J u l y , 1914
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T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
mean velocity head is needed; a n d t o obtain a correct mean velocity head i t is necessary t o average t h e square roots of all t h e velocity head readings taken, throughout t h e cross-sectional area of t h e pipe, a n d t h e n t o square t h e average. There is a t present no royal road t o obtaining accurate velocity measurements b y means of t h e pitot t u b e . Investigators, t o obtain accurate results, must have recourse t o t h e painstaking methods adopted b y Rowse. We are told t h a t , for a 12-inch pipe, results within 2 per cent of correct m a y be obtained b y using 0.8 of t h e velocity head in feet of gas a t t h e center of t h e pipe. Lacking similar factors for pipes of other sizes, velocities must be calculated from readings t a k e n a t ntimerous points in t h e cross-sectional area of t h e pipe. I n recapitulation, it m a y be observed t h a t : I--As a result of t h e work of Rowse, engineers m a y now select a t y p e of pitot t u b e which m a y be used, under certain conditions, with confidence. 11-Further investigation needed, t o render t h e pitot t u b e more generally available as a means of measuring gas velocities, includes: I-A means of determining accurately t h e velocity of gases i n pipes whose length is less t h a n 20 times t h e diameter. a-The establishment of definite relations between t h e velocity head a t t h e center of a pipe, a n d t h e mean velocity head, for pipes of various sizes a n d shapes. NOTE-I a m indebted t o t h e a u t h o r of t h e original paper which constitutes t h e basis for this one-Mr. W. C. Rowse-for his courtesy in reading this manuscript, a n d for valuable suggestions offered b y him a n d adopted herein. COPPERHILL, TENNESSEE
THE NON-UNIFORMITY OF DRYING OVEN TEMPERATURES By LORIN H. BAILEY Received April 13, 1914
While testing t h e accuracy of t h e h e a t control of a new electric drying oven, i t was observed t h a t there
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I n t h e above tests only those thermometers were used which h a d been standardized b y t h e Bureau of Standards. With ovens having glass doors t h e thermometers were placed on t h e shelves a n d t h e temperatures read b y making t h e observations through t h e glass in t h e door without opening t h e oven. With t h e other ovens t h e following scheme was adopted: Six 50 cc. Erlenmeyer flasks were filled with clean, dry sand, stoppered, a n d through t h e stoppers t h e thermometers were inserted so t h a t t h e bulbs were held in t h e middle of t h e flasks. These flasks with their thermometers were t h e n placed in t h e various positions i n t h e ovens a n d after having remained there long enough t o come t o equilibrium they were removed a n d t h e thermometers read as quickly as possible. While this method is not absolutely accurate, i t is sufficiently so t o indicate whether or not there is a n y great variation in temperature. After testing these various drying ovens t h e writer's attention was called t o a n article t o R . G. Grimwoodl on t h e "Analysis of Crude Glycerine b y T h e International Standard Methods, 191 I . " I n this article t h e author mentions difficulty when using a drying oven which complies with t h e International Standard Methods specifications a n d which shows a maximum variation of 16" C. on one shelf a n d between t h e t w o shelves a maximum variation of 4 j o C. H e t h e n describes a special electric drying oven which has proved satisfactory for drying glycerin. This oven showed a maximum variation of z . z o C. from 160' C., t h e temperature desired. The results shoa-n b y Mr. Grimwood are very much better t h a n t h e writer was able t o obtain with a n y oven not surrounded b y boiling water a n d steam. T h e results in t h e above table show t h a t elaborately designed a n d expensive ovens are no more reliable t h a n t h e most simple a n d inexpensive ones. Of t h e ovens tested. only those surrounded b y boiling mater a n d steam are capable of maintaining even approximately uniform temperatures. I wish t o t h a n k Dr. J. A. LeClerc for his interest
Reading of Maximum range of temperature on Maximum thermometer . variation Approximate inserted through ___* top of oven top shelf lower shelf on both shelves inside dimensions in. O C OC O C OC No. Kind of oven 107 99-114 90-108 24 12 X 12 X 14 1 Electrically heated and controlled.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 89-100 88-103 15 12 X 13 X 18 2 Electrically heated and controlled. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 94-104 89-102 15 9 x 9 x 1 5 3 Electrically heated and controlled.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 8892 92-102 14 11 X 12 X 16 4 Gas heated porcelain lined., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 94- 96 105-118 24 8 X 10 X 11 ... 5 Gas heated air jacketed.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 9699 9598 4 13 X 15 X 15 6 Gas heated constant level water and steam jacketed., . . . . . . . . . . . . 100 101-101 101-102 2 15 X 8 in. diam. 7 Steam jacketed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 100-100 100-100 0 15 X 8 in. diam. 8 Gas heated constant level water and steam jacketed vacuum oven. .
was a wide range of temperature i n different p a r t s of t h e oven, not only between different shelves b u t also between different positions on t h e .same shelf. Ordinarily t h e temperature recorded on t h e thermometer inserted through t h e t o p of a n oven is t a k e n as t h e temperature a t which t h e drying is made, b u t i t was seen t h a t such could not be done with this particular oven. Following this observation a number of differe n t t y p e s of drying ovens were tested as t o their uniformity of temperatures throughout t h e drying chambers. T h e variations i n temperature are shown in t h e accompanying table.
a n d suggestions in connection with t h e testing of these various ovens a n d t h e writing of this report. LABORATORY OF PLANT CHEMISTRY, BUREAU OF CHEMISTRY u. s. DEPARTMENT O F .kGRICULTURE, m ' A S H I N G T O N
A MANOSTAT FOR USE IN GAS ANALYSIS By HARVEYN. GILBERT Received April 14, 1914.
I n t h e combustion of gases confined over mercury in a combustion pipette, more or less difficulty is always experienced b y t h e operator i n avoiding a difference of pressure due t o t h e difference of level between t h a t 1
J. SOC.Chem. I n d . , 38, 22.
