Humidity. Humidity Refers to the water vapor content in air or other gases. Humidity measurements can be stated in a variety of terms and units. The three commonly used terms are absolute humidity, dew point, and relative humidity (RH).
Absolute Humidity. Absolute humidity is the ratio of the mass of water vapor to the volume of air or gas. It is commonly expressed in grams per cubic meter or grains per cubic foot (1 grain = 1/7000 lb.). It can be calculated from known RH, temperature, or wet bulb, or it can be measured directly. Refinements in thermistor technology in the 1960s led to the development in the 1980s of a thermal conductivity principle that permits absolute humidity ?easurements at elevated temperatures (>200°C) even in a polluted environment. The system uses two thermistors in a bridge configuration.
Dew Point. Dew point, expressed in °C or °F, is the temperature and pressure at which a gas begins to condense into a liquid. Chilled mirror hygrometers have reliably made dew point measurements since the early 1960s, but the development of stable thin film capacitive sensors in the 1980s now allows measurement of dew points as low as –40°F at a fraction of the chilled mirror cost. Calibration data for each specific sensor are stored in nonvolatile memory for improved accuracy. In contrast, chilled mirrors measure dew point in real time and do not require stored data for measurements.
Relative Humidity. Abbreviated as RH, relative humidity refers to the ratio (stated as a percent) of the moisture content of air compared to the saturated moisture level at the same temperature and pressure. In the early 1900s, RH was derived from measuring a physical change that moisture absorption caused in certain natural materials such as silk or human hair. Nylon and other synthetics were subsequently used. Since the 1940s, most mechanical methods have been replaced by electronic RH sensors due to their greater accuracy and dependability and their lower cost. In the fairly recent past, specialized polymer-based resistive and laser-trimmed capacitive sensors with monolithic signal conditioners have been introduced.
important specifications to keep in mind when selecting a humidity sensor are:
Ability to recover from condensation
Resistance to chemical and physical contaminants
Additional significant long-term factors are the costs associated with sensor replacement, field and in-house calibrations, and the complexity and reliability of the signal conditioning and data acquisition (DA) circuitry. For all these considerations to make sense, the prospective user needs an understanding of the most widely used types of humidity sensors and the general trend of their expected performance. Definitions of absolute humidity, dew point, and relative humidity are provided in the sidebar, "Humidity Basics").
Sensirion’s family of relative humidity and temperature sensors have become established as the industry standard - mainly due to their high performance and integration (CMOSens® Technology) in a miniature format. The capacitive humidity sensors provide digital and fully calibrated output which allows for easy integration without the need for additional calibration. The excellent long term stability has been very well perceived and the cutting edge low energy consumption is unachieved and makes them the right choice for any remote application.
The digital humidity sensors are provided in different packaging types: SMD type (SHT1x series), pin type (SHT7x series) and the new DFN type (SHT2x series). The SHT1x and SHT2x are reflow solderable while pin type humidity sensors are used for devices where flexible integration is crucial or easy exchange is necessary. The three series are subdivided further according to different accuracy levels of humidity reading.
The relative humidity sensors by Humirel are based on capacitive cell technology using a patented solid polymer structure. They are fully interchangeable with no calibration required in standard conditions. Suitable for linear voltage or frequency output circuitry.An NTC thermistor is used for temperature measurement.
The RH modules are based on the HS1101LF capacitive humidity sensor and are voltage output devices. The RH/Temperature modules include a NTC thermistor for temperature measurement and are voltage or frequency output.
• High reliability and long term stability
• Instantaneous de-saturation after long periods in saturation conditions
• Fast response times
• Patented solid polymer structure
The TDK ultrasonic humidifi er unit was the fi rst such product in the world to be developed. In addition
to such home uses, these ultrasonic humidifier units have numerous advantages for humidification of vegetable
showcases, preservation and growth of agricultural products, industrial applications, etc.
NB-59S-09S-0: • Compact with highly reliable circuitry • Separate transducer and drive circuit sections provide superior layout versatility NB-80E-01-H: • Compact with highly reliable circuitry • Separate transducer and drive circuit sections provide superior layout versatility • Because the ultrasonic frequencies used are higher than with typical household-type units, mist particle size is extremely fi ne. This unit is thus ideal for products intended for smaller spaces.
Rapid advancements in semiconductor technology, such as thin film deposition, ion sputtering, and ceramic/silicon coatings, have made possible highly accurate humidity sensors with resistance to chemicals and physical contaminants?at economical prices. No single sensor, however, can satisfy every application. Resistive, capacitive, and thermal conductivity sensing technologies each offer distinct advantages. Resistive sensors are interchangeable, usable for remote locations, and cost effective. Capacitive sensors provide wide RH range and condensation tolerance, and, if laser trimmed, are also interchangeable. Thermal conductivity sensors perform well in corrosive environments and at high temperatures. For most applications, therefore, the environmental conditions dictate the sensor choice.
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Thursday, December 2, 2010
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