An instrument for measuring velocity in water mains
Richard Van Vleck
From Scientific Medical & Mechanical Antiques, No. 26
By the turn of the century, city water works had taken advantage of technological advances in filtration and hydraulics. But, while huge pumping engines were providing a surplus of water, many city water systems were losing over 50% of their capacity to leaks in already aging iron mains. The many miles of iron pipe buried under streets were frequently ignored, at great expense to the water departments. But, with the invention of the portable pitometer, underground water pipes could be monitored at will. A standard 1" corporation cock fitting allowed introduction of the pitot tube and permanent “street connections” provided an instant setup of the pitometer. The recording pitometer made a continual record of the water velocity as downstream valves were closed to determine where leaks or illegal bypasses existed.
Origin of the Pitometer
The principle of the Pitot tube was discovered by the French engineer, Pitot, around 1732. The idea was carried a step further by D’Arcy, to measure the current in open channels (illust at right). The first patent for use of a pitot tube device for measuring velocity in pipes was granted to Henry Fladd, of St. Louis, in 1889. His “velocimeter” utilized a Pitot tube facing into the flow of water and a second tube facing the opposite direction. The forward facing tube is affected by the impact of the water, while the rear facing tube is affected by the draft of the flowing water. The tubes are connected to a u-tube containing a slightly heavier liquid, and the difference in height of the meniscus in each leg of the u-tube is read. The greater the water velocity in the test pipe, the greater will be the difference in the height of the two columns. He used
a clock driven revolving drum recorder fitted with photosensitized paper and a slit along the length of one leg of the u-tube that created a continual recording of the height of the meniscus. The liquid in the u-tube was colored so as to block the light falling on the chart paper. The u-tube was positioned beneath the water pipe, simplifying adjustment of the meniscus. However, he did describe an alternative setup in which the u-tube was inverted and a liquid of lower specific gravity than water was used.
|A photo-pitometer adapted for measuring stream velocity
John R. Freeman, of Lawrence, MA, also developed a Pitot tube device for recording water velocity in pipes in the early 1890's. In 1895, Edward Cole, of Chicago, began working on the Pitometer. He patented a form of self recording manometer in 1899, which he used with a front and rear facing Pitot tube similar to Fladd, but, with the addition of a third tube to measure static pressure in the pipe. Static pressure was recorded on the chart by a simple linkage arm which crossed the light path at the lower end of the slit. An oil lamp was provided for lighting the u-tube and notches in the slit caused parallel reference lines to be printed on the sensitized paper.
|The simplest form of Pitot tube (left) is a curved tube directed into the current. The addition of a second tube, with its opening perpendicular to the flow (center) is used for open channel work. The tube above the top valve is fitted with a mouthpiece for drawing the water column up the manometer. A Pitot tube used in a closed pipe (right). From Carpenter, Experimental Engineering.
In 1903, John Cole patented a simple and inexpensive manual recording device for a manometer, which did not rely on the use of sensitized paper. Edward Cole patented a prism device for better illumination of the meniscus in his Pitometer in 1908. The prism housing was secured to one leg of the u-tube by clips. In 1908, Edward Cole patented an electromagnetic recording device to replace the photo recorder.
In 1908 Smith & Lanham, of Washington, DC, patented an improved pitometer, in which they addressed two problems of Cole’s device. The oil lamp or electric conversion in the Cole Pitometer could heat the u-tube, causing movement of the meniscus. Their improvement was a water jacketed u-tube.
They also used an inverted u-tube, to enable the use of an indicating liquid having a specific gravity slightly less than that of water, and an improved viscosity over the heavier liquids previously used. Cole had recommended a mixture of gasoline and carbon tetrachloride, having a specific gravity of 1.25.
While the Pitometer was designed for measuring water velocity in pressurized pipes, it can also be used to measure stream flow. When placed above the water level in an open system, air must be withdrawn from the u-tube bleeder valves with a small hand pump in order to draw water up to the manometer. Very small velocities can be measured with sensitive pitometers, compared to the impeller type instruments.
|Cole's Photo-Pitometer, marked "No. 464, The Pitometer Company, Chicago". The clockwork driven 10" drum can be set to revolve once per day or once per week. The photosensitive chart paper is exposed by the illumination of an oil lamp mounted in the rear cover. A dark indicating liquid in the U-tube blocks the light below the meniscus and an adjustable slit controls the exposure.
PITOMETER RELATED PATENTS
Author: Richard Van Vleck. © 1996, Greybird Publishing.