PACIFIC SOUTHWEST ASSOCIATION O F CHEMISTRY TEACHERS DESIGN AND OPERATION OF A THERMOMETER COMPARATOR G. ROSS ROBERTSON University of California, Los Angeles, California
INEDUCATIONAL
research laboratories, with periodic turnover of junior personnel, there is frequent need for convenient procedures in calibration of liquid-in-glass thermometers. The usual request for loan of anofficially-certified standard thermometer involves great hazard of breakage by inexperienced workers. Method of use of the standard instrument is uncertain. Furthermore, the cost and delay involved in replacement of a broken standard thermometer are now very serious. Figures 1 and 2 show an outfit which has been operated in the author's laboratory for several years, and is constantly available for immediate use by any research worker. I n this comparator a standard calibrated precision thermometer, ranging from 0" to 10O0C., is installed permanently, mith closely adjoining space where a second thermometer may be inserted and compared a t any point from room temperature to 90'. Since most calls for calibration involve u temperatures below 100°, this comparator CM. eniovs the distinct advantaee of usine Figure 1 diitiiled water as circulating liquid. The high specific heat of water affords maximum speed of temperature response. Minimum damage to graduation markings occurs, in contrast mith use of an oil bath. One must be careful, however, to test only clean thermometers and to exclude all electrolyte in view of the presence of a bare-wire immersion heater, used because of freedom from temperature lag. Concentration of many small accessories on one 90for compactness in shelf mount,cm. (3&in.) ing, has made it desirable not to attempt a picture of the whole assembly in one figure. Figure 1 indicates the general appearance of the main body of the comparator, exclusive of overflow flask and electric power connections. Figure 2 gives more specific detail needed
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by glass and machine-shop workers who might be concerned with the problem of construction. The principal vessel of the outfit is virtually a large Pyrex test tube, with sidearm connected to a simple overflowflask. I n this vessel a section of Pyrex tubing, open a t both ends and with a large hole near C (Figure 2) is held by the upturned edges of a piece of heavy, rigid, Monel or bronze screen E in such a position that water may circulate freely downward past heating element D, and thence upward through the outer space to C and return. D is a spiral of Chromel-A resistance wire (20 per cent Cr, 80 per cent Ni) of about 24 ohms resistance, which may be made of a section of about 38 feet or about 11.5 meters of No. 20 B and S gage wire. Such a resistance unit will draw about five amperes, normal capacity of the common hand-regulated variable laboratory transformer for 110-120-volt domestic service. Connection to the transformer is at D, further electric details of switches, etc., being omitted from the figures. This resistance unit must not be electrically operated unless immersed in the distilled water. Item A is a model KYAF "Barcol Small Motor,"L a neat, silent-operating device of low cost; factory list price of recent date is $3. Incidentally, this induction motor, like other Barcol motors of similar tvne and assortkd sizes, is a very economical unit of low fractional horsepower for actuating numerous small laboratory outfits, including Hershberg stirrers, ventilating devices, and even small thermostatic baths. I n the present assembly, a long tubular bearing C, held firmly by the cork a t the upper end, ensures quiet operation of the stirrer without side play. B is a simplified representation of a double clamp for holding two thermometers in close proximity. If one can obtain a specimen of the old-time cast-brass double buret clamp, no longer manufactured, the two threefinger clamp units may be cut off and used in an assembl~ B. is simp1e box, attached the base the 1
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Barber-ColmsnCO.,~ookfordIllinois.
VOLUME 33, NO. 1, JANUARY 1956
ringstand as in Figure 1. I n this box a block of plaster of Paris or magnesia-asbestos lagging, molded with hemispherical seat, supports the main comparator tube. I n Figure 1 are seen the two sections of a length of common magnesia-asbestos steam-pipe insulation about 12 cm. in outside diameter,, Each hollow hemicylindrical part of this dusty material is wrapped compactly in asbestos paper, which is wetted and pressed snugly into proper place, then dried. The wrapped unit is mounted in a piece of cardboard mailing tube split lengthwise. Such heat insulation is needed for calihration a t higher temperature ranges. A horizontal rod or section of thin metal tubing may be bolted to the half cylinder and held by standard laboratory clampholder to the rod back of the comparator, mostly out of sight with respect to the views shown in Figures 1 and 2. As fina,lly assembled and filled to normal capacity with distilled water, the comparator requires from about 12 to 14 seconds per centigrade degree of temperature elevation when the transformer is set to deliver the full five amperes of heater current. When the comparator has been brought to the desired calibration temperature, the transformer is not turned off; instead it is merely reset a t a "holding" voltage, taken from an empirical chart devised to suit the particular comparator. For example, at normal laboratory temperature i t reauires about 32 volts to hold the author's comparator at 60°C. It would be natural to criticize this instrument, with preference for a large, mide heating bath with general random stirring movement. Actually the comparator as shown here has a special advantage related to its slender design. When the first period of heating is concluded, the mercury column of the thermometer does not merely "crawl" t o the final value. Instead, the last portion of water which passes the immersion heater before turndown of the electric current is a t relatively high temperature, and causes a sudden rise above the ultimate figure to be read. This surge is immediately followed by a colder portion of water, and the mercury co:umn falls. Until the last portion of heated water has time to mix with the main body of the bath, oscillation of temperature occurs. The variations in this oscillation rapidly dwindle, leading promptly to constancy and establishing confidence in the temperature reading which immediately follows. Workers are warned t o adjust clamp B so that the bulbs of the thermometers do not touch either the outer or inner tubes of the comparator proner. A minor additional convenience may be added in the form of a common pharmacist's atomizer bulb with check valve, for returning overflow water to the comparator after the bath has cooled. It is also desirable to adjust the thermometer positions so that the two mercury columns are a t exactly the same height at the temperature of calibration. This eliminates concern over parallax.
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Obviously the thermometer being tested may also be set a t any degree of partial immersion if desired. For comparison with corresponding technique of the National Bureau of Standards, see the excellent chapter by Busse2 who describes the more costly Bureau equipment, and also gives an interesting critical discussion of problems in use of liquid-in-glass thermometers.
The present writer has constructed and put into service two additional comparators, one for general use up to 300°C., and one specially designed for Anschiitz thermometers. These employ silicone oil as heat-transfer liquid. Description of these is deferred until after more adeauate trial of the devices.
