XVI.

A DRAFT AND BLAST PRESSURE GAUGE.

BY J. H. KINEALY, ST. LOUIS, MO.

(Member of the Society.)

25

20

15

I have long wanted a draft and blast pressure gauge that would enable me to easily obtain pressure to within 0.01 of an inch of water. After considerable investigating and some experimenting I finally devised the gauge shown to the society, a drawing of which is shown in Fig. 25. Mounted on a board there is a Utube A in which is placed a quantity of water. To one leg of the U-tube is fastened a 3-way cock H, and to the other is fastened' the scale I and the micrometer screw T. The end of the U-tube to which the micrometer screw is fastened is open to the air, so that the pressure in that end is. · always equal to the atmosphericpressure. The other end of the U-tube is connected, by means of a rubber tube fitting over R, to the place whose pressure it is. desired to measure. E,is an opening in the 3-way cock H that is open to the atmosphere. By turning the handle of H E and R may be put in connection, R and the U-tube may be put in connection, or E and the Utube may be put in connection. If R is in connection with the U-tube and E cut out, the water in the left leg of the U-tube will rise or fall according as the pressure to be measured is less or greater than the atmospheric pressure, and the water in the right leg of the U-tube will fall or rise. Both legs of the tube are of the same diameter, so that the

Fig. 25.

rise or fall will be the same in each, and the difference between the levels of the liquid in ne legs will be equal to twice the rise or fall in one leg..

The scale is graduated to tenths of an inch, the micrometer screw T has ten threads to an inch, and the head O is divided into ten equal parts, so that the graduations on the head O represent hundredths of an inch.

To use the instrument, the board on which it is fixed is fastened to a wall or other place of support, so that the legs of the Utube are vertical; R is connected to the place whose pressure it is desired to measure and the 3-way cock turned so as to connect E with the U-tube and cut out R. The screw T is then turned by means of the milled head C until the point on its end just touches the surface of the water in the U-tube, and the readings of the scale I and head O are taken. Next, the 3-way cock is turned so that the U-tube is in connection with R and E is cut out, and the screw T is run down or up, as the case may be, until the point is again just in contact with the surface of the liquid. The readings of the scale I and head O are again taken. The difference between the first and the last readings of the scale I and the head O give the rise or fall of the water in the right leg of the U-tube, and this multiplied by two will give the pressure of the blast or draft in inches of water.

I have found by experiments that there is no trouble in determining when the point of the screw T touches the surface of the water and in reading the scale and micrometer head to .003 of an inch, although I seldom take a reading closer than .005 of an inch. The end of the screw must be kept slightly oiled so that a drop of water will not gather on it.

In the gauge which I show the society the U-tube is made of a glass tube whose internal diameter is of an inch. I have found, however, that this is not large enough to prevent the interference of capillary attraction. If one of the legs is dirtier than the other, there is a marked difference in the capillary attraction; and to get absolutely correct results, I find that the tube should be made larger. I think it should be about 3 of an inch internal diameter.

The glass work has not been very well done in the gauge which I show the society, and therefore the range of pressure wth which the gauge can be used is not as great as it should be.

It is evident that if the gauge is to be used only to measure the pressure of blasts of fans the scale could be graduated to tenths of an ounce and the micrometer head to hundredths of an ounce.

DISCUSSION.

Prof. Carpenter:-I am interested in this gauge which Prof. Kinealy has shown, as I think it excellent. I have, however, been using for the past year a gauge of a somewhat different type, which I will take pleasure in showing later. It reads as close as Prof. Kinealy's gauge, and is adapted for use as an anemometer as well as gauge, so that it can be used not only for measuring pressures but velocities of air. On the train last night I unfortunately stepped on the glass portion of the tube and broke it. I left the gauge at a shop to be repaired, but it will be up here this afternoon. If the discussion can be taken up later, I should like

to show it to the society.

Mr. Cary:-I have seen Prof. Carpenter's instrument and I am very much pleased with it, but while this matter is under discussion I might show a type which I have used. (Mr. Cary then made a sketch on the blackboard of the instrument to which he referred. This gauge was first described by Mr. J. C. Hoadley in a report of a series of trials of a warm blast apparatus, which was published in Vol. VI. of the transactions of the American Society of Mechanical Engineers, from which the following description of the gauge is taken.)

"Two glass tubes (Fig. 26), about 30 inches long, about 0.4 inch diameter inside and 0.7 inch outside, are connected at each end by means of stuffing boxes, to suitable brass attachments through which they are secured to a backing of wood.

These attachments, at top and bottom, have each a tubular opening, with a stop-cock in the middle of its length, which can be turned at will to establish a free communication between the glass tubes, or to shut off all communication. Directly over each tube a brass drum-shaped vessel is placed, 4.25 inches in length and of equal diameter. The heads of these drums, at both ends, are formed of plate glass, properly secured with screwrings and made tight with suitable packing. A tubular opening extends up from each glass tube to the drum above it, and there is a hole in each drum, directly in line with the axis of the glass tube, each fitted with a stop-cock and nipple for attaching a flexible pipe. Two sliding scales are arranged between the glass tubes to measure.

Fig. 26.

one the depressions, the other elevations of the surface of a liquid. filling the lower half the tubes, indicated in the cut, (Fig. 26), near the middle of the height. Both stop-cocks are represented in the cut as closed.

"The lower one being opened the two tubes, in communication at their lower ends, are filled up to about the middle of their height with a mixture of alcohol and water, care being taken to avoid wetting the interior of the upper end of the tube poured through, the pouring being done through a small glass tube inserted through the hole at the top of the drum, from which the stop-cock is removed for this purpose. The filling-tube is now to be raised so that its lower end is a little above the surface of the alcohol and water, the lower stop-cock is to be closed, and the upper one opened, and crude olive oil is to be carefully poured in until it fills the first tube up to the upper cross-tube into the second tube, and so finally fills both tubes and rises to about the middle of both drums.

"The crude olive oil is of an olive-green color and forms with the colorless alcohol and water a beautiful and very deep meniscus, if the tubes are clean and the filling has been done with sufficient care, making the line of demarkation very distinct. Neither liquid discolors the glass, and if up-and-down motions are made cautiously and slowly, the liquids do not mix and the common surfaces remain undisturbed. The specific gravity of the oil should be determined in advance, as it may vary a little, although it has been found quite uniformly 0.916. The specific gravity of the alcohol and water may be made anything desired between that of water, 1.000, and that absolute alcohol, 0.813, but must always be made greater than that of olive oil.

"Where extreme delicacy is desired the difference may be as small as one per cent; that is, if the oil be as above, .916, the mixture of alcohol may be .926. If the difference be much less than one per cent, the upper and lower liquids have a tendency to get into confusion and do not constantly maintain a distinct line of demarkation at their common surface. For many purposes a difference of specific gravity as great as two per cent will give sufficient sensitiveness-fifty times as much range as a water column and is more convenient

to use.

"The method of using this instrument to ascertain the force of chimney draft or other air current is as follows: If both the stopcocks between the tubes are opened and both the small stop-cocks on the top of the drums are also opened, so that the surface of the oil in both drums alike is open to the air, both liquids will come to a level; the oil in the drums, very obviously, and the heavier mixture

below the oil as certainly, if not quite as obviously; since, if higher in one tube than in the other, the united weight of the two liquids in that tube must be greater than in the other and must cause the liquid to sink down and flow into the other tube, raising the surface. of the oil in the drum over that other tube and causing it to flow across to the first tube, until both liquids are brought to a coincident height in the two tubes.

"A slight difference will, however, commonly be found in the height of the lower liquid, owing to the unequal capillarity of the tubes, since these can rarely be obtained sufficiently near alike in caliber to avoid, when in equilibrium, a small, but sensible difference of level, which must be ascertained and allowed for.

"If, now, the upper stop-cock between the tubes be closed, the lower one being left open, the surfaces of the two liquids will retain their respective heights in the two tubes, so long as the surface of the oil in the two drums remains subject to equal pressure. But if one drum be put in communication with a flue or chimney, by means of a flexible or other tube connected with the nipple of the small stopcock-this stop-cock being open-while the other drum remains open to the air through its open stop-cock, the diminished pressure, due to chimney draft, upon the surface of the oil in that drum, will cause the oil to flow up into the drum under the preponderating weight of the air on the surface of the oil in the other drum.

"The surface of the oil in the drum is about 100 times as large as the inside cross section of the glass tubes, and in the same proportion will the rise of the lower liquid on the one side and its depression on the other exceed the corresponding rise and depression of the upper surface of the oil.

"If now, when equilibrium has been restored, the lower stopcock be closed and the upper one opened and the connection with flue or chimney be severed, say, by removing the flexible tube from the nipple, the lower liquid will be kept immovable, while the oil will flow through the upper cross-tube, and come to a common level in the two drums. On connecting the nipple again with the flue or chimney, and again closing the upper stop-cock and opening the lower one, a diminished repetition of the former action will take place; the lower liquid will rise a little in one tube and fall a little in the other, and the surface level of the oil in the two drums will again become slightly unequal. This inequality, which will be much less than before, may be again removed by the same method; and a very few repetitions of this process will bring the difference in level of the surface of the lower liquid in the two tubes (corrected.