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A multiple-tube manometer
A MULTIPLE-TUBE MANOMETER .* BY
A . F. ZAHM, Ph.D. Bureau of Construction and Repair, Navy Department .
Preface .-For the measurement of pressure distribution over aerodynamic models, it is convenient to have a manometer composed of numerous tubes arranged side by side and adjustable to various sensibilities . Figs . i and 2 give the external appearance of such an instrument developed in the Aerodynamical Laboratory of the Bureau of Construction and Repair, early in 1917 . Description of lnstrunzent.-A cylindrical alcohol tank rotatable in axial bearings carries at one end a tee-shaped pipe which supports a bank of twenty glass tubes mounted on a graduated metal plate . The glass tubes are sealed in sockets opening into ,he bottom branch of the tee, and are extended above the top branch by brass nipples for connection with rubber tubes leading to the aerodynamic surface to be studied. As the bore of the glass tubes is /,, inch, the alcohol tank is made 5o inches long by 6 inches in diameter, so that the change of hydrostatic head therein is very slight when the meniscus in the tubes travels a distance of two feet, this being the probable range required in general wind tunnel use . For the measurement of large pressure differences the tubes are set vertical ; for slight differences they are set on a slope of t in to by rotation of the system in its bearings ; and to prevent shifting of the zero in this operation the surface of the alcohol is kept in the axis of the instrument by timely adjustment . By means of the leveling screws the axis of the tank is set horizontal, and the whole apparatus can be given a slight rotation, if need be, to correct the slope of the inclined tubeholder . The clamping of the tube system is effected by means of the spring latch shown screwed to the baseboard and engaging a slot in the flange at the near end of the tank . Three such slots are provided ; one for holding the tubes vertical ; one for holding them on a i to 5 slope ; and a third for holding them on a i to io slope . The graduations on the plate back of the glass tubes, when vertical, read in inches and tenth inches of water, positive and negative . * Communicated by the Author . 771
772
1. F. ZAti+i .
[j, F. [ .
Fu: . I,
Multiple tube manometer, upright.
Fin . 2 .
Multiple tube manometer, inclined (by rotation on a" pipe running endwise through the tank and resting in end bearings) .
. i 9 Dec
I
.\ \It t .TIPLE-TI-BE
M_1\o11ETER .
%73
J) tree of Accuracv .-The individual tubes are calibrated with a hook gauge truly to'/ inch water head referred to the zero position of the meniscus . When so mane tubes arc used that the surface of the alcohol in the cistern sinks appreciably, the amount of this fall is noted on an open-ended tube and applied as a correction . This never is large . For the surface area of the fluid in the tank is ten thousand times the projected area . of one Fm . 3 .
Meniscus . Hence, when one tube only is used, the error due to neglecting the fall in the tank is or per cent ., if the tube be vertical ; o ; per cent ., if sloped r in ~ ; and ro per cent ., if sloped r in to. Tf ten tubes are used at once, all under pressure or all under suction, the error is ten times as much, and may require correction, as above indicated . _1pplicntion .-Figs . 2 and 3 illustrate the manner of connecting the multiple-tube manometer to an aerofoil whose pressure distribution is to be studied at various wind speeds and incidences . From the manometer nipple f/8-inch rubber tubing is conducted
774
A . F. ZAHM .
15 . F. 1.
down the back of the tube-holder ; thence through the floor of the room and the ceiling of the wind tunnel ; thence down a streamline strut to brass reduction couples, from the other ends of which fine spectacle tubes run to still finer nipples inserted in the aerodynamic model, to collect the pressure at a series of neighboring points . Due care is taken to locate the fine collector tubes in such manner as not to disturb the air flow over the portion of the surface under study . In the apparatus here illustrated the brass nipples are of 0 .025inch bore and % inch long ; the spectacle tubes are of ' ./,,-inch bore and average nearly a foot long ; and the whole tube system dampens the play of the alcohol columns sufficiently for convenient reading. Ten readings at a time may be taken for ten successive angles of incidence, and all completed within an hour, if the aerofoil be rotated from without, while the wind speed remains unchanged . Except at the trailing edge of the aerofoil, each brass nipple collects the same pressure when the other nine are present as when they are absent . In an actual experiment each of the two rearmost nipples collected pressures which were false by o .oi inch to 0 .02 inch of water when the other nine were present . This error in the two rearmost pressures may entail an error of about i per c ent . i n the resultant pressue for the entire surface .
The Action of Carbonic Acid on Sulphides . (Scientific Proceedings Royal Dublin Society, vol . xv, p . 171, 1917 .)-E . A . Letts and Florence W . Rea,in the course of a study of the chemistry of foul mud deposits, made some studies of the actions of solutions of carbonic acid on sulphides, the results of which are interesting to analytical chemists and geologists . It has long been believed that sulphates may be reduced by the action of organic natter to sulphides, and this, according to Beyerink and Van Delden, takes place under the influence of specific microbes : Microspira desulphuricans in fresh water, and M . estuarii in salt water . Letts and Rea found that a current of carbonic acid passed into a solution of sodium sulphide produces a rapid evolution of hydrogen sulphide, the whole of the sodium salt being ultimately converted into sodium acid sulphate . Calcium sulphide is decomposed to nearly fifty per cent ., the soluble, so-called bicarbonate, being formed . Freshly precipitated ferrous sulphide is slowly decoinposed by carbonic acid, the final result being the soluble ferrous H. L. " bicarbonate ."
A . F. ZAHM, Ph.D. Bureau of Construction and Repair, Navy Department .
Preface .-For the measurement of pressure distribution over aerodynamic models, it is convenient to have a manometer composed of numerous tubes arranged side by side and adjustable to various sensibilities . Figs . i and 2 give the external appearance of such an instrument developed in the Aerodynamical Laboratory of the Bureau of Construction and Repair, early in 1917 . Description of lnstrunzent.-A cylindrical alcohol tank rotatable in axial bearings carries at one end a tee-shaped pipe which supports a bank of twenty glass tubes mounted on a graduated metal plate . The glass tubes are sealed in sockets opening into ,he bottom branch of the tee, and are extended above the top branch by brass nipples for connection with rubber tubes leading to the aerodynamic surface to be studied. As the bore of the glass tubes is /,, inch, the alcohol tank is made 5o inches long by 6 inches in diameter, so that the change of hydrostatic head therein is very slight when the meniscus in the tubes travels a distance of two feet, this being the probable range required in general wind tunnel use . For the measurement of large pressure differences the tubes are set vertical ; for slight differences they are set on a slope of t in to by rotation of the system in its bearings ; and to prevent shifting of the zero in this operation the surface of the alcohol is kept in the axis of the instrument by timely adjustment . By means of the leveling screws the axis of the tank is set horizontal, and the whole apparatus can be given a slight rotation, if need be, to correct the slope of the inclined tubeholder . The clamping of the tube system is effected by means of the spring latch shown screwed to the baseboard and engaging a slot in the flange at the near end of the tank . Three such slots are provided ; one for holding the tubes vertical ; one for holding them on a i to 5 slope ; and a third for holding them on a i to io slope . The graduations on the plate back of the glass tubes, when vertical, read in inches and tenth inches of water, positive and negative . * Communicated by the Author . 771
772
1. F. ZAti+i .
[j, F. [ .
Fu: . I,
Multiple tube manometer, upright.
Fin . 2 .
Multiple tube manometer, inclined (by rotation on a" pipe running endwise through the tank and resting in end bearings) .
. i 9 Dec
I
.\ \It t .TIPLE-TI-BE
M_1\o11ETER .
%73
J) tree of Accuracv .-The individual tubes are calibrated with a hook gauge truly to'/ inch water head referred to the zero position of the meniscus . When so mane tubes arc used that the surface of the alcohol in the cistern sinks appreciably, the amount of this fall is noted on an open-ended tube and applied as a correction . This never is large . For the surface area of the fluid in the tank is ten thousand times the projected area . of one Fm . 3 .
Meniscus . Hence, when one tube only is used, the error due to neglecting the fall in the tank is or per cent ., if the tube be vertical ; o ; per cent ., if sloped r in ~ ; and ro per cent ., if sloped r in to. Tf ten tubes are used at once, all under pressure or all under suction, the error is ten times as much, and may require correction, as above indicated . _1pplicntion .-Figs . 2 and 3 illustrate the manner of connecting the multiple-tube manometer to an aerofoil whose pressure distribution is to be studied at various wind speeds and incidences . From the manometer nipple f/8-inch rubber tubing is conducted
774
A . F. ZAHM .
15 . F. 1.
down the back of the tube-holder ; thence through the floor of the room and the ceiling of the wind tunnel ; thence down a streamline strut to brass reduction couples, from the other ends of which fine spectacle tubes run to still finer nipples inserted in the aerodynamic model, to collect the pressure at a series of neighboring points . Due care is taken to locate the fine collector tubes in such manner as not to disturb the air flow over the portion of the surface under study . In the apparatus here illustrated the brass nipples are of 0 .025inch bore and % inch long ; the spectacle tubes are of ' ./,,-inch bore and average nearly a foot long ; and the whole tube system dampens the play of the alcohol columns sufficiently for convenient reading. Ten readings at a time may be taken for ten successive angles of incidence, and all completed within an hour, if the aerofoil be rotated from without, while the wind speed remains unchanged . Except at the trailing edge of the aerofoil, each brass nipple collects the same pressure when the other nine are present as when they are absent . In an actual experiment each of the two rearmost nipples collected pressures which were false by o .oi inch to 0 .02 inch of water when the other nine were present . This error in the two rearmost pressures may entail an error of about i per c ent . i n the resultant pressue for the entire surface .
The Action of Carbonic Acid on Sulphides . (Scientific Proceedings Royal Dublin Society, vol . xv, p . 171, 1917 .)-E . A . Letts and Florence W . Rea,in the course of a study of the chemistry of foul mud deposits, made some studies of the actions of solutions of carbonic acid on sulphides, the results of which are interesting to analytical chemists and geologists . It has long been believed that sulphates may be reduced by the action of organic natter to sulphides, and this, according to Beyerink and Van Delden, takes place under the influence of specific microbes : Microspira desulphuricans in fresh water, and M . estuarii in salt water . Letts and Rea found that a current of carbonic acid passed into a solution of sodium sulphide produces a rapid evolution of hydrogen sulphide, the whole of the sodium salt being ultimately converted into sodium acid sulphate . Calcium sulphide is decomposed to nearly fifty per cent ., the soluble, so-called bicarbonate, being formed . Freshly precipitated ferrous sulphide is slowly decoinposed by carbonic acid, the final result being the soluble ferrous H. L. " bicarbonate ."