Document 0qo6zBZ8eo8zzRpqBRbK414Lb

Heating Ventilating Air Conditioning .Guide 1939 nected in series are called thermopiles. Thermocouples for the:measurement of' high temperatures are calibrated :with, the aid of .the known melting points of pure metals. Resistance thermometers are suitable for temperature measurements up to 1800 F. These thermometers depend for their operation on the change of resistance with temperature of a pjatinum,.nickel,.or copper wire coil, and they are calibrated in the same way as thermocouples. 'Pyrometers of various types may be used, for temperatures above 500 F. Themercurial pyrometer is a thermometer with an inert gas, such as; nitrogen or carbon dioxide/ above the mercury' Column to prevent the mercury from boiling. The radiation pyrometer consists of a thermopile upon which the' radiation from a hot source is focused by a concave^mirror or Tens-' .A sensitive galvanometer or potentiometer with "a'calibrated1 temperature scale indicates the thermo-electromotive force created by the > heat on ;the thermopile. The optical pyrometer measures radiant energy " by comparing the intensity of a narrow spectral band, usually red light emitted by the .object, with that; emitted by a-standard light source (electric lamp). Thermoelectric pyrometers operate on the same principle as thermocouples. - When measuring high temperatures; it is custorhaiy to hold the cold junction at-room temperature and this may cause somb error if, the room temperature is above or below the calibration, point. For extremely precise temperature measurements, the cold junction is usually; immersed in melting rice to fix the cold junction temperature. Various forms of hand-operated and automatic cold junction temperature com pensators are also available. . , ,:In the measuring of room temperature:care must be exercisecl to pre-; vent the; results from being affected by the body heat of the observer,; by air currents from doors, windows and other openings, or by radiant, heat from some local sourcesuch as a radiator, or .wall.. All glass thermo meters should be mercury-thermometers, with engraved stems. The total graduations.of the thermometers should be from 20 to 120.F, in.ope degree graduations. No ten degrees should `occupy a space of less than one-half, inch. The accuracy throughout the whole scale must be within orie-half degree. The operator should take hold of the top and no part of the body, including the hand, should be nearer than 10 in. to the bulb. The ther mometer should not be closer than 5 ft to any door, window, or other opening; should not be closer than 12 in. to any wall; and-should be between 3 and 5 ft from the floor; A sling instrument should be used for extreme accuracy; Thermocouples or resistance thermometers'may also be used for room temperature measurements, an advantage being that the operator can read temperatures from outside the room if desired, and thus eliminate the errors which might be caused by,his presence close to the temperature measuring device. : 1 .; For measuring dUct temperatures a duct thermometer should be used,, with , the: bulb extending into the duct at least 6. in. When the thermo meter is to be permanently located in the duct, a pipe flange or nipple, should'be used to receive the threaded portion of the thermometer stem. When the thermometer is not to.be permanently located, acork or, rubber stopper may be placed around the stem to prevent errors from air leakage. Readings: should be taken at various locations: in a . duct so due con sideration may be given to temperature stratification. Other forms of 826 Chapter 44. Test Methods and Instruments temperature measuring devices may be used, but the active part, must be at least 6 in. from? the duct wall; Recording instruments may be used for testing and for making con tinuous records of operation. Potentiometer and Wheatstone bridge recorders for thermocouples and resistance thermometers respectively may have accuracies of == per cent of their range, or, for example, to =*=' 1 F in a range of 0 to 300 F. This accuracy compares favorably with that of other forms of temperature measuring devices. AIR MOVEMENT MEASUREMENT The quantity, velocity and pressure of air moved by a fan or flowing through a duct or grille may be determined by various methods. The instruments in common use are the Pitot tube,'anemometer, direct reading velocity meter, and Kata.-thermometer, the latter being suitable for low air velocities and being commonly used for measurements at points where the air is not confined in a duct. Electrical anemometers are als6 available," operating on the principle of measurement of the variation of resistance of a hot wire cooled to various degrees by air velocities past the wire. The use of calibrated nozzles, orifice plates, and Venturi meters are recognized methods, which, however, have little application in con nection-with ventilation practice. Pitot Tube This usually consists of two tubes, one within the other, which when properly held in the air stream will register the total or impact pressure and the static pressure, respectively. If these7 tubes are connected to opposite sides of a draft gage, or other type of U tube, the recorded presr sure will be the differential or.velocity pressure. Volume measurements may thus be made in a duct of known area. Pitot tube measurements are preferably used for air velocities exceeding 20 fps; Volumetric determi nations from Pitot tube readings should take into account the barometric pressure and the temperature and humidity of the air measured. Air flow in .ventilation practice is generally`in the turbulent range. When stratification of velocity, vortex motion, or. violent eddy currents of air in ducts exist,-accurate velocity pressure measurements afe difficult. To insure accuracy a straight section of duct from 5 to 10 times its own diameter, is desirable in order to straighten: out the air currents. If it is necessary.to. take Pitot tube readings in shorter sections of straight duct, the results must be considered subject to some doubt.and checked accordingly. For accurate work it is necessary to make a traverse of the duct, dividing its cross-seCtion into a number of imaginary equal areas and taking a reading in the center of each, the average of the velocities cor responding to these pressures giving the true velocity in the duct. A pitot tube of standard design and the traverse method of obtaining average velocity are completely described in the A.S.H.V.E. Standard Test Code for Centrifugal and Axial Fans.4 For precise work the shape, size and calibration of the Pitot tube are important considerations in the determination of the correct air flqw: r Vl. 29, 1923, p. 407. Amended June, 1931. Alao'see Standard Test Code for Centrifugal and Axial Fans. Ed tion of 19R8. ... ..?***.. 827