INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Vol. 15, N o . 8
T h e Measurement of Temperature in Rubber and Insulating Materials by Means of Thermocouples‘ By Ellwood B. Spear and J. F. Purdy THE GOODYEAR TIRFAND RUBBERCo., AKRON,OHIO
SVO methods have The cause of the erroThe thermocouple methods hitherto employed are shown to gioe presented t h e in neous readings obtained by temperature measurements in rubber articles, tires, efc., which are the foregoing method is exselves to various inmuch too low, unless there is little or no temperaturedifferentialfor plained as follows: The vestigators as possible ways seoeral inches along the wiresfrom the thermaljunction. thermocouple wires are very of measuring the operatCuroes are given whereby many temperature measurements regood conductors of heat, ing temperatures of pneucorded in the literature may be corrected. while rubber conducts heat matic tires. One is the deA new thermocouple casing is described which has been found poorly. When thermotermination of the temperconoenient for the measurement of temperatures in solid or pneumatic couple wires are imbedded ature from pressure-volume tires while the latter are in use on the road. changes of the air confined only a short distance in the rubber, as is necessarily the i n t6e inner tube. The other method involves the use of thermocouples. case in a tire, only a small surface of wire is in contact with Considerable attention has been paid to the first-mentioned the rubber. Heat from a thin film of rubber immediately method.2 I n order that reliable results may be obtained surrounding the wires is rapidly conducted away by the it is necessary to measure very accurately changes of pres- wires. Heat from the surrounding mass of rubber is not sure, the expansion of the tire, and leakage of air from the transferred to the wires so rapidly as heat is conducted inner tube during the time the tire was rolling. Considera- away by them. As a result the temperature of the therble calculation is necessary to derive the temperature from the mocouple and the thin film of rubber around it is kept data obtainable. The method is best suited to laboratory lower than that of the surrounding mass of rubber. tests. The greatest objection to this method lies in the fact Experiments in this laboratory have shown that when that only the average temperature in the tube is measured. using No. 28 gage wire to measure the temperature of rubber This is not the temperature of the carcass of the tire. Recent a t least 31/2inches of the wire must be kept a t the same temtests have shown that the carcass of a truck tire may run perature as the rubber a t the point where the temperature is to 60” F. above the temperature of the air inside the tube. I n be measured, in order to secure correct readings. That is to say, many cases it may indicate that one tire is hotter than mother, there must be no temperature differential along the wires for but this is not invariably reliable, especially when testing a t least 31/2 inches from the couple. When this length of wire tires of radically different construction. It does not indicate is in contact with the rubber, enough surface is exposed to the which portion of the tire is the hottest. Some means are nec- rubber to allow suffiessary whereby the carcass of a tire can be explored, and cient heat to be transits actual temperatuye and its hottest portions known. ferred to the wires to To fill this need the thermocouple has been brought into maintain the thermouse. At fist thermocouple? were placed between the plies couple a t the temperof the tires, when building. Very meager results were ob- ature of the surroundtained by this method, for as soon as the tire began to roll ing mass of rubber. In the wires were broken by the flexing action. cases of temperatures More recently means have been employed to insert a ther- higher than those ormocouple into the tire from the outside to some point whose dinarily measured in temperature was to be measured. The thermocouples were rubber compounds, or made of No. 28 gage, copper-constantan wire twisted to- in cases of other artigether and soldered. One way of inserting the thermocouple cles of lower heat conin the rubber was to pass the couple through an eye in the ductivity, it will probhead of an awL3 The awl was thrust into the rubber, carrying ably be necessary to the couple with it. On withdrawing the awl the couple re- insert the thermomained inbedded in the rubber. When tire temperatures couple deeper in order were measured the thermocouple was inserted to a depth of to insure correct temfrom ‘/4 to 11/4 inches. perature reading?. No @ Experiments in this laboratory have shown that thermo- expcrimental work has couples used in the manner described above give temperature been carried out in this readings which are far too low. The error depends upon the regard on substances size wires used, the depth to which they are inserted, the other than rubber. temperature differential along the wire, and the thermal FIG 1-ArrARATUS TO DETERMINE ERROR INSERTED conductivity of the rubber stock whose temperature is being DEPTH OF lNsERT1oK I N READINGSOF THERMOCOUPLES OF THERMoCoUPLE TO VARIOUSDEPTHSI N RUBBER measured. The correction to be applied must be determined NECESSARY A-Rubber cylinder built around a steam for the particular conditions under which the thermocouple pipe; half of cylinder removed t o show The apparatus used to is used. position of thermocouples show the proper depth B-Steam pipe through which live steam was 1 Presented before the Division of Rubber Chemistry a t the 64th Meeting of insertion of a therpassed t o heat the rubber of the American Chemical Society, Pittsburgh, Pa., September 4 t o 8, 1922. mocoup~enecessary to C-Thermocouples parallel t o pipe imbedded Ellenwood, J. Sor Automotive Eng., 10 (1922).
T
2
8 “A Method of Measuring the Temperature a t Different Points in t h e Body of an Automobile Tire.” Research Bulletin, Kew Jersey Zinc Cornpany, November, 1921.
correct is shown in Fig. 1. give
t o various depths in rubber D-Thermocouples perpendicular t o pipe imbedded to various depths in rubber
INDUSTRIAL A N D ENGINEERING CHEiMlSTRY
August, 1923
A cylinder of rubber, A, made of a pure gum-sulfur mix (Fig. I) was built around a 37/16-inchiron pipe, B. After the rubber cylinder was properly cured it was cut in half axially. Therniocouples, C, were placed between the halves. Each
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CORRECTIOX CURVES In order to determine the corrections that must be made on thermocouple readings under conditions usually found when measuring tire temperatures, the apparatus shown in Fig. 3 was constructed. A thin slab of rubber, B, was laid over an electric hot plate, A . Other slabs of rubber, C, of any desired thickness could be laid on top of B. The actual temperature between the slabs B and C was measured by thermocouples having at least 6 inches of wire at the same temperature as the rubber about the couple. Previous experiments, as described above, had shown that such thermocouples would give correct temperature readings. Other thermocouples were inserted in the rubber and their readings compared with the actual temperature.
FIG.3--APPARATWS TO DETERMINE ERROR I N THERMOCOUPLE READINGS A-Electric hot plate B-Thin slab of rubber C-Slab of rubber of a n y desired thickness, through which thermocouples were inserted to slab B D-Thermocouples placed to give actual temperature between slabs B and C E-Tire thermocouple inserted in rubber F-Thermocouples inserted in rubber with a n awl, a s is sometimes done in meamring tire temperatures FIG
READINGS
OF
THERMOCOUPLES IMBEDDED I N RUBBERTO VARIOUS
DEPTHS A-Temperature readings of couples inserted 1 inch in rubber B-Temperature readings of couples inserted 2 inches in rubber C-Temperature readings of couples inserted 33/4, 5 , a n d 8 inches in rubber Curve C represents t h e actual temperature
thermocouple was placed at a carefully measured distance from the pipe. The wires leading from the thermocouples were laid exactly parallel to the pipe. B y placing the wires in this manner, the entire length of the wire imbedded in the rubber was surrounded by a mass of rubber a t the same temperature as that surrounding the thermocouple. Sets of wires were imbedded in the rubber to various depths from 2 to 8 inches. The halves of the cylinder were than cemented together and cured for a short time in order to set the cement. Steam at any desired pressure could be passed through the iron pipe to raise the temperature of the rubber to the desired degree. By this method it was found that the thermocouples of No. 28 gage wire imbedded 31/2inches or more in the rubber gave the same temperature readings a t any given distance from the steam pipe. Those imbedded less than 3*/2 inches gave readings lower than the actual temperature a t the point measured. Fig. 2 shows curves drawn from original data. The curves show the temperature readings of thermocouples imbedded to depths of 1 and 2 inches in the rubber and the actual temperatures as obtained by couples imbedded 3*/2, 5, and 8 inches. Had the rubber not had such a low thermal conductivity, less than 31/2inches insertion would doubtless have sufficed. In the experiment the temperature ranged as high as 300" F., thus covering any temperature usually found in rubber compounds. Thermocouples inserted, as shown by D. Fig. 1, to depths of 2 inches and less, and having a greater temperature differential along the wires than was the case with couples C, Fig. 1, gave readings much lower than the actual temperature.
In this apparatus the thermocouples could be placed much more accurately in any desired position than was possible when using the rubber cylinder (Fig. 1). It also had the advantage of giving a much wider temperature range. Thermocouples, F , Fig. 3, were inserted through slab C by means of an awl. The slab C was then laid in position so that the thermocouples F lay between slabs B and C. ThermocoupIes were made of insulated copper-constantan wires twisted together at the ends and soldered. The temperature gradient through the slab of rubber C represented very closely the temperature gradient in the carcass of a tire. Calibrations were made using two different rubber compounds. Both compounds are used in tires. One had a comparatively
-- TEMPERATURE
DIFFERENTIALOF
ORDINARY THERMOCOUPLES TNSERTED TO VARIOUS DEPTHS I N RUBBERO F VERY Low THERMAL CONDUCTIVITY A-No. 28 gage wire inserted 0 30 inch in rubber B-No. 28 gage wire inserted 0.55 inch in rubber C-No. 28 gage wire inserted 0.85 inch in rubber D-No. 28 gage wire inserted 1.15 inches in rubber E-No. 36 gage wire inserted 0.30 inch in rubber F-No. 36 gage wire inserted 0.55 inch in rubber G--No. 36 gage wire inserted 0.88 inch in rubber Temperature differential is equal t o t h e potentiometer reading minus t h e temperature of the air immediately around the rubber
FIG.4-cORRECTTON
TO
INDUSTRIAL A N D ENGINEERING CHEiWISTRY
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low thermal conductivity, the other had a comparatively high conductivity. Figs. 4 and 5 show the corrections to be applied to thermocouples of No. 28 and 36 gage when inserted as shown by F , Fig. 3.
0
20
40
60 bo 100 120 140 TEnPERATUflE DIFFERENTIAL OF
FIG.5-CORRECTION
TO O R D I N A R Y
160
1.w
200
THERMOCOUPLES I N S E R T E D TO VARIOUS
DEPTHSi N RUBBER S T O C K OF MEDIUM THERMAL CONDuCTrvrTY A-No. 28 gage wire inserted 0 30 inch in rubber B--No. 28 gage wire inserted 0.55 inch in rubber C-No. 28 gage wire inserted 0.85 inch in rubber 17-No. 28 gage wire iqserted 1 15 inches in rubber E-No. 36 gage wire inserted 0.55 inch in rubber F-No. 36 gage wire inserted 1.15 inches in rubber Temperature differential is equal t o the potentiometer reading minus the temperature of the air immediately around t h e rubber
NEWTYPEOF THERMOCOUPLE Fig. 6 shows a thermocouple developed by the authors which has given very satisfactory results. A cylinder of hard maple is turned to the dimensions shown. A thermocouple is made of No. 36 or No. 40'gage copper and constantan wires twisted together and soldered, or fused as desired. A small copper disk "18 inch in diameter and inch thick is soldered to the thermocouple. The disk is then sprung into a depression at the end of the tube. A maple plug is turned to fit tightly into the top of the tube. Two heavy terminals, one of copper and one of constantan, are fixed in the plug. The small t'hermocouple wires are attached to these terminals. Magnesium oxide is sifted lightly around the M Tires until the tube is filled. The plug is then glued into the top of the fube. This type of thermocouple is inserted into a hole drilled in the tread of a tire to some point in the carcass. By using a high-speed drill (1800 toi2400 r. p. m.) a clean hole can be made. The hole is just large enough to admit the thermocouple, thus excluding all air and allowing the copper disk, or heat collector, to press tightly against the bottom of the hole. By using mires of very smail ' * h n I G . 6-TIRE THERMOCOUPLE A-Hard maple tube turned on lathe diameter the heat loss by B - - S O . 36 gage copper and constan- conduction along the wires t a n wires is Kreatlv These " reduced. C-Copmr disk and thPrmocouple fragile -,ires are completely D-Magnesium oxide packed loosely protected by the maple tube. around wires The copper disk soldered E-Copper-constantan terminals
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to the thermocouple acts as a heat collector. It exposes a comparatively large surface to the rubber, thus facilitating the transfer of heat from the rubber to the thermocouple. This compensates to a large degree for the lack of depth of e insertion of the thermocouple in the rubber. The purpose of the magnesium oxide in the tube is to aid in insulating the wires from heat from the sides of the hole. This will mask, if not wholly eliminate, the effect of the temperature gradient in the tire. For any given depth of insertion the temperature will depend only upon the actual temperature a t the point being measured and the temperature of the air immediately surrounding the tire. This makes possible a study of the different temperatures found a t various points in a tire carcass. The thermocouples are easily reproducible, and give identical readings on calibrating. The correction for this type of thermocouple is shown in Fig. 7. The depth of insertion of the thermocouple, the potentiometer reading, and the temperature of the air around the tire are easily found. When they are known the correction to be added to the thermocouple is found from the chart. This chart covers a much wider range of conditions than will ordinarily be found when measuring tire temperatures. Ugually a correction of 6"or 8" F. is the largest that need be made. Because the errors in the readings of this type of thermocouple are small, the difference in the correction to be applied when stocks of different thermal conductivities are measured is so small as to be negligible. In addition to the calibration made in the apparatus shown in Fig. 3, the tire thermocouple was also calibrated in a 40 x 8 inch tire. The tread. and several plies were laid back. Thermocouples were made of No. 36 gage wires. These couples were placed between the plies in such a manner that at least 6 inches of the wires were kept at the same temperature as the thermocouple. The plies were then cemented down and given a short cure, exactly as is done when repairing a tire. The thermocouples between the plies gave the
'
DEPTH O F INSERTION IN IflCHES FIG.7- CORRECTIOK CURVES*OR TIRETHERMOCOUPLES Temperature differential equals the potentiometer reading minus the temperature of the air immediately around the tire
August, 1923
IiND V S T R I A L A N D ENGINEERING CHEXISTRY
actual temperatures. Holes were drilled through the tread of the tire to various depths to admit the tire thermocouple. The tire was heated by a sectional air bag through which steam could be passed a t any desired pressure. The results of the calibration of the thermocouples in this manner checked with those previously obtained. Drilling a hole in the tread of a tire to receive the thermocoupltx is not so severe on the tire as one might suppose. After a year's experience using this method in the laboratory, no tire has been found in which premature separation between the plies has occurred due to drilling the hole. In a tire having twelve plies of fabric, the writers have drilled through nine plies and still operated the tire for an extended time. When this method is used on road tests a rubber plug inserted in the hole will exclude all dirt. No rubber cement is necessary to keep the plug in place. When a
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reading is desired the plug can easily be removed with SE pair of sharp-nose pliers. Two precautions must be taken when using the tire thermocouple in order to secure correct temperature readings. The first is to allow the thermocouple to remain in position long enough to overcome the lag in the thermocouple. This takes about one minute, but varies slightly under different conditions of temperature. The other is the correction necessary for the time of reading after the tire stops. As soon as the tire stops rolling it begins to cool. Since the temperature cannot be taken instantly, it is always necessary to note the exact time of stop and the exact time when the temperature is taken. The rate of cooling of the tire should be observed and from the cooling curve a correction can be made for the drop in temperature between the time when the tire stops rolling and the time when temperature is hken.
Sodiurn Hypochlorite' I-The
Preparation of Concentrated Sodium Hypochlorite Solutions of Great Stability for Use in Food Factories,-Milk Plants, Etc. By Harper F. Zoller THBNIZER LABORATORIES Co., DETROIT,MICH.
ETHODS for the tants. Calcium hypochloA demand for cheap sodium hypochlorite exists in the United preparation of sorite, or chloride of lime, is States because of its rapidly increasing use as a disinfecting agent dium hypochlorite much lesi; used than forin food jactories and elsewhere. are so numerous that it merly, principally because A method for making strong sodium hypochlorite solutions of seems almost unnecessary of the high lime carrying 2 to 5 per cent available chlorine from sodium hydroxide, sodium to add one more to therichly power and the sluggishcarbonate, and drum chlorine is given, and cost data arc presented. laden literature. But the ness of its suspensions. The value of much bu$er material in the holding of the pH around methods heretofore adSodium hypochlorite is 10.5 for the region of maximum stability of concentrated sodium vanced have been of value more effective, is simpler hypochlorite solutions is the principle of a successful method of only t o the textile manuto handle, and will not preparing such solutions. facturw. A specific hypoleave a film of lime a t any The yield of sodium hypochlorite calculated from the weight of chlorite solution was made time through its use. chlorine used is about 100 per cent. This points to a di$erent for a special factory operaMany factories and plants conception of the chlorinating reaction than that which has been tion under a given set of engaged in the production prevalent. working Conditions. No of food,products would use method has been devised for sodium hwochlorite as a the wholesale production of a stable hypochlorite solution sterilizing rinse for their equipment, providedlarge quantities which would be suitable for transportation or storage for use of a uniformly stable quality could be made readily available. in other industries. This study of hypochlorites was under- The present market price is prohibitive to a factory of any taken for the express purpose of determining the true nature size. of hypochlorites in solution, their stability and activity, and 'A careful search into the merits of hypochlorites as sterilof producing a method for their preparation in large or small izing agents under conditions where organic matter was quantrties for industrial plants. mainly absent, a t once revealed their value, and a series of Our conceptions of the constitution and the physical and studies was undertaken to determine suitable methods of chemical behavior of hypochlorite have very materially preparing the one logical hypochlorite (sodium) in large changed in the last few years. Recently, there have been a quantities. These studies led to a consideration of the few attempts to regulate the stability and bleaching prop- practical stability of this substance under the conditions to erties of chlorinated alkali solutions by studying their alka- which it would be subjected in food factories and elsewhere, linity in an empirical fashion. Among these investigations as distinguished from the properties expected of it as a are the observations of Higgins2that alkalies stabilize sodium textile bleaching agent. hypochlorite solutions and lower the velocity of bleaching, A method is here outlined which will enable the average while neutral salts increase the bleaching rate and decrease factory to prepare its own effective disinfecting agent simply the stability. Renewed interest in hypochlorites, especially and cheaply. In subsequent papers the writer will take the alhdi hypochlorites, for bleaching purposes in the textile up the activity of sodium hypochlorite in the presence of industry has been strongly manifested in England during the organic matter and its relative bactericidal efficiency. The last decade. I n the United States the pendulum seems to have method described in this paper is based upon the hydrogenswung in another direction; we have become deeply interested ion control system of manufacture. The solution is heavily in hypochlorites as sterilizing agents and general disinfec- buffered in order that the reaction of the solution will not rapidly change to yield an unstable solution. A sodium 1 Received December 18, 1922. hypochlorite solution containing 5 per cent available chlorine * J . SOC.Chem. Ind., 27, 185 (1911).
