Document 8592JyYRL10V8dKzeOQ95DZJy

1112 CHAPTER SI 1954 Guide the observed relative humidity by the ratio of the observed to the standard atmospheric pressure. For air temperatures below 32 F, the water on the wick may either freeze or super-cool, and its state must be known and a proper table or chart used, since the wet-bulb' temperature is different for ice and for water. Some operators remove the wick from the wet-bulb for freezing conditions and dip the bulb in water a few times, allowing the water to freeze on the bulb between dips and to form a film of ice: Since wet-bulb depression is slight at low temperatures, precise temperature readings are essential. In the ventilated or aspirated psychrometer, the thermometers remain stationary, and a small fan or blower or a syringe is used to move the air across the thermometer bulbs. . Various designs have been employed in the laboratories, and several commercial models are available. The Dew-Point Hygrometer In the usual form of these instruments, means are provided for cooling, and of observing the temperature of, a surface which is exposed to air. The temperature at which visible condensation occurs on the surface is con sidered the dew-point of the air. With the dew-point temperature known, the relative humidity and other properties of the air can be taken from tables and charts (See Chapter 3). A bright surface or metallic mirror is usually employed to improve the visibility of the dew deposit, and various means areUsed to cool the mirror from the back, including evaporating ether or another refrigerant, or a stream of air passed through dry ice. Dew-point temperatures, in some cases, are observed by means of thermom eters in fluids in contact with the back of the mirror, but in modem instru ments thermocouples are used, and are soldered or welded to the mirror itself. The dew-point apparatus is not so commonly used as the psy chrometer, probably because it is less convenient. It is usable, however, for higher temperatures than the wet- and dry-bulb psychrometer, and should be considered for dew-points near or above the boiling point, as in the case of flue gases. Special apparatus for high precision has been con structed, in which the photronic cell and a light source are used for dew or frost detection instead of visual Inspection. Hair Hygrometers Many materials, especially organic materials, change in dimensions with changes in humidity, and many devices have been designed in efforts to utilize this action in simple and effective humidity indicators, recorders and controllers. Unfortunately, no material has been found which can be relied upon to perfectly reproduce its action when exposed to repeated identical changes in humidity. The field has been well explored, and in strument and control manufacturers are practically unanimous in the selection of human hair for this service. The hair hygrometer consists of from one to several strands of hair with a mechanism whereby changes in length of the strands, due to changes in humidity, cause an indicator to move across a dial. In the recording instru ment, a pen is moved across and marks a moving paper ribbon, indexed in relative humidity. In a controller, or humidistat, the motion makes or breaks an electric contact governing the air conditioning equipment. Such devices require initial calibration and, for precise work, frequent recalibratiori or setting, especially if they are exposed to extremes of either high or low humidity. For continuous operation, with only slight changes in humidity, some operators report satisfactory reproducibility of results. Instruments and Measurements 1113 Electrolytic Hygrometers The dampness, and therefore the electrical resistance of a salt film, varies with the humidity of the atmosphere to which the film is exposed,.and at least two types of instruments based , on this fact havq been developed. The Dunmore hygrometer was originally designed for use in radio-sondes or small balloons, and means were devised whereby the. device transmits humidity data back to earth in the form of. a radio signal. In the more usual form, this hygrometer consists of a dual, winding.of small wire on a non-conducting tube. The whole is coated with an electrolytic film, usually containing a salt such as lithium chloride, which forms an electric connec tion between the windings. Means are provided for determining the.elec trical resistance of the film, which is an indication of the humidity. In the radio-sonde, variations in the resistance of the film affect the frequency of an oscillating circuit. These hygrometers are usually calibrated by com parison with a wet- and dry-bulb psychrometer. For calibration for some purposes, particularly for use at sub-zero temperatures, means have been provided for producing atmospheres of known humidity.43 Advantages of instruments of this type for some scientific and industrial purposes are becoming apparent, and some forms of them are on the market. The Weaver type hygrometer is particularly useful for determining the humidity of air or other gas in pipes or closed vessels at various pressures and temperatures. It consists of a threaded plug carrying a central elec trode insulated from the plug, except for a gelatinous electrolytic film. The film is exposed to the air or gas from the pipe or vessel, and provision is made for determining its electrical resistance. The hygrometer, or detector, is mounted in a manifold equipped with valves whereby the film can be alter nately exposed to the test gas, and to a standard gas having a known abso lute humidity . The pressure of the standard gas is varied until contact with it establishes the same resistance in the film as the test gas, and the humidity of the test gas is then determined by computation based on the gas laws. Chemical Hygrometry The humidity of an atmosphere can be measured directly by extracting and weighing the water vapor from a known sample. For precise labora tory work, powerful desiccants such as sulphuric acid and phosphorus pentjde are used for the extraction process, while for some purposes, calcium chloride, lithium chloride or silica gel are satisfactory. Freezing the water vapor out of a measured stream of air or gas with solid carbon dioxide, and weighing the resulting ice, is a similar operation. A thermal conductivity method for gas analysis can be used for temperatures above 212 F, or for very low humidities.44 HEAT TRANSFER THROUGH BUILDING MATERIALS Thermal Conductivity Use of the guarded hot plate apparatus for determining the thermal con ductivity (k value) of homogeneous materials was adopted by A.S.H.Y.E. m 1942, and has become practically universal, and the apparatus is de scribed in an A.S.T.M. publication.45 * 46 It consists essentially of an elecncally heated plate and two water cooled plates. Two identical speci mens or slabs of a material are required for a test, and one is mounted on ach side of the hot plate. A cold plate is then pressed against the outside each specimen by a clamp screw. Hot plate apparatus accommodating Pecimens on the order of one foot square and an inch or more thick, is