c15_145 (Влагомеры)

REFERENCE AND APPLICATION DATAHumidity SensorsHIH SeriesPSYCHROMETRICS & MOISTUREMOISTURE measurements involve different terms and units.Moisture terms and units all fall under the area of psychrometrics, the study of water vapor concentration in air as afunction of temperature and pressure. Selecting a moisture termdepends on the application at hand.Dew points and frost points are often used when the dryness ofthe gas is important, (moisture condensation from gas at lowprocess temperatures must be avoided). Dew point is also usedas an indicator of water vapor in high temperature processes,such as industrial drying.Mixing ratios, volume percent, and specific humidity are usuallyused when water vapor is either an impurity or a defined component of a process gas mixture used in manufacturing.

Mixingratios are also used, like dew point, in industrial drying.Relative humidity is most commonly used in HVAC applicationswhere it directly impacts human comfort and indoor air qualityissues. Relative humidity is also of interest to process controlpersonnel as low RH can cause brittleness and static electricityproblems, while high RH can cause swelling and clumpingregardless of temperature.MOISTURE TERMS, DEFINITIONS AND UNITSTermDefinitionAbsoluteHumidity,(Vaporconcentration)Mass, VaporVolumeMixing RatioRelativeHumidityDew PointVolume RatioMass RatioPPM byvolumeMass, VaporMass, dry gasMass, actual vaporMass saturated vaporActual vapor pressureSaturation vapor pressurePartial pressure, vaporVapor pressure waterTemperature of saturation(condensation)Partial pressure, vaporPartial pressure, dry gasSame as Mixing RatioVolume, vapor x 106Volume, dry gasPPM by weight PPMV ×Mole weight of waterMole weight ofcarrier gasPSYCHROMETRICS deals with the thermodynamic propertiesof moist gases while the term Humidity simply refers to thepresence of water vapor in air or other carrier gas.Psychrometrics concerns mixtures of water vapor and dry air.Much of it also applies to other carrier gases since the thermodynamic characteristics of water vapor are fairly independent ofthe carrier gas.

In addition, as the composition of atmosphericair is fairly constant, dry air is treated as a homogeneous gaswith a molecular weight of 28.9645. The molecular weight ofwater is 18.01528.PARTIAL PRESSURE: The gas laws say that the total pressureof a gas mixture is the sum of the partial pressures of theconstituent gases. Also the volume ratios of constituent gasesare equal to the ratios of their partial pressures. For example,atmospheric pressure is the sum of the partial pressures of dryair and water vapor (ratm J r J ra + rW).WATER VAPOR PRESSURE: When a mixture of air and watervapor is in equilibrium with liquid water or with ice, it is considered to be saturated (RH J 100%).

The water vapor saturationpressure over ice for the temperature range of −148 to 32°F isgiven by:UnitIn(rWS) JGrains/ft3Grams/m3wherelb/lb, grains/lb,kg/kg, grams/kg%where% by volumePPM by weight,PPMWC1 J −1.0214165E +04 C5 J 3.5575832E −10C2 J −4.8932428E +00 C6 J −9.0344688E −14C3 J −5.3765794E −03 C7 J 4.1635019E +00C4 J 1.9202377E −07The saturation pressure over liquid water for the temperaturerange of 32 to 392°F is given by:In(rWS) J°F or °CC1+ C2 + C3T + C4T2 + C5T3 + C6T4 + C7 In(T)TC8+ C9 + C10T + C11T2 + C12T3 + C13 In(T)TC8 J −1.0440397E +04 C11 J 1.2890360E −05C9 J −1.1294650E +01 C12 J −2.4780681E −09C10 J −2.7022355E −02 C13 J 6.5459673In both of the above equations,rWS J saturation pressure, psiaT J absolute temperature, °R J °F + 459.67PPM by volume,PPMVPPM by weight,PPMWInstrument for measuring moisture in gas (fromGreek hygros – wet, moist)PsychrometerInstrument using wet/dry bulbs to measuremoisture in gas (from Greek psychros – cold)Honeywell 1 MICRO SWITCH Sensing and Control 1 1-800-537-6945 USA 1 F1-815-235-6847 International 1 1-800-737-3360 CanadaReference/IndexHygrometer145REFERENCE AND APPLICATION DATAHumidity SensorsSIMPLIFIED FORMULATIONS: The preceding equations arevery accurate, but may be overly cumbersome for real timecalculation.

The following equations are less accurate, but aregenerally suitable for mid-range calculations as used in HVACapplications, for example.For dew points higher than ice point:e J [1.0007+P×3.46E−6] × 6.1121 × exp 17.502 × T  240.9 + T HIH SeriesIf RH and dry bulb temperature are known, dew point can bederived by first calculating saturation pressure at the dry bulbtemperature and then multiplying by the RH ratio to obtain rW,the partial water vapor pressure.

Now apply the following:For the dew points in the range of 32 to 200°F:Td J C14 + C15a + C16a2 + C17a3 + C18rW0.1984And for dew points below 32°F:Td J 90.12 + 26.412a + 0.8927a2For dew points at or below ice point:e J [1.0003+P×4.18E−6] × 6.1115 × exp 22.452 × T  272.55 + T epw vapor pressure in millibars(one psi J 68.94745 millibars)P J total pressure in millibars (1 atm J 1013.25 millibars,14.696 psia)T J temperature in °C(°FJ°C×1.8+32)RELATIVE HUMIDITY: the ratio of the partial vapor pressure tosaturation vapor pressure at the dry bulb temperature:rWrWS(Td)RH JrWS J rWS(T)where rWS(Td) is saturation pressure at the dew point temperature and rWS(T) is saturation pressure at the dry bulb temperature.

Relative humidity is moisture and temperature dependentbut independent of total pressure. If dew point and dry bulbtemperatures are known, then RH can be derived by calculatingsaturation vapor pressure for dew point and for dry bulb, thenapplying the RH definition above.DEW POINT is the temperature at which a given sample of moistair is saturated. If the sample is cooled below dew point, thenwater vapor begins to condense. This phenomenon is the basisfor various chilled sensor type dew point meters.Frost Point: If measurements are made below the freezingpoint of water – that is if the indicated dew point is below thefreezing point of water, then the equilibrium occurs at thevapor pressure of ice (not water), which is less than that ofwater. That is, the frost point is a bit higher than dew point.146Where for both expressions:Td J dew point, °FC14 J 100.45C16 J 2.319C18 J 1.2063a J In(rW), rW in psiaC15 J 33.193C17 J 0.17074VOLUME RATIO (also called mixing ratio by volume, or ppmv)is the ratio of water vapor volume to dry air volume (Vw/Va).Because the volume ratios of mixed gases are the same as theirpartial pressures, volume ratio can be expressed as:VR JrW; r J r W + raraBecause total pressure is the sum of partial pressures, the partialpressure of dry air can be readily derived (once vapor pressureis known), by measuring total pressure directly or by assumingone atmosphere (14.696 psia) total pressure.

Multiply the ratioby one million to obtain ppmv (parts per million by volume).HUMIDITY RATIO (also called mixing ratio by weight, or ppmw)is the ratio of the mass of water vapor to the mass of dry air. Tocalculate this, multiply the volume ratio by the ratio of the molecular weights:WMW 18.01528 rWrWJJ 0.62198raMa28.9645 raThe humidity ratio, in common use, is expressed in lb/lb, grains/lb, kg/kg, or g/kg. (There are 7000 grains in one pound.) Multiplythe ratio by one million to obtain ppmw (parts per million byweight).

An engineer may, for example, combine a humidityratio value with the reading from a mass flow meter to calculatethe mass of water vapor flowing through a dryer exhaust ductper unit time.Honeywell 1 MICRO SWITCH Sensing and Control 1 1-800-537-6945 USA 1 F1-815-235-6847 International 1 1-800-737-3360 CanadaREFERENCE AND APPLICATION DATAHumidity SensorsVOLUME PERCENT: equivalent to 100 times the mole fraction;the ratio of water vapor volume to total volume, VW/V or VW/(VW +Va) expressed as a percentage. Like the volume ratio, it can becalculated in terms of partial pressures:V% JrW; r J r W + rarSPECIFIC HUMIDITY: normally expressed as a percentage, isthe ratio of the mass of water vapor to the total mass, and canalso be calculated in terms of the humidity ratio:qJMWWMW + M a J 1 + WABSOLUTE HUMIDITY: (or water vapor density) is the ratio ofthe mass of water vapor to the total volume:Mv217.6 × edv JV J Tdb + 273.16dv J absolute humidity expressed in grams H2O per cubicmeter of dry air and vapor mix (divide by 16,018.46 forlb/cu ft; divide by 2.28835 for grains/cu ft)HIH SeriesSTANDARD ATMOSPHERIC DATA: Normal atmosphericpressure variations have small effects on calculations that require a value for total pressure.

However, at higher altitudes(Denver, for example), atmospheric pressure variations becomesignificant. The following standard data is adapted from NASA.At sea level, standard temperature is 59°F and standard barometric pressure is 29.921 in. Hg.STANDARD ATMOSPHERIC PRESSURE DATAAltitudeTemp.Pressureft°Fin Hg.psia059.029.92114.69650057.229.3814.430100055.428.8614.175200051.927.8213.664300048.326.8213.173400044.725.8212.682500041.224.9012.230Tdb J dry bulb temperature in °C (°F J °C × 1.8 + 32)Reference: 1993 ASHRAE Handbook of Fundamentals, published by American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 1791 Tullie Circle, N.E., Atlanta, GA30329.

Telephone 404-636-8400.ENTHALPY: the measure of the energy content per unit mass.The enthalpy of a gas mixture equals the sum of the individualpartial enthalpies of the components, (dry air and water vapor).In the English system, the specific enthalpy of dry air is assigneda value of zero at 0°F and standard atmospheric pressure. Tocalculate moist air enthalpy in Btu/lb dry air:NOTE: Most of the ASHRAE formulations are based on thethermodynamic temperature scale, which differs very slightlyfrom practical scales (ITS-90) used for physical measurements.The boiling point of water is 211.95°F on this scale rather than thetraditional 212°F.

The slight difference is negligible for any practical application.h J 0.240T + W(1061 + 0.444T)SATURATION VAPOR PRESSURES OF WATER Pwe J rW vapor pressure in millibars (1 psi J 68.94745 millibars)whereT J dry bulb temperature, °FW J humidity ratio of the moist airTemperature(°C)SaturationTemperatureVapor(°C)Pressure (mmHg)SaturationVaporPressure (mmHg)-200.860149.5-101.970233.880355.304.6109.22017.53031.84055.490525.95092.6100760.0Reference/IndexHoneywell 1 MICRO SWITCH Sensing and Control 1 1-800-537-6945 USA 1 F1-815-235-6847 International 1 1-800-737-3360 Canada147.