Force sensors | isweek - industry sourcing & wholesale industrial products

Force sensors are initially rigid links between two shapes that are able to measure transmitted forces and torques. The rigidity of force sensors is conditional, in the sense that force sensors can be broken if a certain condition arises (e.g. if a force or torque threshold is overshot). Following figure illustrates an application using a force sensor:

[Force sensor object (green) measuring the force and torque exerted by a beam (blue) anchored in a wall (red)]

A force sensor measures a pair of 3 values representing the force on the sensor along the x-, y- and z-axis, and the torque on the sensor about the x-, y- and z-axis:

[Forces and torques measured by a force sensor]

        Initially, a force sensor acts as a rigid link. During simulation, a force sensor can however be broken when a specified force/torque threshold is overshot or when some other user-defined conditions are met. Following figures illustrate the broken state of a force sensor:

[Broken force sensor]

A force sensor is only operational during simulation if it is dynamically enabled. For more information on dynamically enabled force sensors, refer also to the section on designing dynamic simulations. Joints are also able to measure a force or a torque, however only along/about their z-axis.

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Gas Sensors Penetrate Smartphones | ISweek - industry sourcing & wholesale industrial products

NAPA, Calif.—The world's first gas sensor small enough for any smartphone was shown at the MEMS Executive Congress 2015 (held here, Nov. 4-6). Manufactured by Cambridge CMOS Sensors Ltd. (U.K.), the tiny 1 millimeter square MEMS-CMOS die are small and cheap enough to become ubiquitous—for the first time beating Apple in new types of MEMS sensors in smartphones.

Cambridge also announced its first design-win in K-Free Wireless Ltd. (Shenzhen, China). Carriers apply their own name to K-Free's white-box smartphones.

Only Cambridge CMOS Sensors claims to have a Microelectromechanical system complementary metal oxide semiconductor (MEMS-CMOS) hotplate-based gas sensors that can scale to smaller sizes along with the International Technology Roadmap for Semiconductors (ITRS) using the top metal oxide to tuned it to sense specific gases after being heated up by the hotplate below it.
(Source: Cambridge CMOS Sensors, used with permission)

Cambridge CMOS Sensors' design win at K-Free one-ups Apple with its new all-digital construction of a MEMS-CMOS CCS811 sensor. The sensor can be configured to sense volatile organic compounds (VOC) such as carbon monoxide from cheap heaters, formaldehyde in cheap furniture, or even used as a breathalyzer since driving-drunk is a jail-sentence offense in China, according to Brown.

Announcing today its MEMS-CMOS design-wins with K-Phone and K-Free white-box smartphone that will be branded by Chinese carriers for their home markets where sensing dangerous chemicals in the air is critical to one's health as they move about the incredibly polluted cities, homes and businesses.
(Source: Cambridge CMOS Sensors, used with permission)

In the U.K., a relatively new type of sealed house construction practice—putting plastic under the entire house, including the actual foundation—has opened a market for sensing noxious gas build-ups inside houses where there is no ongoing ventilation.

For other worldwide markets, the all-digital CCS811 sensor can be configured with different top-metal oxides or filters to detect only CO2, only ethanol, or nearly any other noxious gas. It can also be configured to measure the outside air quality, including nitrogen dioxide [NO2], in cities from Beijing to Los Angeles.

"It can tell you when to open window for ventilation while inside where very low levels of VOCs have been found to make people's minds 61 percent less efficient in decision making," claimed Brown.

But the biggest benefit of the MEMS-CMOS gas sensor, according to Brown, is that it can be scaled continually as the International Technology Roadmap for Semiconductors (ITRS) scaled silicon chips to smaller and smaller sizes.

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CO2 sensor (Carbon dioxide sensor) -- isweek.com is industry sourcing & wholesale industrial products

A carbon dioxide sensor or CO2 sensor is an instrument for the measurement of carbon dioxide gas. The most common principles for CO2 sensors are infrared gas sensors (NDIR) and chemical gas sensors. Measuring carbon dioxide is important in monitoring indoor air quality, the function of the lungs in the form of a capnograph device, and many industrial processes.

NDIR sensors are spectroscopic sensors to detect CO2 in a gaseous environment by its characteristic absorption. The key components are an infrared source, a light tube, an interference (wavelength) filter, and an infrared detector. The gas is pumped or diffuses into the light tube, and the electronics measures the absorption of the characteristic wavelength of light. NDIR sensors are most often used for measuring carbon dioxide. The best of these have sensitivities of 20–50 PPM.[1] Typical NDIR sensors cost in the (US) $100 to $1000 range.

New developments include using microelectromechanical systems to bring down the costs of this sensor and to create smaller devices (for example for use in air conditioning applications). NDIR CO2 sensors are also used for dissolved CO2 for applications such as beverage carbonation, pharmaceutical fermentation and CO2 sequestration applications. In this case they are mated to an ATR (attenuated total reflection) optic and measure the gas in situ.

Another method (Henry's Law) can be also be used to measure the amount of dissolved CO2 in a liquid, if the amount of foreign gases is insignificant.

Chemical CO2 gas sensors with sensitive layers based on polymer- or heteropolysiloxane have the principal advantage of a very low energy consumption, and can be reduced in size to fit into microelectronic-based systems. On the downside, short- and long term drift effects as well as a rather low overall lifetime are major obstacles when compared with the NDIR measurement principle.Most CO₂ sensors are fully calibrated prior to shipping from the factory. Over time, the zero point of the sensor needs to be calibrated to maintain the long term stability of the sensor.

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Fiber optic sensor | isweek.com - industry sourcing & wholesale industrial products

A fiber optic sensor is a sensor that uses optical fiber either as the sensing element ("intrinsic sensors"), or as a means of relaying signals from a remote sensor to the electronics that process the signals ("extrinsic sensors"). Fibers have many uses in remote sensing. Depending on the application, fiber may be used because of its small size, or because no electrical power is needed at the remote location, or because many sensors can be multiplexed along the length of a fiber by using light wavelength shift for each sensor, or by sensing the time delay as light passes along the fiber through each sensor. Time delay can be determined using a device such as an optical time-domain reflectometer and wavelength shift can be calculated using an instrument implementing optical frequency domain reflectometry.

Fiber optic sensors are also immune to electromagnetic interference, and do not conduct electricity so they can be used in places where there is high voltage electricity or flammable material such as jet fuel. Fiber optic sensors can be designed to withstand high temperatures as well.

Optical fibers can be used as sensors to measure strain,temperature, pressure and other quantities by modifying a fiber so that the quantity to be measured modulates the intensity, phase, polarization, wavelength or transit time of light in the fiber. Sensors that vary the intensity of light are the simplest, since only a simple source and detector are required. A particularly useful feature of intrinsic fiber optic sensors is that they can, if required, provide distributed sensing over very large distances.

Temperature can be measured by using a fiber that has evanescent loss that varies with temperature, or by analyzing the Raman scattering of the optical fiber. Electrical voltage can be sensed by nonlinear optical effects in specially-doped fiber, which alter the polarization of light as a function of voltage or electric field. Angle measurement sensors can be based on the Sagnac effect.

Special fibers like long-period fiber grating (LPG) optical fibers can be used for direction recognition. Photonics Research Group of Aston University in UK has some publications on vectorial bend sensor applications.

Optical fibers are used as hydrophones for seismic and sonar applications. Hydrophone systems with more than one hundred sensors per fiber cable have been developed. Hydrophone sensor systems are used by the oil industry as well as a few countries' navies. Both bottom-mounted hydrophone arrays and towed streamer systems are in use. The German company Sennheiser developed a laser microphone for use with optical fibers.

A fiber optic microphone and fiber-optic based headphone are useful in areas with strong electrical or magnetic fields, such as communication amongst the team of people working on a patient inside a magnetic resonance imaging (MRI) machine during MRI-guided surgery.

Optical fiber sensors for temperature and pressure have been developed for downhole measurement in oil wells.The fiber optic sensor is well suited for this environment as it functions at temperatures too high for semiconductor sensors (distributed temperature sensing).

Optical fibers can be made into interferometric sensors such as fiber optic gyroscopes, which are used in the Boeing 767 and in some car models (for navigation purposes). They are also used to make hydrogen sensors.

Fiber-optic sensors have been developed to measure co-located temperature and strain simultaneously with very high accuracy using fiber Bragg gratings.This is particularly useful when acquiring information from small or complex structures.Fiber Bragg grating sensors are also particularly well suited for remote monitoring, and they can be interrogated 250 km away from the monitoring station using an optical fiber cable. Brillouin scattering effects can also be used to detect strain and temperature over large distances (20–120 kilometers).

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Fiber optic sensor

A fiber optic sensor is a sensor that uses optical fiber either as the sensing element ("intrinsic sensors"), or as a means of relaying signals from a remote sensor to the electronics that process the signals ("extrinsic sensors"). Fibers have many uses in remote sensing. Depending on the application, fiber may be used because of its small size, or because no electrical power is needed at the remote location, or because many sensors can be multiplexed along the length of a fiber by using light wavelength shift for each sensor, or by sensing the time delay as light passes along the fiber through each sensor. Time delay can be determined using a device such as an optical time-domain reflectometer and wavelength shift can be calculated using an instrument implementing optical frequency domain reflectometry.

Fiber optic sensors are also immune to electromagnetic interference, and do not conduct electricity so they can be used in places where there is high voltage electricity or flammable material such as jet fuel. Fiber optic sensors can be designed to withstand high temperatures as well.

Optical fibers can be used as sensors to measure strain,temperature, pressure and other quantities by modifying a fiber so that the quantity to be measured modulates the intensity, phase, polarization, wavelength or transit time of light in the fiber. Sensors that vary the intensity of light are the simplest, since only a simple source and detector are required. A particularly useful feature of intrinsic fiber optic sensors is that they can, if required, provide distributed sensing over very large distances.

Temperature can be measured by using a fiber that has evanescent loss that varies with temperature, or by analyzing the Raman scattering of the optical fiber. Electrical voltage can be sensed by nonlinear optical effects in specially-doped fiber, which alter the polarization of light as a function of voltage or electric field. Angle measurement sensors can be based on the Sagnac effect.

Special fibers like long-period fiber grating (LPG) optical fibers can be used for direction recognition . Photonics Research Group of Aston University in UK has some publications on vectorial bend sensor applications.

Optical fibers are used as hydrophones for seismic and sonar applications. Hydrophone systems with more than one hundred sensors per fiber cable have been developed. Hydrophone sensor systems are used by the oil industry as well as a few countries' navies. Both bottom-mounted hydrophone arrays and towed streamer systems are in use. The German company Sennheiser developed a laser microphone for use with optical fibers.

A fiber optic microphone and fiber-optic based headphone are useful in areas with strong electrical or magnetic fields, such as communication amongst the team of people working on a patient inside a magnetic resonance imaging (MRI) machine during MRI-guided surgery.

Optical fiber sensors for temperature and pressure have been developed for downhole measurement in oil wells. The fiber optic sensor is well suited for this environment as it functions at temperatures too high for semiconductor sensors (distributed temperature sensing).

Optical fibers can be made into interferometric sensors such as fiber optic gyroscopes, which are used in the Boeing 767 and in some car models (for navigation purposes). They are also used to make hydrogen sensors.

Fiber-optic sensors have been developed to measure co-located temperature and strain simultaneously with very high accuracy using fiber Bragg gratings.This is particularly useful when acquiring information from small or complex structures.Fiber Bragg grating sensors are also particularly well suited for remote monitoring, and they can be interrogated 250 km away from the monitoring station using an optical fiber cable.[12] Brillouin scattering effects can also be used to detect strain and temperature over large distances (20–120 kilometers)


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Mass flow sensor -- industry sourcing, wholesale industrial products from isweek

A mass flow sensor (MAF) is used to find out the mass flowrate of air entering a fuel-injected internal combustion engine.

The air mass information is necessary for the engine control unit (ECU) to balance and deliver the correct fuel mass to the engine. Air changes its density as it expands and contracts with temperature and pressure. In automotive applications, air density varies with the ambient temperature, altitude and the use of forced induction, which means that mass flow sensors are more appropriate than volumetric flow sensors for determining the quantity of intake air in each cylinder. (See stoichiometry and ideal gas law.)

There are two common types of mass airflow sensors in use on automotive engines. These are the vane meter and the hot wire. Neither design employs technology that measures air mass directly. However, with additional sensors and inputs, an engine's ECU can determine the mass flowrate of intake air.

Both approaches are used almost exclusively on electronic fuel injection (EFI) engines. Both sensor designs output a 0.0–5.0 volt or a pulse-width modulation (PWM) signal that is proportional to the air mass flow rate, and both sensors have an intake air temperature (IAT) sensor incorporated into their housings for most post OBDII vehicles. Vehicles prior to 1996 could have MAF without an IAT. An example is 1994 Infiniti Q45.

When a MAF sensor is used in conjunction with an oxygen sensor, the engine's air/fuel ratio can be controlled very accurately. The MAF sensor provides the open-loop controller predicted air flow information (the measured air flow) to the ECU, and the oxygen sensor provides closed-loop feedback in order to make minor corrections to the predicted air mass.

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Electrochemical gas sensor -- industry sourcing,wholesale industrial products,ndir co2 sensor | isweek.com

Electrochemical gas sensors are gas detectors that measure the concentration of a target gas by oxidizing or reducing the target gas at an electrode and measuring the resulting current.

Beginning his research in 1962, Mr. Naoyoshi Taguchi became the first person in the world to succeed in the development of a semiconductor device which could detect low concentrations of combustible and reducing gases when used with a simple electrical circuit. Devices based on this technology are often called "TGS" (Taguchi Gas Sensors).

The sensors contain two or three electrodes, occasionally four, in contact with an electrolyte. The electrodes are typically fabricated by fixing a high surface area precious metal on to the porous hydrophobic membrane. The working electrode contacts both the electrolyte and the ambient air to be monitored usually via a porous membrane. The electrolyte most commonly used is a mineral acid, but organic electrolytes are also used for some sensors. The electrodes and housing are usually in a plastic housing which contains a gas entry hole for the gas and electrical contacts.

The gas diffuses into the sensor, through the back of the porous membrane to the working electrode where it is oxidized or reduced. This electrochemical reaction results in an electric current that passes through the external circuit. In addition to measuring, amplifying and performing other signal processing functions, the external circuit maintains the voltage across the sensor between the working and counter electrodes for a two electrode sensor or between the working and reference electrodes for a three electrode cell. At the counter electrode an equal and opposite reaction occurs, such that if the working electrode is an oxidation, then the counter electrode is a reduction.

The magnitude of the current is controlled by how much of the target gas is oxidized at the working electrode. Sensors are usually designed so that the gas supply is limited bydiffusion and thus the output from the sensor is linearly proportional to the gas concentration. This linear output is one of the advantages of electrochemical sensors over other sensor technologies, (e.g. infrared), whose output must be linearized before they can be used. A linear output allows for more precise measurement of low concentrations and much simpler calibration (only baseline and one point are needed).

Diffusion control offers another advantage. Changing the diffusion barrier allows the sensor manufacturer to tailor the sensor to a particular target gas concentration range. In addition, since the diffusion barrier is primarily mechanical, the calibration of electrochemical sensors tends to be more stable over time and so electrochemical sensor based instruments require much less maintenance than some other detection technologies. In principle, the sensitivity can be calculated based on the diffusion properties of the gas path into the sensor, though experimental errors in the measurement of the diffusion properties make the calculation less accurate than calibrating with test gas.

For some gases such as ethylene oxide, cross sensitivity can be a problem because ethylene oxide requires a very active working electrode catalyst and high operating potential for its oxidation. Therefore gases which are more easily oxidized such as alcohols and carbon monoxide will also give a response. Cross sensitivity problems can be eliminated though through the use of a chemical filter, for example filters that allows the target gas to pass through unimpeded, but which reacts with and removes common interferences.

While electrochemical sensors offer many advantages, they are not suitable for every gas. Since the detection mechanism involves the oxidation or reduction of the gas, electrochemical sensors are usually only suitable for gases which are electrochemically active, though it is possible to detect electrochemically inert gases indirectly if the gas interacts with another species in the sensor that then produces a response.Sensors for carbon dioxide are an example of this approach and they have been commercially available for several years.

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【industry sourcing】IPHONE 6 BAROMETER SENSOR FEATURES | isweek.com

Apple has equipped it’s latest flagships, the iPhone 6 and iPhone 6 Plus, with a new sensor-
barometer sensor. I’m talking about the barometer! This addition is welcomed by all iOS users, as it comes to complete an array of detectors which include ambient light measurer, accelerometer, gyroscope, digital compass, proximity sensor, Touch ID finger print reader and iSight camera sensor.

Barometer has the role to measure air pressure. This reading is important because it helps your iPhone determine altitude, as well as relative elevation changes. This way your iOS device can track your vertical movement, without the need of GPS assistance. More, it transforms your iPhone in an actual barometer, because air pressure reading is used for weather forecasting!

How To Use iPhone Air Pressure Readings
This new sensor opens up a lot of important features for iOS users. Developers have already released new apps and adjusted current ones to be able to access, keep track and efficiently use your iPhone’s air pressure readings.
They are usually recorded using the hectopascal (hPa) unit and can be used to determine elevation, forecast weather and even track the number of stairs climbed:

1. Elevation Estimation

Your iPhone reads the current air pressure and compares it with the sea level value to calculate your current elevation. You can also obtain relative altitude readings by setting altitude to 0 feet (or meters) when on ground level. Then let your iOS device show you how much you climbed when you’re at the 20th story of a skyscraper. The great thing is that these readings are adjusting almost instantly, so you can actually track your elevation as you’re in the elevator. And, yes this is done without a GPS, so don’t worry about lack of signal!

2. Weather Forecasting

Old school weather forecasting can be accomplished with an iOS Barometer app. In meteorology significant pressure change means, also an alteration of weather. If air pressure drops suddenly, it signals that a low pressure system looms at a horizon and there are increased chances of rain. On the other hand, fast pressure raises are the equivalent of sunshine! Any change of 3.5 hPA or greater in air pressure is forecast of changing weather. Thanks to the barometer sensor you can easily let your iPhone track these readings and be your own meteorologist.

Update: Dark Sky is one of the first iOS weather apps that uses the iPhone 6 and iPhone 6 Plus barometric sensor for forecasts based on crowdsourcing!

3. Physical Activity Tracking

Step counters have already implemented iPhone’s barometer readings and are now able to keep track of your physical activity, even on the vertical. This was possible prior to iPhone 6 and iOS 8 only with the help of the GPS. But using this system around the clock, significantly reduces battery life and exposes the user to additional electromagnetic field radiation. Now, with the help of the barometer sensor, this extra load is removed from the A8 chip and your pedometer can easily track your stair climbing 24/7 and show your progress whenever you slide down the iOS Notification Center.

iPhone Barometer Sensor Apps
Here are a few iOS apps that use your smartphone’s built-in barometer sensor to accomplish the functions described above. We’ve tested them out for you and here are the results:

1. Barometer & Altimeter (Weather, Elevation) – Another great free app that estimates elevation and also predicts the weather, while analyzing your iPhone’s air pressure readings. As a bonus it offers widget integration for iOS 8 users.

2. Pedometer++ (Movement Tracking) – A simple and very efficient step counter that tracks your movement 24/7 and counts the amount of steps you made. If you own an iPhone 6, it will also track the amount of stairs climbed and transform them in floors. Accuracy is pretty good. We’ve tested it on a ten-storey building and Pedometer counted 9 floors for me. Widget function is included!

3. Barometer plus Altimeter (Weather, Elevation) – This one is similar to the first placed app in this list. The minus comes from the lack of widget support. If you’re not an iPhone 6 user yet you have to use the GPS based altimeters.

4. Barometer + Altimeter (Air Pressure, Elevation) – Is one of the simplest software available. It provides air pressure readings and can measure relative altitude, which means that you can measure how much you climb or decline, when setting a desired location as starting point.

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Liquid Level Sensors Information ｜isweek.com

Liquid level sensors are used to detect liquid levels or interfaces between liquids such as oil and water or liquids and solids. They can be defined as sensors or transducers, or as integrated systems with instrumentation and control capabilities.

Types of Sensors

Liquid level sensors provide measurement of the height or position of a fluid surface using a variety of different technologies and methods. 

• Air bubbler systems involve a fixed flow of air passed through a tube with an opening below the surface level of the measured liquid. Pressure is proportional to the depth and density of the liquid.
• Capacitance sensors apply radio frequency signals to a capacitance circuit to sense materials with dielectric constants ranging from 1.1 to 88 or more.
• Conductivity or resistance sensors use a low-voltage power source applied across separate electrodes. Level is interpreted based on the amount of resistance or conductance of the substance which carries the current.
• Float sensors involve the opening or closing of a mechanical switch through either direct contact or magnetic operation from a device which floats on the surface of the measured liquid.
• Optical sensors detect the decrease or change in transmission of infrared light emitted from a diode through a material.
• Pressure membrane and differential sensors measure the pressure or change in pressure in a vessel such as a holding or storage tank.
• Radar or microwavesensors measure using the emittance and detection of microwave pulses. 
• Ultrasonic or sonic liquid level sensors measure the length of time it takes for a reflected sound wave to return to a transducer from a target surface.
• Vibrating or tuning fork liquid level sensors use a piezoelectric crystal or other technology to vibrate a probe and then monitor the presence, absence, increase or decrease of that vibration.

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Humidity Sensor - Wholesale sensors & Transmitters from isweek.com (industry sourcing)

Humidity is the presence of water in air. The amount of water vapor in air can affect human comfort as well as many manufacturing processes in industries. The presence of water vapor also influences various physical, chemical, and biological processes. Humidity measurement in industries is critical because it may affect the business cost of the product and the health and safety of the personnel. Hence, humidity sensing is very important, especially in the control systems for industrial processes and human comfort.

 

Controlling or monitoring humidity is of paramount importance in many industrial & domestic applications. In semiconductor industry, humidity or moisture levels needs to be properly controlled & monitored during wafer processing. In medical applications, humidity control is required for respiratory equipments, sterilizers, incubators, pharmaceutical processing, and biological products. Humidity control is also necessary in chemical gas purification, dryers, ovens, film desiccation, paper and textile production, and food processing. In agriculture, measurement of humidity is important for plantation protection (dew prevention), soil moisture monitoring, etc. For domestic applications, humidity control is required for living environment in buildings, cooking control for microwave ovens, etc.  In all such applications and many others, humidity sensors are employed to provide an indication of the moisture levels in the environment.

 

RELEVANT MOISTURE TERMS

To mention moisture levels, variety of terminologies are used. The study of water vapour concentration in air as a function of temperature and pressure falls under the area of psychometrics. Psychometrics deals with the thermodynamic properties of moist gases while the term “humidity’ simply refers to the presence of water vapour in air or other carrier gas.

 

Humidity measurement determines the amount of water vapor present in a gas that can be a mixture, such as air, or a pure gas, such as nitrogen or argon. Various terms used to indicate moisture levels are tabulated in the table below:

 

S.No	Term	Definition	Unit
1	Absolute Humidity (Vapor Concentration)	Ratio of mass(vapour) to volume.	grams/m3
2	Mixing Ratio OR Mass Ratio	Ratio of mass(vapour) to mass(dry gas)	grams/m3
3	Relative Humidity	Ratio of mass(vapour) to mass(saturated vapour) OR ratio of actual vapor pressure to saturation vapor pressure.	%
4	Specific Humidity	Ratio of mass(vapour) to total mass.	%
5	Dew Point	Temperature(above 0°C) at which the water vapor in a gas condenses to liquid water)	°C
6	Frost Point	Temperature(below 0°C) at which the water vapor in a gas condenses to ice
7	Volume Ratio	Ratio of partial pressure(vapour) to partial pressure (dry gas)	% by volume
8	PPM by Volume	Ratio of volume(vapour) X 106 to volume(dry gas)	PPMV
9	PPM by Weight	PPMV X	PPMW

 

Most commonly used units for humidity measurement are Relative Humidity (RH), Dew/Frost point (D/F PT) and Parts Per Million (PPM). RH is a function of temperature, and thus it is a relative measurement. Dew/Frost point is a function of the pressure of the gas but is independent of temperature and is therefore defined as absolute humidity measurement. PPM is also an absolute measurement.

 

Dew points and frost points are often used when the dryness of the gas is important. Dew point is also used as an indicator of water vapor in high temperature processes, such as industrial drying.

Mixing ratios, volume percent, and specific humidity are usually used when water vapor is either an impurity or a defined component of a process gas mixture used in manufacturing.

 

Correlation among RH, Dew/Frost point and PPM_v is shown below:

HUMIDITY SENSING – CLASSIFICATION & PRINCIPLES

According to the measurement units, humidity sensors are divided into two types: Relative humidity(RH)sensors and absolute humidity(moisture) sensors. Most humidity sensors are relative humidity sensors and use different sensing principles.

 

A table showing important parameters of different types of humidity sensors is given below:

Active Material	Thermo-set Polymer	Thermoplastic Polymer	Thermoplastic Polymer	Bulk Thermoplastic	Bulk AlO3	Lithium Chloride Film
Substrate	Ceramic or Silicon	Ceramic or silicon	Polyester or mylar film	N/A	N/A	Ceramic
Sensed Parameter	Capacitance	Capacitance	Capacitance	Resistance	Resistance	Conductivity
Measured Parameter	%RH	%RH	%RH	%RH	%RH	%RH
RH Change	0% to 100%	0% to 100%	0% to 100%	20% to 100%	2% to 90%	15% to <100%
RH Accuracy	±1% to ±5%	±3% to ±5%	±3% to ±5%	±3% to ±10%	±1% to ±5%	±5%
Interchangability	±2% to ±10% RH	±3% to ±20% RH	±3% to ±20% RH	±5% to ±25% RH	poor	±3% to ±10% RH
Hysterisis	<1% to 3% RH	2% to 5% RH	2% to 5% RH	3% to 6% RH	<2% RH	very poor
Linearity	±1% RH	±1% RH	±2% RH	poor	poor	Very poor
Risetime	15 s to 60 s	15 s to 90 s	15 s to 90 s	2 min to 5 min	3 min to 5 min	3 min to 5 min
Temperature Range	-40 °C to 185 °C	-30 °C to 190 °C	-25°C to 100 °C	10 °C to 40 °C	-10 °C to 75 °C	-
Long Term Stability	±1%RH/5 yr	±1%RH/yr	±1%RH/yr	±3%RH/yr	±3% RH/yr	>1% RH/°C

 

·         Sensing Principle

Humidity measurement can be done using dry and wet bulb hygrometers, dew point hygrometers, and electronic hygrometers. There has been a surge in the demand of electronic hygrometers, often called humidity sensors.

 

Electronic type hygrometers or humidity sensors can be broadly divided into two categories: one employs capacitive sensing principle, while other use resistive effects

 

Sensors based on capacitive effect:

Humidity sensors relying on this principle consists of a hygroscopic dielectric material sandwiched between a pair of electrodes forming a small capacitor. Most capacitive sensors use a plastic or polymer as the dielectric material, with a typical dielectric constant ranging from 2 to 15. In absence of moisture, the dielectric constant of the hygroscopic dielectric material and the sensor geometry determine the value of capacitance.

 

At normal room temperature, the dielectric constant of water vapor has a value of about 80, a value much larger than the constant of the sensor dielectric material. Therefore, absorption of water vapor by the sensor results in an increase in sensor capacitance.

 

At equilibrium conditions, the amount of moisture present in a hygroscopic material depends on both the ambient temperature and the ambient water vapor pressure. This is true also for the hygroscopic dielectric material used on the sensor.

 

By definition, relative humidity is a function of both the ambient temperature and water vapor pressure. Therefore there is a relationship between relative humidity, the amount of moisture present in the sensor, and sensor capacitance. This relationship governs the operation of a capacitive humidity instrument.

 

Basic structure of capacitive type humidity sensor is shown below:

On Alumina substrate, lower electrode is formed using gold, platinum or other material. A polymer layer such as PVA is deposited on the electrode. This layers senses humidity. On top of this polymer film, gold layer is deposited which acts as top electrode. The top electrode also allows water vapour to pass through it, into the sensing layer .  The vapors enter or leave the hygroscopic sensing layer until the  vapour content is in equilibrium with the ambient air or gas.Thus capacitive type sensor is basically a capacitor with humidity sensitive polymer film as the dielectric.  

 

Sensors based on Resistive effect:

Resistive type humidity sensors pick up changes in the resistance value of the sensor element in response to the change in the humidity. Basic structure of resistive type humidity sensor from TDK is shown below

Thick film conductor of precious metals like gold, ruthenium oxide is printed and calcinated in the shape of the comb to form an electrode. Then a polymeric film is applied on the electrode; the film acts as a humidity sensing film due to the existence of movable ions. Change in impedance occurs due to the change in the number of movable ions.

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