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 SensorHumidity 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 PPMv 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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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 SensorHumidity measurement in industries is critical because it may affect the business cost of the product and the health and safety of the personnel.
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