C.4.2 Vertical Axis Current Meters
Three models of vertical axis meters are in general use in Canada: the Price 622AA meter, the WSC winter meter, and the Pygmy meter. This section describes these meters.
The Price 622AA meter (Figures C-14 and C-15) is the most common vertical shaft meter and is often considered the "standard" for discharge measurement. It has been subjected to extensive research and experimentation and shown to be well suited to a wide variety of field conditions. The Price 622AA is the principal meter used by the WSC and many other agencies to determine discharge.
Figure C-14. Components of the Price 622AA current meter.
Figure C-15. Price 622AA meter.
Only one bucket wheel assembly is required for the entire range of velocities encountered during normal stream gauging operations. Providing that it is properly maintained, the meter responds accurately to velocities ranging from 2.0 to 300 cm/s. Although the Price 622AA has been in use for 80 years and has an established record of dependability and durability, this meter must be treated with the same care and attention given to any scientific instrument. Damaged components can cause erroneous measurements, which can go undetected for long periods of time.
The main components of the current meter are the pivot and rotor, the contact chamber, and the yoke and tail assembly. The rotor has six cone-shaped elements and is 125 mm in diameter. The letter "T" stamped on the inner portion of the frame indicates the top side of the bucket wheel. When in use, the rotor moves in a counterclockwise direction.
The key feature of the Price meter is the location of both the upper and lower bearing surfaces in fairly deep, inverted cavities which trap air when the meter is submerged. This effectively excludes water-borne silt from the bearing surfaces, which eliminates undue wear. Worn bearings or any other damage to the cup wheel will result in change in the meter rating. This meter may be attached to the standard USGS top-setting rod or the Columbus weight hanger. Mounting adapters, with or without the relocating device (Section D.2), are available from the manufacturers of the 20-mm bridge rods.
On the upper extension of the rotor is a chamber where cup rotation produces an electrical pulse for conversion of angular motion to stream velocity. This contact chamber is fitted with a bearing, a penta gear, and two insulated binding posts. Each post has a fine contact wire.
The top of the rotor shaft is rounded to provide a smooth surface where it comes in contact with the bottom of the chamber cap. Immediately below the rounded end, an eccentric is cut in the shaft. This is the means by which the shaft makes contact with the upper contact wire once during each revolution of the rotor. The next section of the shaft fits into the contact chamber bearing lug. A short section of acme thread is cut into the shaft below the bearing section. This meshes smoothly with the penta gear fitted in the bottom of the contact chamber. The penta gear has two tabs, each of which brushes the lower contact wire once during every five revolutions of the rotor.
Generally, the standard Price 622AA meter does not provide a signal suitable for use with an electric pulse counter in lower velocity regimes. When an eccentric makes a single contact with the " cat's whisker", several pulses may be generated and registered on the counter. The usual method of operation is to time the revolutions of the rotor either visually, or by means of an electrically generated audio signal. The Price meter can, however, be modified for use with a counter; see the next section.
The design and use of current meter vanes in cable suspension assemblies may be a problem in certain forms of turbulent flow. The problem lies with the suspension point (hanger bar) being equidistant between the rotor axis (of a vertical axis meter) and the hydrodynamic centre of the directional vanes, as is the case for the standard Price meter. This meter is inherently sensitive to lateral turbulent fluctuations due to low degree of directional stability, and can introduce an error to the measurement of velocity. The effect the hanger, rotor and vane relationship can only be assessed during field tests in streams over a complete range of turbulent length scales, these conditions can not be duplicated during tow tank calibrations.
The standard Price 622AA meter can be modified in two ways, using retro-fit kits, to compensate for the low-velocity limitations.
A 13-mm long permanent magnet is embedded in the top portion of the rotor shaft. This shaft fits into the centre of a special contact chamber (Figure C-16). A magnetic reed switch, which is accessible from the top of the assembly, is located in a chamber adjacent to the rotor chamber. The binding post and the insulating bushing seal this chamber. During each revolution of the rotor shaft, the magnet passes the chamber and closes the reed switch for a moment. Price 622AA current meters supplied with magnetic reed switches are usually referred to as low velocity models. However, in this case the maximum measurable velocity depends on the pulse rating (pulse/s) of the counter unit employed. The electronic revolution counters manufactured by OTT, Sherlock, and Braystoke, for example, have a maximum counting frequency of 20 pulses/s while some earlier electro-mechanical units were limited to 10 pulses/s In either case the maximum velocity measurable exceeds 5 miles/s while low velocity measurement accuracy is increased due to the absence of friction caused by contact wires.
Figure C-16. Price 622AA meter with retro-fit kit containing magnetic reed switch contact chamber.
The counter may be set to read direct velocity (display averaging 10 s for the 622AA, and 5 s for the Pygmy), or seconds vs. revolutions. Also, a calibration mode allows the continued use of a damaged meter for a limited time period, by electronically compensating for a meter that does not rotate according to its original specifications.
Note: This feature does not absolve an organization, working to Provincial Standards, of the requirement of regular calibration of current meters by the National Water Research Institute.
One of the most important features of this retrofit is its ability to measure accurate low velocities while using both the 622AA or Pygmy current meters which, in their standard contact chamber design, have poor low velocity response. Because the fibre-optic sensor does not require that physical contact be made and broken to produce a signal, as in the "cat's whisker" type, nearly all friction has been eliminated. This means that velocities lower than 3 cm/s can be accurately determined while the counter is in the seconds/revolutions mode. The maximum velocity range of a retrofitted current meter is 4.5 m/s.
Note: The fibre optic sensor should not be used with reel or handline suspension systems using the suspension cable as the sensor signal wire. Problems have been encountered in connecting to, and signal strength through, the Ellsworth type two conductor cable used by these systems. The fibre optic sensor, which attaches to the replacement meter head, is housed at the end of a two conductor cable. The sensor to counter cable connection is supplied in a standard length of 3 m for use with a wading rod but may be ordered in lengths up to 300 m. Ordering a factory 7-m or 10-m sensor cable for use with a bridge rod will also expand the capabilities of the retrofit for use with a handline or reel.
Figure C-17. Fibre optic retrofitted Price 622AA.
