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208 CHAPTER 11 1946 Guide heated thermometer anemometer 2o. This consists of a mercurial glass thermometer with a resistance winding on the bulb. Current is supplied from an external source in a measured amount. The difference between the temperature of this heated thermometer and that of an ordinary thermometer at the same location, together with the current supplied, makes it possible to calculate the non-directional velocity of the air stream. The heated thermocouple anemometer,employs a thermocouple instead of a thermometer21. Another instrument is the hot wire anemometer which has been made in several patterns. In general, a measured current is supplied to raise the temperature of a fine bare wire above the temperature of the surrounding air. With the use of a very fine wire, minute fluctuations in velocity may be measured, and the area exposed to radiant exchange with heated or cooled surfaces is at a minimum. This instrument is easily adapted to remote reading or recording. A group of them may be connected together to give the average velocity in a space, or the velocity at individual points within a test space, by suitable switching arrangements **. Deflecting Vane Anemometer The deflecting vane anemometer consists of a pivoted vane enclosed in a case, against which air exerts a pressure as it passes through the instrument from an up-stream to a down-stream opening; The move ment 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. When used in fluctuating velocities, it is necessary to average visually the swings of the needle to obtain average velocities. N This instrument is very useful for studying mixing of air in a room24 and in locating and measuring peak velocities that may be objectionable. Various attachments are available, such as the double tube arrangement for obtaining velocities in ducts, and a device for measuring static pres sures. Each instrument and the attachments for it must receive individual calibration. , Propeller or Revolving Vane Anemometer The propeller or revolving vane anemometer consists of a light re volving wheel connected through a gear train to a set of recording dials that read the linear feet of air passing in a measured length of time. It is made in various sizes, 3 in., 4 in., and 6 in. being most common. Each instrument requires individual calibration. At. low velocities the friction drag, of the mechanism is considerable. In order to compensate for this, a gear train that overspeeds is commonly used. For this reason the correction is often additive at the lower range and subtractive at the upper range with the least correction in the middle range of velocities, Most of these are not sensitive enough for use below 200 fpm. 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 accuracy 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; successfully on most of the common, types of supply grilles a: ". The core Instruments and Measurements 209 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 get 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. On exhaust openings, the anemometer traverse is made as described previously. The air volume may be determined by multiplying the corrected 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 27. When a propeller type anemometer is held in a stream of varying velocities, 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 minimum velocity. This is the main reason for the large difference, in ratings of unit ventilators by the anemometer method and by air volume measurements in a duct approach to the inlet. Any of the other anemometers described 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 grilles29. On modern air conditioning grilles the core area is used without a cor rection 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 may also be used on exhaust grilles if proper grille factors are applied50. While hardly a quantitative instrument, smoke is very useful in studying air streams and currents. The application of a more accurate instrument is often made more exact by a preliminary exploration with smoke. A mixture of potassium chlorate and powdered sugar in equal portions gives a very satisfactory non-irritating smoke. It is fired by a. match, and since considerable heat is evolved, it should be placed in a pan away from, inflammable objects. AIR CHANGE MEASUREMENTS Atmospheric air contains a certain amount of carbon dioxide. Its concentration is increased within enclosures by the-carbon dioxide given off by occupants. The total air change through open windows, infiltra tion, and mechanical ventilation, may be measured by the carbon dioxide concentration but absorption of COj by walls and. other materials may -introduce errors 51 and consequently some investigators prefer hydrogen to CO2 for such tests. Since occupants also give off moisture, the increase in humidity may also be used as an index of ventilation within a space. Usually,more direct methods of measuring air supply and air distribution are in favor; MEASUREMENT OF RELATIVE HUMIDITY Wet- and dry-bulb mercurial thermometers are usually-used to deter- mine relative humidity. The.sling psychrometer is a common mounting of the thermometers to permit swinging. The wet-bulb wick and water