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1108 . CHAPTER 51 1954 Guide near 0.60. The exact value depends on the location of the connections, the pressure drop, the diameter ratio of orifice to pipe, and the sharpness of the edge.22'23 Other information on orifices and their use is contained in Chap ter 4, Fluid Flow. Shaped orifices or nozzles have the advantage, if well made, that their discharge coefficients are close to unity so that the probability of large errors is less. Orifices of this type have been adopted for several specific purposes, and designs are.described in the A.S.H.V.E. Unit.Heater1 and Unit Venti lator Codes,2: and in A.S.R.E. Circular 133 entitled "Standard Methods of Rating and Testing Air Conditioning Equipment". In some instances nozzles are used in multiple so that the capacity of the testing equipment can be changed by shutting off the flow through one or more nozzles. An apparatus designed for testing the air flow and capacity of air conditioning equipment is described by Wile24 in an article in which pertinent informa tion on nozzle discharge coefficients, Reynolds numbers, and the resistance Fig. 5. Expansion Factob fob Air and Otheb Diatomic Gases Applicable to Flange, Radius and Vena Contbacta Taps of perforated plates is also presented. Such apparatus in some laboratories is commonly referred to as a code tester. The Venturi meter is like the nozzle, except for the addition of a downstream transition section that reduces the pressure drop through the meas uring apparatus. In some cases air velocity through a duct, heater coil, or heating unit may be most conveniently estimated by computation from the heat given up by the coil, and the temperature rise (measured by thermocouples) of the air passing through. It is essential to have a uniform flow over the entire inlet and outlet of the heater at the plane of temperature measurement. Propeller or Revolving Vane Anemometer The propeller or revolving vane anemometer consists of a light revolving wind-driven 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 correc- Instruinents and Measurements tion 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. Anemometers of this type are practically standard for wind measurements, and may be .used in large 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 a room,25 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.26' 27 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. On 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..28 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.29 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.30 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