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Again the difference in character of the hydrated cements was very apparent, fine sulfoaluminate needles being abundant in the hydrated commercialcement while none were found in the white cement. Lastly, the crystalline grains, as centers of gelatinous particles, lasted for a much longer time in the white cement than they did in the commercial cement. Possibly these differences may be due to the small amount of ground gypsum added to the commercial cement and also to its greater percentage of magnesium and ferric oxides. The presence of silicates seemed to have influenced the speed of disintegration of hydrated tricalcium aluminate crystals, since in the tests with the two varieties of cement
Vol. 18, No. 8
and solutions of sodium sulfate these crystals lasted for a much shorter time than they did in the tests with pure hydrated tricalcium aluminate. Solutions of magnesium sulfate were especially destructive in their action on crystals of hydrated tricalcium aluminate, regardless of the presence or absence of calcium silicate. The character of the layers surrounding crystals of hydrated lime varied with the sulfate solutions used. In sodium sulfate the layers were apparent only on the edges of the lime crystals and were made up of granular masses. In magnesium sulfate solutions the lime crystals were heavily enveloped, the coating frequently consisting of a transparent homogeneous layer as well as granular masses.
The Measurement of Surface Temperatures' I I-Comparison
of Various Methods
By F. W. Adams and R. H. Kean MASSACHUSETTS INSTITUTE OF
A
TECHNOLOGY, SCHOOL
OF
CHEMICAL
LL problems of the flow of heat from solid bodies involve a determination of the temperature of the
ENGINEERING PRACTICE. EASTERN MANUFACTURING C O . , BANGOR, ME.
recently calibrated, millivoltmeter which could be read to closer than 0.5' C. The different methods of measurement tested were in detail as follows:
surface of the solid, and there is a surprising lack of information in the literature on this subject, below the range of optical and radiation pyrometers. Since a large propor(a) T h e compensated thermocouple devised by Boyer and tion of heat-transfer probIems falj in this range, the present Buss at this station, consisting essentially of a thermocouple investigation was conducted to determine the reliability of held against the surface and compensated for heat losses by various methods of surface tempereture measurement and to means of a small electric heating element. ( b ) A thermocouple whose hot junction was imbedded in compare their precision and general applicability. the wall of the steam chest, as recommended by Foote, Fairchild, The investigation included a ktudy of the precision ob- and Harrison.3 The wires were inserted into a small hole drilled tainable in the measurement of surface temperatures by the about 2 mm. into the surface and held in place by a soft copper plug driven in with them. use of a mercury thermom(c) A thermocouple whose eter, of thermocouples with h o t j u n c t i o n was soldered soldered hot junctions, of a onto the surface. The results obtained by various methods of measurthermocouple in which the ( d ) A device made of two ing the temperature of a metal surface between 100' No. 14 B. & S. gage thermometal surface served as a and 150' C. are presented and discussed. A thermocouple wires clamped into a hot junction, and of a thercouple compensated for heat losses was found to be wooden handle leaving about mocouple compensated for 3 mm. of each protruding. the most accurate method. This device has been sucheat lossesl2and was limited These ends were sharpened cessfully applied to the measurement of the temand pressed firmly against to a range of temperatures peratures of a wide variety of metallic and nonmetallic t h e s u r f a c e t o make the c o n v e n i e n t 1y obtainable surfaces, including moving surfaces. c o n t a c t between the wires with steam as a heating t h r o u g h t h e metal of the steam chest itself. medium. Experimental Procedure
The measurements were made upon the surface of an iron steam chest between 100' and 150' C., and the results compared with the temperature of the steam within, and with the calculated surface temperature. The steam chest was equipped with a pressure gage and thermometer well, giving a knowledge of the conditions within, as a basis for the estimation of the precision of the various measurements. The steam temperature could be maintained constant within 0.3O C., and during the tests care was taken to eliminate disturbing air conditions which might distort the readings. The thermocouples used in the investigation were all made of chrome1 and cope1 alloys, and except where otherwise mentioned were of No. 22 B. & S. gage. They were all calibrated before use against an accurate thermometer, which was used for determining the temperature in the steam chest. The e. m. f.'s generated by the thermocouples were measured on a new, Received March 12, 1926. 1 Boyer and Buss, TXISJOURNAL, 18, 728 (1926). 1
(e) A thermocouple whose hot junction was placed under a n asbestos pad held firmly against the surface. (f) A thermometer whose bulb was placed under a n asbestos pad held firmly against the surface.
Results
The results obtained on the surface of the steam chest by the various methods are presented in Table I, and in Figure 1. The temperature of the surface (Curve 2, Figure 1) was calculated from the temperature of the steam4 to be between 0.3' and 0.6' C. below the steam temperature. The temperature indicated by the compensated couple is shown to fall on the same line as the calculated surface temperature (Curve 2). The imbedded couple (Curve 3) falls about 1O C. below this line, while the couple soldered to the metal surface gives a reading about 3" C. low (Curve 4) over the range studied. The other methods give results further away from the true surface temperature (Curves 5, 6, and 7 ) . a 4
Bur. Standards, Technologic P a P n 170, p. 298. Walker, Lewis, and McAdams, "Principles of Chemicat Engineering; '
1923, p . 170.
August', 1926
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of Surface (" C.) of S t e a m C h e s t as Measured by Various I n s t r u m e n t s ( d ) SEPARATE WIRES (a) COMPENSATED COXXECTING cf) TABRMOMETER COUPLE ( b ) IMBEDDED COUPLE ( 6 ) SOLDERED COUPLE THROUGH SURFACE (e) COUPLEUNDER PAD UNDER PAD Recorded Error Recorded Error Recorded Error Recorded Error Recorded Error Recorded Error 102.8 -1.9 101.7 -3.0 105.0 -3.6 108.3 -1.4 106.7 -3.0 103.9 -5.8 98.3 -11.4 111 1 +0.3 112.2 -1.9 113.3 -1.4 111.1 -3.6 107.8 -6.9 102.8 -11.9 116 1 -0.3 113.9 -4.9 118.3 -1.1 116.7 -2.7 112.8 -6.6 107.2 -12.2 118.3 -5.0 124 4 0 123.3 -1.1 121.1 -3.3 117.8 -6.6 111.1 -13.3 $1.1 126 1 130 0 +0.6 128.3 -1.1 126.1 -3.3 122.8 -6.6 122.2 -7.2 115.0 -14.4 134 4 0 133.3 -1.1 131.1 118.9 -15.5 -3.3 127.2 -7.2 130.6 -4.9 137 2 0 132.8 -5.6 140 0 $0.6 138.3 -1.1 136.7 -2.7 132.2 -7.2 123.3 -16.1 141 6 0 144 4 143.3 -1.1 141.1 -3.3 136.1 -8.3 128.3 -16.1 140.6 -6.0 147 8 +o. 1 147.8 -1.0 141.1 -8.3 132.2 -17.8 150 6 $0.6 148.3 -1.7 -0.5 150 0 142.2 -8.3 T a b l e I-Temperature
Steam 105.0 108.9 110.0 111.1 114.4 115.0 116.7 119.4 120.0 123.9 125.0 125.6
130.0 135.0 136.1 137.8 138.9 140.0 142.2 145.0 147.2 148.3 150.0 150.6 151.1
Surface calcd. 104.7 108.6 109.7 110.8 114.1 114.7 116.4 118.8 119.4 123.3 124,4 125.0 129.4 134.4 135.5 137.2 138.3 139.4 141.6 144.4 146.6 147.7 149.4 150.0 150.5
I n Table I are given the temperatures recorded by the different methods together with the deviation in degrees Centigrade of each measurement from the calculated temperature of the surface. As mentioned by Boyer and Buss, there is some tendency toward localized heating of the surface beneath the "head" of the compensated couple device, which, however, can be eliminated by a slow continual movement of the instrument over the surface. This heating is undoubtedly aggravated by the fact that the head of the present experimental instrument covers about 58 sq. cm. and it should be largely eliminated by reducing this to a few square centimeters as is planned in the construction of future instruments. The approximate error in the calculation of convection and radiation losses from a metal surface a t 132' C. to a room a t 20" C. for each of the methods is given in Table 11. Thus, for determining heat losses under these conditions with a precision of 10 per cent, any of the methods except the use of a thermometer is available.
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Table 11-Error in Calculated Heat Loss ( b y Radiation a n d Convection) f r o m Body at 1 3 2 O C. t o R o o m a t Z O O C. Error Method of temperature measurement Per cent ( a ) Compensated couple 0 ( b ) Imbedded couple 1 ( c ) Soldered couple 4 ( d ) Separate wires connecting through surface 7 (P) Couple under pad 9 c f ) Thermometer under pad 18
I n the calculation of film coefficients where an accurate knowledge of the temperature of the metal surface is required, the compensated couple is preferred. The two methods utilizing the metal surface as a hot junction are available where less precision is desired, but are limited by the fact that metallic contact is necessary. Furthermore, as metal surfaces are usually covered with a layer of oxide, paint, or dirt, nearly all such surfaces encountered in practice should be classed as nonmetallic and hence their temperature can be accurately measured only with the compensated couple. This device has further advantages in that on metal it obriates the necessity of drilling into the surface, and that it is available for single measurements a t many points on any plane surface. An additional marked advantage is its availability for use on moving surfaces, to which it has, in fact, already been successfully applied. A calibration obtained on a rotating, steam-heated, iron drier roll galre a surface temperature of 132.8" C. when the steam temperature was 133.9' C. In this case the surface temperature calculated from the coefficient of the steam film and the conductivity of the metal was 133.1" C. The ease of manipulation and portability of the cornpensated couple device make it a very
57"am Temperahre "C Figure 1 1-Steam temperature 4-Soldered couple Calculated surface temperature 5-Wires connectin(:) through {Compensated couple (a) surface ( d ) 3-Imbedded couple ( b ) 6-Couple under pad (e) 7-Thermometer under pad (f)
wooden and glass doors of drying lofts, magnesia lagging, and moving sheets of pulp. The temperatures covered have ranged from 25' C. to 150" C., beyond which it has not been possible to go with the present experimental instrument owing to the use of tin solder in its construction. However, with brazed or silver-soldered connections its range should be much extended.
Secretary of Agriculture Reduces Charge for Grading Rosin Seven cents per package for grading rosin under the Naval Stores Act, with a minimum charge of $3.00, will be charged by graders of the United States Department of Agriculture in the future in accordance with regulations signed by the Secretary of Agriculture on July 17, 1926. This charge is a reduction from ten cents per package.
