Document MGkjO50ZrXdnzxmEK9w73E37

1250 CHAPTER 52 1958. Guide ducts where the air flow is not seriously altered by the presence of the instrument itself. Deflecting Vane Anemometer The deflecting vane anemometer consists of a pivoted vane enclosed in a case. Air exerts a pressure on the vane as it passes through the instrument from an up-stream to a down-stream opening. The movement of the vane is resisted by a hair spring and a damping magnet. The instrument gives instantaneous readings of directional velocities on an indicating scale. With fluctuating velocities, it is necessary to average visually the swings of the needle to obtain average velocities. This instrument is very useful for studying motion of the air in aroom,26 and in locating objectionable drafts. Various attachments are available, such as the double tube arrangement for determining velocities in ducts, and a device for measuring static pres sures. Each instrument, and the attachments for it, must receive indi vidual calibration. For determining average velocity in a duct, it is neces sary to traverse the duct as is done when using the Pitot tube. Measurement of Velocities at Inlets and Outlets of Ducts In the field it is often desirable to make volume measurements at the face of the supply openings. It is rare to have access to the interior of duct sections where the flow is sufficiently uniform for measurement. For ac curacy, the instrument and its application should be checked on a similar approach and grille in the laboratory before use in the field. Tests have shown that the propeller type anemometer can be used suc cessfully on most of the common types of supply grilles.27 28 The core area is divided into equal squares, and the anemometer is held against the face of the grille for the same length of time in each. To obtain the air volume in cubic feet per minute, the average corrected velocity in feet per minute thus obtained is multiplied by the average of the gross and net free area of the grille (core) in square feet. Qn exhaust openings, the anemometer traverse is made as described previously. The air volume may be determined by multiplying the cor rected velocity in feet per minute by the gross core area of the grille in square feet, and by a coefficient for average conditions of 0.85.29 When a propeller type anemometer is held in a stream of varying veloc ities, it tends to indicate higher than the true average, that is, the speed of the propeller is nearer to the top velocity in its area than it is to the mini mum velocity. This is the main reason for the large difference in ratings of unit ventilators by the anemometer method, and by air volume measure ments in a duct approach to the inlet.30 Anemometers can be used within their range at the face of supply grilles when properly applied. In principle, it is a case of finding the velocity at many points, and using the average thus found with the correct discharge area at that cross-section. The deflecting vane anemometer equipped with a jet on the end of a rubber tube has been found especially convenient and accurate on supply grilles.31 On modem air conditioning grilles, the core area is used without a correction coefficient when the jet is held one inch away from the face of the grille. At this distance, the constriction due to the thin bars has disappeared, since the small air jets have reunited and the air stream has not yet spread beyond the core dimensions. With deflecting grilles the exploring jet should be turned to the angle giving a maximum reading. With suitable traversing tips and calibration, this instrument Instruments and Measurements 1251 may also be used on exhaust grilles if proper grille factors are applied.32 Those contemplating such measurements should consult the references cited. Smoke is a qualitative tool which is very useful in studying air move ments. Satisfactory smoke can be obtained from titanium tetra-chloride (which, however, is very irritating to nasal membranes) or by mixing potassium chlorate and powdered sugar (a non-irritating smoke) and firing the mixture with a match. This latter process evolves considerable heat, and it should be confined in a pan away from flammable materials. The titanium tetra-chloride smoke lends itself to spot determinations, particu larly for leakage through casings and ducts, as it can be easily handled in a small pistol-like ejector. The fumes of aqua ammonia and of sulfuric acid, if permitted- to mix, form a white precipitate which is useful for some purposes. Two bottles, one containing ammonia water and the other acid, are connected to a common nozzle by means of rubber tubing. Air is forced over the surfaces of the liquids in the bottles by means of a syringe, and the two streams, upon mixing at the nozzle, form a white cloud. The Kata Thermometer The measurement of air current velocities within enclosed spaces, such as the rooms of a house, is usually a tedious undertaking. However, useful data can be obtained by using the instruments described in following para graphs if they are maintained in calibration, and the user understands the operation and limitations of the instruments.33 One of the instruments useful for determining the velocity of air currents in free spaces is the Kata thermometer which is essentially an alcohol thermometer with a large bulb. The instrument is heated above 100 F, and then the time in seconds required' for it to cool from 100 to 95, when located in the air current, gives a measure of the air speed. It is important to have the Kata thermometer dry before taking the reading. Each Kata has its own factor etched on the stem; and this factor must be used with its cooling formula or chart for obtaining the velocity. The Kata thermom eter is useful in exploring ventilated spaces to determine whether the proper air movement and distribution are being maintained. It is also used in determining the cooling power of the atmosphere, since it loses heat by radiation and convection when dry, and by radiation, convection, and evaporation when the bulb is equipped with a wetted cloth covering.34 Thermal Anemometers ^ ^ suitable sensing element is heated electrically at a fixed rate and an ajr stream) the temperature difference between the element and the stream becomes a measure of velocity by calibration. In the not-vnre anemometer, a very fine heated wire is employed as a resistance- thermometer element whose temperature may be determined accurately.M. 36 jn the heated-buU) thermometer type, a heating wire is wound around the bulb of a mercury-in-glass thermometer, and the temperature mnerence between this thermometer and a similar unheated one serves fi5 t2i air speed.37 The heated-thermocouple anemometer is cali- i/vi k lve velocity in terms of the differential emf between heated and nheated thermo-junctions exposed to an air stream.35- 38 Combined easurenientg of air temperature and velocity are particularly useful for ffistnbution studies, and automatic recording potentiometers or resist- ee-thermometer devices facilitate spatial traverses. A correct calibra-