The USGS Type AA Current Meter is commonly known as the Price-Type Current Meter and has proven to be a versatile instrument with a long history of accuracy, reliability, and durability. The precisely balanced bucket wheel is mounted on a vertical pivot and is rotated by water flow. The rate of rotation is proportional to water velocity – streamflow velocities from 0.1 to 25 feet per second (0.03 to 7.6 meters per second) can be measured.
The main features of this meter are the uniquely designed bucket wheel shaft bearings and the two post contact chamber. The bucket wheel has six conical shaped cups, is five inches (12.7 cm) in diameter and rotates on a vertical axis inside a yoke. The tungsten carbide bearings for the bucket wheel shaft are located in deeply recessed inverted cups. When the meter is in use, these cups become air chambers and the entrapped air effectively excludes water and silt from the bearing surfaces giving extremely low starting velocities and minimal friction in the bearings. The standard contact chamber houses a penta gear and two binding posts, each having a fine platinum alloy contact wire (whisker hair). One wire makes contact with the bucket wheel shaft once every revolution; the other is used when fast velocities are encountered, and makes contact with the penta gear once every five revolutions of the bucket wheel.
The AA Current Meter can be suspended by means of a sounding weight while wading a stream, or by a cable and wading rod from an overhead structure. A tailpiece assembly assures proper alignment of the meter to water flow when suspended by a cable. Velocity is determined by counting the number of revolutions of the bucket wheel over a given period of time. Revolutions can be monitored by several means. A headset can be used to manually count the revolutions via an audible tone produced every revolution. Number of revolutions over a period of time are then compared to a rating chart (provided with each meter) which determines water velocity. The standard Type AA can be used with all counters as well as the AquaCalc Pro Digital Flow Computer.
The meter is made from brass and stainless steel and all exposed surfaces are chrome plated for corrosion-free service. Each AA Current Meter is provided with a standard rating table to convert bucket revolutions to stream velocity in English (feet per second) and Metric units (meters per second), spare parts, instrument oil, cleaning cloth, screwdriver and a rugged carrying case.
The Type AA-MH magnetic head current meter is the same basic design as the standard Type AA meter with the exception of the contact chamber and shaft. The Type AA-MH has a reed switch and magnetic shaft that allows for reduced friction in the contact chamber for better low velocity response and produces a cleaner signal for the AquaCalc Pro, AquaCalc 5000, AquCount & AquaPulse. Any Type AA meter can be converted to a Type AA-MH by just replacing the contact chamber and shaft and requires no special tools.
Pygmy Current Meter
Also available is the standard Pygmy Current Meter for shallow, slow wading survey applications. The Pygmy Meter is scaled two-fifths the size of the standard Type AA current meter. It does not have a tailfin assembly and is used only for measuring shallow streams with a wading rod. Its range of operation is 0.1 to 4.9 feet per second (0.03 to 1.5 meters per second). The 2-inch (5.1 cm) diameter bucket wheel contains six cups; a single contact closure is made each revolution of the bucket wheel. Each pygmy meter is shipped with a carrying case, standard rating table (English & Metric), spare parts, screwdriver, instrument oil and cleaning cloth.
The Pygmy-MH meter is a magnetic head current meter with the same basic design as the standard Pygmy meter with the exception of shaft and binding post contact. The Pygmy-MH has a miniature reed switch and magnetic shaft that allows for reduced friction in the contact chamber for better low velocity response and produces a cleaner signal for the AquaCalc Pro, AquaCalc 5000, AquaCount & AquaPulse.