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anemometers

Значение термина anemometers в knolik

anemometers - anemometers (the determining of air currents)
anemometers - See also chimneys; ventilation; etc.

The various methods that have been employed for this purpose, may be divided into three groups.

First. - By moving at the same velocity as the current, and noting the distance passed over in a unit of time.

Second. - Determining from observation the rate at which small floating particles are carried along by the current, and assuming their velocities to be identical with that of the air-current itself. Smoke from exploded gunpowder, burning turpentine or amadou, small pieces of down, and small balloons filled with hydrogen, have been all more or less employed for this purpose.

Third. - By using anemometers, or apparatus of various forms; and these may be divided into three classes: (a) Anemometers having vanea or wands, made to revolve by the current of air impinging upon them, the rate at which they revolve being indicated by pointers on dials forming a part of the instrument - the pointers being made to revolve by means of wheels connecting them with the axis of the vanes or wands. The anemometers of Biram and Davis are instances of this class of instruments now in use in this country, all of which require a correction for friction. (b) Instruments which are affected by the force or impulse of the wind, without being subjected to any continuous revolving motion, such as Dr. Lind's, Henaut's, Bougier's, and Dickinson's anemometers.

In modern practice determination of air velocities are made only by the anemometer, and it is therefore unnecessary to give further particulars of the first and second methods.

Benjamen Biram's Anemometer

The simplest form is that invented by Benjamen Biram, shown in Fig. It consists of a series of vanes, D, E, which revolve with the action of the air-current - the number of revolutions, or rather numbers proportional to the revolutions, being registered by a pointer, P, on the face of a dial forming a part of the instrument itself. It is made of three sizes, 4, 6, and 12 in.; is very portable, and is not, with proper care, liable to get out of order, especially the smaller size. A certain force of current is required to overcome the friction, and put the instrument into motion. Some of these instruments will continue to revolve in a current as low as 30 ft. a minute; but with most of them a velocity of about 50 ft. is required.

Every one who has occasion to use this anemometer should be aware that it does not register the actual velocity of the air, especially in feeble air-currents, nor yet the number of revolutions, but only a number proportional to the latter; and although it is of great value, as indicating an increase or decrease in the velocity, from time to time, such as the periodical variations in any particular current, it is of comparatively little value as generally used for ascertaining real velocities, such, for instance, as occur in changing or splitting air-currents, when it is of great importance to know the actual quantities. To obtain with this instrument accurate results, available for all purposes, it is necessary, as with Combes' anemometer, to apply a formula to its recorded revolutions, or rather to the number indicated by the index, in order to ascertain the actual velocity of any current; each particular instrument requiring special experiments to be made with it, in order to determine the value of the constants required to be employed in the formula. These constants remain the same for the same instrument, so long as it remains in the same condition, and are independent of the velocities of the currents of air in which it is employed. These adjustments are carefully made by the manufacturer.

Lind's Anemometer

(a) The raising of a column of fluid above the general level of its surface is the principle of Dr. Lind's anemometer, Fig. It consists of two glass tubes about 9 inches long and 4/10 of an inch in diameter, connected at their lower extremities by another tube of glass only 1/10 of an inch in diameter. The upper extremity of one tube is either bent over as shown, or is fitted with a thin metal cap, at right angles, so that its mouth may receive the current of air in a horizontal direction. Water is poured in at the mouth till the tubes are nearly half full, and a scale of inches and parts of an inch is placed between the tubes. When the wind blows in at the mouth of the tube the column of water is depressed in this tube, and elevated to a similar extent in the other tube, so that the distance between the surfaces of the fluid in each tube is the length of a column of water, the weight of which is equal to the force of the wind upon a surface equal to the base of the column of fluid. The object of the small tube which connects the two larger ones is to prevent the oscillation of the fluid by irregular blasts of wind. The absolute velocity of the wind is deduced from the height of the column of water, or it may be ascertained from the tables constructed for the purpose. Thus, according to Dr. Lind, a column of water 0.025 in. high, exerts a pressure of rather more than 2 oz. 1 dr. upon a sq. ft. of surface, and balances the effect of a gentle wind moving at the rate of about 5½ ft, in a second, or not quite 4 miles an hour. When the column of water is 1 in. high, the force of the wind on a sq. ft. is nearly 5½ lb., its velocity 32½ miles an hour, and its character a high wind. When the column marks 3 in. the force is upwards of 15½ lb. on the sq, ft. the velocity above 56¼ miles per hour, and of the character of a storm. At 9 in. the force on the sq. ft. is stated to be 46 lb. 14 oz.; the velocity 97½ miles an hour, producing a most violent hurricane. Thus, it will be observed that in the greatest storms, the difference between the atmospheric pressures on the windward and leeward sides of any object does not amount to 1/50 of the pressure of the leeward side.

From numerous experiments, Dr. Lind considered that the pressure of the wind in direct impulse is nearly proportional to the square of its velocity. The following Table is calculated from this, but considerably enlarged by other experiments.

Borda, however, found that the force of the wind was greater by 1/10 part than Eouse's Table gives. Button also showed that the forces at very great velocities increased in a somewhat higher ratio than the squares of the velocity.

Henaut's Anemometer

Henaut's Anemometer is similar in its principle and action to that of Dickinson (See Fig); in the latter the impulse is received on a plain surface A, of oiled skin about 3 inches square, suspended from the top p, the variations of which, from the perpendicular pbdg, are noted on a scale ddn, which is marked off by direct experiments. This instrument is extremely portable, and not easily put out of order; but whilst it possesses the great value, with other instruments of this class, of not requiring any watch or other means of noting the time, it is in common with them subject to the great disadvantage of vibrating continually, especially in a rapid current, and of not recording the variation of the velocity within limits of 20 ft. per minute; it is however, very useful in steady currents of from 200 to 700 ft. per minute. The supports g g, are secured to a base ct, which is levelled by screws rv.

A simple anemometer

in Fig.: a is the pressure-plate, exactly 6 in. square made of galvanized iron and fastened to the pillar a 10 lb. spring balance 6; the cylinder of the balance being fixed to the vertical tube c, which carries the vane, etc. To the end of the iron rod, in the balance, is attached a wire which passes over a wheel inside c, shown in the drawing, but, of course, in the tube, and so that the copper wire can go down centre of c to the weight of d, which must have a slit in one side to run over a wire soldered inside f; this keeps lower wire straight and prevents torsion. The wire is joined up in two at the bottom of d, and the two wires should be continued down to the bottom. The vane is of the usual form, but should form a balance for the other side, and must be weighted to form the balance necessary. The wire is continued down into the room in which dial is fixed. Here make a dial 12 in. diameter and divide it into 36 divisions each division being a full inch from the, next: these divisions will indicate pounds, and if you divide the spaces between each into 5 you will have everything you require. The centre wheel has a groove, and its circumference should be exactly the length of the rod in the balance, or rather the length from 1 to 9 lb., every pound giving 4 lb. on. dial. Now fix at the end a thin wire or watch-chain, the latter would be better, the old chains used in our grandfathers' verge watches - this, by the bye, must be connected to upper as well as lower wires; to the bottom one then hang a weight just sufficient to keep finger in place. Having all done, get another similar balance and fix it up against dial-plate, letting it mark 1 lb.; this will be 4 lb. on dial down below; mark this on the dial; then let upper balance be pushed on to 21b., mark 8 below, and so on till the whole dial is marked, then divide and mark pounds. You may go over this several times until you have carefully marked the dial, and your anemometer will be finished.

A multiplying anemometer

A multiplying anemometer, applicable to the measurement of the velocity of air-currents, to meteorological observations, and to the determination of waterflow, consists of a tube formed as two truncated conical tubes, the smaller ends of which are of the same area (Venturi's tubes). In this tube a much smaller one of similar construction is placed, as shown in Fig. If greater delicacy be required, a third may be added, the whole system being eccentric (Fig.). The constricted part of the outer compound tube is surrounded by a hollow jacket, and connected with it by the small interval which separates the two truncated cones. This jacket is in connection with a U water-gauge, which indicates the velocity of the current to be measured. This arrangement for a single compound tube is shown in Fig. The utility of the instrument depends upon the fact that in such a case, as shown in Fig., the reading of the manometer attached to the jacket is several times that indicated by a manometer at the orifice of the tube. The former is of course negative, whilst the latter is positive. The relation between the two may be, for example, 6:1. In an instrument consisting of two compound tubes, and in one of three tubes, the readings were related to those at the orifice in the proportions 20:1 and 80: 1 respectively. The instrument is simple, rigid, portable, and inexpensive; it affords a check on the ventilating apparatus of mines, and by a simple clockwork arrangement could be made to indicate defective ventilation; lastly, its multiplied reading conduces to great accuracy. (Atkinson and Dalgleish.)

Table of the force and velocity of different winds for the graduation of anemometers

Height of the column of water in Dr. Lind's Anemometer	Force on a sq. ft. avoirdupois		Computed from Bouse's Experiments		Computed from Dr. Mutton's Experiments		Character of the Winds.	Author
	lb.	lb. oz. dr.	ft/sec	mph	ft/sec	mph
0.0009515	0.005	00 00 1.280	1.43	1	1.63	1.11	Hardly perceptible	Rouse
0.0038060	0.020	00 00 5.120	2.93	2	3.26	2.22	Just perceptible	Rouse
0.0083732	0.044	00 00< 11.264	4.40	3	4.84	3.30
0.0133210	0.079	00 01 4.224	5.87	4	6.52	4.44	Gentle winds	Rouse
0.023	0.123	00 01 15.488	7.33	5	8.09	5.51
0.025	0.130	00 02 1.280	7.55	5.14	8.33	5.67	A gentle wind	Lind
0.050	0.260	00 04 2.560	10.67	7.27	11.77	8.00	Pleasant wind	Lind
0.092	0.492	00 07 13.952	14.67	10.00	16.16	11.01	Pleasant brisk gale	Rouse
0.10	0.521	00 08 5.378	15.19	10.35	16.66	11.35	Fresh breeze	Lind
0.11	1.107	01 01 11.392	22.00	15.00	24.30	16.57	Brisk gale	Rouse
0.368	1.968	01 15 7.808	29.34	20.00	32.39	22.00	Very brisk	Rouse
0.5	2.604	02 09 10.624	33.74	23.00	37.26	25.40	Brisk gale	Lind
0.585	3.075	03 01 3.200	36.67	25.00	40.51	27.62	Very brisk	Rouse
0.84	4.429	04 06 13.824	44.01	30.00	48.60	33.13	High wind	Rouse
1.0	5.208	05 03 5.248	47.73	32.54	52.70	35.93	High wind	Lind
1.146	6.027	06 00 6.912	51.34	35.00	56.69	38.65
1.5	7.873	07 13 10.688	58.68	40.00	64.79	44.00	Very high	Rouse
1.9	9.963	09 15 6.528	66.01	45.00	72.89	49.69	Great storm	Denham
2.0	10.417	10 06 10.496	67.50	46.02	74.53	50.81	Very high	Lind
2.68	12.300	12 04 12.800	73.35	50.00	81.02	55.24	Storm or tempest	Rouse
3.0	15.625	15 10 0.000	82.67	56.37	91.28	62.23	Storm	Lind
3.37	17.715	17 11 7.040	88.02	60.00	97.20	66.27	Great storm	Rouse
4.0	20.833	20 13 5.248	95.46	65.08	105.40	71.86	Great storm	Lind
4.08	21.435	21 06 15.360	96.82	66.00	106.92	74.79	Great storm	La Condamina
5.0	26.041	26 00 10.496	106.72	72.76	117.84	80.10	Very great storm	Lind
6.0	31.490	31 07 13.440	117.36	80.00	129.59	88.54	Hurricane	Rouse
6.0	31.250	31 04 0.000	116.91	79.71	129.09	88.01	Hurricane	Lind
7.0	36.548	36 08 12.288	126.43	86.20	139.65	95.21	Great hurricane	Lind
8.0	41.667	41 10 10.752	135.00	92.04	149.07	101.63	Very great hurricane	Lind
9.0	46.875	46 14 0.000	143.11	97.57	168.11	107.80	Most violent hurricane	Lind
9.36	49.200	49 03 3.200	146.70	100.00	162.04	110.48	Hurricane that tears up	Rouse
10.0	52.083	52 01 5.248	150.93	102.90	166.66	113.63	trees and throws down	Rouse
11.0	57.293	57 04 11.008	158.29	107.92	171.72	117.08	buildings	Rouse
11.12	58.450	58 07 3.200	160.00	109.00	176.55	120.37		Rochon
12.0	62.500	62 08 0.000	165.34	112.73	182.57	124.47

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