The Venturi Water Meter.
At the recent meeting of the New England Water-works Association. R. A. Robertson, Jr., treasurer of the Builders’ Iron Foundry, Providence, read a paper on the Venturi water meter, which itself is of extremely simple construction. It is merely a contraction introduced in a pipe line to produce an abrupt depression in the hydraulic gradient, from the measurement of which the quantity of water can be accurately determined. In Mr. Herschel s experiments gauges were connected with the meter, one at the throat and one at some convenient point at the up-stream side of the contraction. The difference in pressure indicated by these gauges was the head on the meter. Substituting this value of the head in formula, a coefficient approximating to unity was obtained. The coefficient showed a remarkable constancy whether applied to a rough meter and 9-inch pipe, or a smooth meter and i-inch pipe, with velocities ranging from 1 foot to 6 feet per second.
The original meters used by Mr. Herschel at Holyoke were of wood, built up inside a metal tube and lined at the throat with brass. The use of wood, however, is not to be recommended on account of the swelling and warping sure to take place, as well as on account of its lack of strength. The Holyoke meters were found at the close of the experiments to be in a very poor condition. Recently the East Jersey Water Company has received from the Builders’ Iron Foundry, Providence. R. I., a 36 inch meter made entirely of cast-iron and carefully worked to the required shape. The throat is lined with brass, and the pressure at the head or up-stream end is taken from an air chamber around the body of the lube above the beginning of the cone, and connected with the interior by clearly drilled holes. The throat is surrounded by a similar chamber, and vent holes are drilled in it in a plane perpendicular to the axes and with clean, sharp edges.
The rounding of the up-stream intersection of the cone with the cylinder of this meter, as compared with the Holyoke meters, and the greater smoothness of the cylinder and the superiority of the metal surface over the wood, contribute materially toward reducing the friction and consequent loss of head, and will also produce a greater uniformity of flow, thereby increasing the precision of the indications.
The length of the meter is about ioj£ times the diameter of the trunk. One to nine has been taken as the ratio of the area of throat to that of the pipe, and where the loss of two or three feet of head is of no consequence, Mr. Robertson states that it will be best to use that ratio The reason for this is that with high velocities through the throat of the meter, greater accuracy is obtained.
With the maximum velocity which is allowed in pipe lines by good practice, say, 6 feet per second, there will be a velocity of 54 feet per second through the throat of a meter whose ratio is 1 to 9. The loss of head when passing this quantity will not, it is claimed, exceed xo feet. With a flow of 5 feet per second and a corresponding velocity of 45 feet through the throat of the meter, the loss of head will not exceed 4½ feet.
Mr. Robertson does not claim that this meter will successfully operate for very low velocities. By the use of very sensitive gauges, measurements of extreme accuracy can be made, and this meter can thus be employed in some tests in place of weirs. But the commercial success of the meter for use in mains depends upon the use of an instrument capable of indicating the amount of water passing at any time, and of recording the total quantity passed.
It has proved a quite serious problem to devise a satisfactory indicating and recording instrument for this purpose. Mr. Herschel and other engineers have made several ingenious appliances, but all failed to record at a glance the quantity of water that had passed. II. D. Pearsall of London, Eng., suggested the use of an ordinary house meter, to be set on a by-pass leading from the up-stream connection to the throat of the meter. The flow through it would be proportional to the flow through the Venturi, and, if properly rated, its dial should indicate the volume passing through the meter. The latest and, according to Mr. Robertson, the best instrument has been found in a device of F. N. Connet, for which he has applied for a patent.
In a loop of pipe connecting the throat and up-stream end of the meter is inserted a gauge column of non-conducting material and a closed reservoir containing mercury. The gauge column is of sufficient length to permit the mercury, as it is displaced from the reservoir and rises in it, to balance any difference of pressure that may at any time exist between the throat and up-stream end of the meter. Through the shell of the gauge lube wires are inserted, each of which is connected with one or more buttons spaced about the periphery* of a switch dial. An arm ts made to revolve over the face of the switch dial, and is connected to the mercury in the gauge tube by wire, through a counter and battery.
The velocity of flow in the throat of the meter is determined by the excess of pressure at the up-stream end over that at the throat. Those who care to examine further into the theory of this meter will find a particularly lucid explanation in Prof. Merriman’s “Treatise on Hydraulics.” When the Connet register is employed, this head on the Venturi is indicated by the difference between the level of the mercury in the reservoir and that in the tube. The conductors inserted through the shell of the tube are spaced in such a manner that the number in contact with the mercury is always proportionate to the velocity of flow through the meter. The switch arm revolves at a constant rate, and in one revolution touches every button on the switch dial. There will be as many circuits made and broken in this manner and indicated by the counter as there are contacts with the mercury in the column. In this manner the readings of the counter are directly proportional to and indications of the rate of flow through the meter.
With the sectional area of the throat and the number of revolutions of the switch arm known, the train of gearing in the counter is made such that a positive volumetric record is made of the flow. The counter adds up the flow for successive cycles of an indefinite period, its dial reading to feet or gallons. The switch arm may be driven either by clockwork or by an electric regulator clock, which, with the counter and batteries, maybe removed to a considerable distance if desired.
A recording apparatus of this pattern has been made to accompany a 16 inch meter furnished to the town of Montclair, N. J., where the maximum velocity of flow through the throat does not exceed 25 feet per second, roughly corresponding to 2,500,000 gallons in 24 hours. A variation of 002 foot in head on the Venturi was readily indicated, according to Mr. Robertson, although such accuracy was unnecessary because the intervals between the conductors inserted in the gauge column arc greater than this, being made to correspond to increments of one-fourth of a foot in velocity through the throat above four feet. The minimum flow is about five feet, though the recording instrument is arranged to indicate flows as low as three-fourths of a foot per second. Mr. Robertson considers that the readings of velocities Inflow a)£ feet per second will be somewhat in error.
The Venturi meter is, it is understood, now being applied to the measurement of large quantities of gases, and the results of the experiments will be awaited with interest.