say, a steady consumption of gas by the burners, it follows that no change can be made in the number of burners without altering either their average individual output or else the adjustment of the regulator. Special star burners.-The essential feature of the form of gas burner which is to be used with glass beakers (but also is suitable for use with other types of boiling-vessel) is the arrangement of the multiple-flame jets upon the curved upper surfaces of the six rays, as indicated in the drawings, figures 92 and 93. Desirable but not indispensable features are, first, the unobstructed knife-edged disk nozzle for the injection of the gas into the mixing chamber, which

FIGURE 93.-Special star burner as constructed with water jackets and disk nozzle, but without micrometer and needle valve.

is readily removable for cleaning or exchange: and second, the watercooling of the nozzle chamber and nozzle and the mixing chamber. [13] The disk nozzles are to be properly formed of corrosion-resistant material, such as monel metal or stainless steel. A number of interchangeable nozzles with assorted diameters of orifice should be provided to facilitate adjustment of the burner output. For a given rate of discharge, the orifice of a thin disk nozzle presents maximum clearance for the flow of gas and minimum probability of clearance contraction through accumulation of dirt, tar, or polymerization waxes. Hence the disk nozzle provides stability and reliability of heat out

FIGURE 94.-Stove assembly with the burner arrangement illustrated in Figure 93. To indicate the correct relations among the beaker, burner, and top plate, the operating position of the boiling-vessel assembly is illustrated in broken outline. WI represents the water inlet, WO the water outlet, and GI the gas inlet.

put by the burner. At some sacrifice of these features, but at little sacrifice of accessibility for cleaning when deposits appear, provision can be made for adjustment of the output capacity of a burner with disk nozzle by installing in the nozzle chamber a long-taper needle valve with a maximum thrust diameter somewhat less than the

FIGURE 95.-Stove assembly with the burner arrangement illustrated in Figure 94. The boiling-vessel assembly is in position for the carrying out of a candy test.

diameter of the orifice, which it is arranged to penetrate concentrically. The arrangement is indicated in figure 92.

In figures 92 and 94 the nozzle chamber into which the gas inlet, GI, leads is enveloped by its water jacket, H1, through which the current of water passes from its inlet, WI, to the transfer duct leading to the mixing-chamber water jacket, H2, whence it flows to waste through the outlet, WO. By means of the vertical adjustment provided for the nozzle chamber assembly, the cap, C, can be brought to

act as a shutter against the mouth of the venturi of the mixing chamber to control the uptake of primary air. Thus an annular inlet, open at the bottom, is formed for the primary air between the jacket of the nozzle chamber and the skirt of the mixing chamber.

Stoves, or support assemblies.-The functions of the stove, figures 94 and 95, are to support the boiling vessel and burner in constant locations and thus maintain the correct spatial relations between them; to shield the flames and hot combustion gases from the disturbances caused by cross drafts (provided they are not too strong) and otherwise to assist in directing the hot gases against the bottom and sides of the vessel below the surface level of the solution; and finally, to divert the hot gases away from the sides of the vessel above the solution.

The top plate, of transite board with centrally located hole into which the beaker fits, is supported on three strap-iron legs which also support the truncated conical flame shield of polished sheet aluminum. The lower ends of the legs are bolted to the wide iron base upon which the burner is mounted, which in turn is supported upon one fixed foot and two leveling screws.

In stoves for use with glass beakers, which for any particular nominal size vary slightly in diameter, the central hole is cylindrical and just large enough to avoid any binding when the largest beaker in the set is inserted. The beakers are supported at the correct height above the burner (and at the correct insertion through the plate) by means of three thin Monel metal or stainless steel brackets attached to the under side of the plate. The boiling-vessel assembly should be set into the stove with the center of the thermometer bulb in a vertical plane bisecting the angle between two adjacent rays of the burner.

Inclined manometers (or gages). For the convenient observation of the service pressure and the regulated pressure of the gas in the making and inspection of the adjustment of the flow manostat and burners, provide a pair of oil- or water-filled hydrostatic gages with scales about 100 cm long and inclined at suitable (and therefore different) angles. Other types of heating equipment.-Electric-resistance heaters, for example, may be used in place of the special star burner with flow manostatic control described above, provided ample comparative candy tests on samples of a sufficient range of quality have demonstrated that they yield results equivalent to the results obtained with the burner.

Adjustment of the heating device.-The heating device is adjusted in the following manner: A minimum of four standard simple barleysugar tests (basic method, p. 372) are run on a pure sucrose sample (p. 385), and the heating device is so adjusted that the mean duration of the time of cooking, between the temperatures 30° and 176.7° C, shall be 20 minutes 20 seconds. Make similar tests at sufficiently frequent intervals to assure that adjustment is not changing.

The first (or coarse) adjustment is made by installing the proper disk nozzle in the burner. The nozzle is of such a size as to yield nearly the required interval. Final adjustment is made by changing the level of hydrostatic liquid in the flow manostat. As a preliminary criterion of adjustment, it is convenient to heat 300.0 g of water in the

903232 O-50-26

600-ml beaker through the 50-degree interval from 30° to 80° C (or the 90-degree interval from 86° to 176° F). The cooking interval will be approximately six times the time required to heat the water through the specified interval, i. e., the water should heat in approximately 3 minutes 20 seconds as the mean of a minimum of four tests. (3) THERMOMETERS.-Mercury-in-glass thermometers are suitable for all ordinary candy-test procedures, provided the instruments are properly constructed. They may be graduated in either the centigrade or the Fahrenheit scale, as specified by the user, but must not be graduated in both scales on the same thermometer. They should conform preferably with the specifications presented below, which provide an instrument of low lag. Alternatively, thermometers conforming with ASTM Specifications D 183-25 or thermometers for general use may be employed.

SPECIFICATIONS FOR CANDY-TEST THERMOMETERS
TYPE.-Etched stem, glass.
LIQUID.-Mercury.

RANGE AND SUBDIVISION.-Minus 10° to plus 200° C, in 1.0°, with expansion chamber at the top to permit heating to 50° above the upper limit of the scale, or plus 14° to 390° F, in 2.0°, with expansion chamber at the top to permit heating to 90° above the upper limit of the scale.

TOTAL LENGTH.-340 to 360 mm.

STEM.-Plain front, enamel back, made of suitable thermometer tubing. Diameter 6 to 7 mm.

BULB. Corning normal or equally suitable thermometric glass. Diameter not less than 4 mm and not greater than 5 mm. Length not less than 10 mm and not greater than 15 mm.

DISTANCE FROM BOTTOM OF BULB TO -10° C OR 14° F MARK.-80 to 100 mm. DISTANCE FROM 200° C OR 390° F MARK to Top of THERMOMETER.-30 to 70 mm. LENGTH OF UNCHANGED CAPILLARY between top of bulb and first graduation mark, 60-mm minimum; and between the last graduation mark and the expansion chamber at the top, 10-mm minimum.

TOP FINISH.-Glass ring.

SPACE ABOVE MERCURY.-Filled with nitrogen or other suitable gas. GRADUATION. All lines, figures and letters are to be clear cut and distinct. Maximum width of graduation lines, 0.1 mm. On the centigrade scale, all graduation lines at multiples of 5° are to be longer than the remaining graduation marks. Graduations are to be numbered at each multiple of 10°. On the Fahrenheit scale, all graduation lines at multiples of 10° are to be longer than the remaining graduation marks. Graduations are to be numbered at intervals of 20° beginning at 20°.

IMMERSION, 76 mm.-The words "76-mm immersion" and a line around the stem at a distance of 75 to 77 mm above the bottom of the bulb are to be etched on the stem.

SPECIAL MARKING.-The words "NBS CANDY TEST", a serial number and the manufacturer's name or trade-mark shall be etched upon the stem. The marking "C" or "F", as the case may be, shall be etched on the front of the stem above the scale.

SCALE ERROR.-The error at any point on the scale, when the thermometer is standardized at 76-mm immersion in a room at 25° C temperature, shall not exceed 1° C or 2° F.

CASE.-The thermometer shall be supplied in a suitable case, on which shall appear the marking "NBS CANDY TEST, -10° TO 200° c" or "NBS CANDY TEST, 14° TO 390° F", as the description may require.

NOTE. For the purpose of interpreting these specifications, the following definitions apply:

The total length is the over-all length of the instrument, as supplied.
The diameter is that measured with a ring gage.

The length of the bulb is the distance from the bottom of the bulb to the beginning of the enamel backing.

The top of the thermometer is the top of the finished instrument.