. As the detector assembly moves in the plastic pipe, affection from buildup, chemicals or gas is limited.
(NOHKEN product: Capacitive Level Sensor CL)

Electrode level sensors have electrodes to detect the level. When the measured material reaches the electrodes, electric current flows between the electrodes. Due to the simple design and easy assembling of electrodes as well as available various electrode materials, electrode sensors are used in a wide range of applications including water purifying plants and wastewater treatment facilities.
(NOHKEN products: Electrode Level Sensor FE, CE (electrode band type))

Conductivity level sensors can measure virtually any conductive liquids, and one of the most simply designed sensors. The sensor has an electrode. When the material reaches the electrode, an electric circuit is formed and current flows. The sensor is widely used in steel, food, chemical, and pharmaceutical industries, and agricultural water, water purifying plants, and wastewater treatment facilities.
Conductivity sensors are divided into two groups depending on electrode location: electrode switches and leak sensors.

Leak sensors are a type of conductivity sensors. Leak sensors detect presence of current flow between the electrode and the earth. They can accommodate up to 10 electrodes on their single bar-shaped body, enabling measurement in small spaces. They are widely used in wastewater treatment facilities.
(NOHKEN product: Electrical Conductivity Level Sensor MT10)

Through-beam level sensors are comprised of a transmitter and a receiver. The transmitter transmits microwave and the receiver receives it. Attenuation indicates the material presence.

Plum bob level sensors detect the level mechanically. A weight and cable is lowered in the tank. The sensor detects the change in cable tension when the weight reaches the material surface, and gives a signal output proportional to the distance the weight have descended.

Rotating paddle level sensors are mechanical solid level sensors that uses a rotating paddle. When the paddle rotation/reciprocation is hindered by the material, the sensor reports material detection.
There are two types of rotating-paddle level sensors. One has a vane that rotates, and the other has a circular reciprocating-puddle. The two types have features in common and are used in same or similar kinds of applications.

Rotating-vane level sensors are a mechanical solid level sensor with a rotating vane, and used to be one of the most common level sensors. They are insensitive to measured materials or ambient conditions. Easy sensitivity adjustment makes them an ideal solution for a wide range of application.
(NOHKEN products: Rotating-puddle Level Sensor R7, RB)

Reciprocating-puddle level sensors are a type of rotating-puddle level sensors. The sensor reports a level when reciprocation is hindered by the measured material.
The detection elements include a motor, paddle, shaft, and spring. These components are in linkage mechanism. The motor rotation is converted to reciprocation, and the shaft and the paddle slowly repeat reciprocation. Without the material around the paddle, the spring is not pressed. With the material around the paddle, the paddle reciprocation is hindered and stops. After reciprocation stops, the motor keeps rotating, pressing the spring, causing the microswitch to send a signal and the motor to stop rotation.
When the material level falls and the paddle is no longer covered, the spring returns to its former shape This results in the linkage mechanism and the microswitch returning to their former state. The paddle then resumes reciprocation.
(NOHKEN products: Reciprocating-puddle Level Sensor C5/B3 (Bin leveler))

Vibrating level sensors have a vibrating detector. They use the difference in vibration between with and without the material covering the detector. These sensors include tuning fork, vibrating rod, and vibrating plate sensors.

Tuning fork level sensors have a piezoelectric crystal inside the detector that causes the fork to oscillate at a frequency. When the fork is immersed in water, the oscillation frequency changes. The crystal detects the change in frequency to report level detection.
(NOHKEN products: Liquid Fork Sensor SG, Mini-squing SG10)

Vibrating rod level sensors have a rod to detect material presence. When the rod is covered by the material, the incorporated relay changes states, indicating level detection.
The detection assembly includes a rod, a vibration plate, a permanent magnet and an electromagnet. The rod and vibration plate form a kind of cantilever. The electro magnet keeps attracting and repelling the permanent magnet mounted on the vibrating plate, causing the detection rod to vibrate. When the detection rod is covered by the material, the rod vibration is attenuated. This causes change in current flowing in the electromagnet. The sensor detects the change in current and reports level detection.
The vibrating rod sensors are suitable for detection of solids, granular materials, bulk solids, and sludge blanket at a desired level, and trigger an alarm or control solenoid valves and other devices.
(NOHKEN products: Vibrating Level Sensor VL, High-Sensitive Vibrating Level Sensor VH (piezoelectric crystal is supported at two places), Vibrating Level Sensor VM (with acceleration pickup)

Vibrating plate level sensors are compact, lightweight and flush mounted sensors with a vibrating plate. Two piezoelectric crystals are mounted on the plate. Applying voltage pulse to the crystals causes the plate to strain, followed by ringing. The ringing attenuates as the time passes, slowly with the material and rapidly without material on the plate. The sensor uses the speed of ringing attenuation to determine material presence.
These sensors are suitable for small hoppers or integration into other machines.
(NOHKEN products: Vibrating Level Sensor VP)

Optical level sensors have two optical semiconductors, a transmitter and a receiver, in a pipe, and measure the level or interface. Modulated infrared is used for the light source to eliminate affection from contamination on the pipe liquids with low clearness, or external light.
(NOHKEN product: Optical Interface Measurement Sensor OX100)

Optical continuous level sensors have two optical semiconductors, a transmitter and a receiver, in a pipe, and continuously measure the level or interface. Modulated infrared is used for the light source to eliminate affection from contamination on the pipe liquids with low clearness, or external light.
(NOHKEN product: Optical Interface Measurement Sensor OX100)

Optical level switches have two optical semiconductors, a transmitter and a receiver, and measure light amplitude between the semiconductors. The light amplitude is at its maximum when no material is between the semiconductors. When the material reaches the semiconductors, the light is attenuated or blocked. The sensors amplify the photoelectric current to operate the relay circuit, and indicate level detection. Optical level switches are suitable for interface measurement.
(NOHKEN products: Optical Interface Measurement Sensor OX100)