MEASUREMENT OF DISCHARGE BY CONVENTIONAL CURRENT METER METHODS
5.3 INSTRUMENTS AND EQUIPMENT
Current-meter measurements usually are classified in terms of the method used to cross the stream during the measurement, such as wading, cableway, bridge, boat or ice. Instruments and equipment used in making current-meter measurements will vary depending on which of these measurement types are being used. Current meters, timers, and counting equipment are generally common to all types of current-meter measurements. This section describes all of the equipment used in making conventional current meter measurements, including current meters, timers, electronic field
Figure I.5.2. Computation notes of a current-meter measurement by the midsection method
DISCHARGE MEASUREMENT NOTES
STATION ... METRE NO ...
CHECKED BY ...
DATE ... DATE ...
WIDTH ... AREA ...
VEL ... DISCHARGE ...
METHOD ... CHANGE IN STAGE ... IN ... HOURS...
VEL COEFFT ... HORZ ANGLE COEFFT ... METRE NO...
SUSP ... SUSP COEFFT ...
METER RATING NO... DATE ...
WADING, CABLE, ICE, BOAT BRIDGE NUMBER OF VERTICALS ...
CONDITIONS:
WEATHER ... FLOW ...
RECORDER ... OUTSIDE GAUGE ...
CONTROL ... INTAKE ...
REMARKS:
GAUGE READINGS
Time Outside Recorder Inside
Start
Finish Weighted MGH
Figure I.5.3. Discharge measurement summary sheet
notebooks, sounding equipment, width-measuring equipment, equipment assemblies and miscellaneous equipment.
5.3.1 Current meters, general
A current meter is a precision instrument calibrated to measure the velocity of flowing water. Several types of current meters are available for use, including rotating-element mechanical meters, electromagnetic meters, acoustic meters and optical meters. All of these meters, when properly maintained and calibrated, are considered suitable and accurate for measuring the velocity of streamflow.
The principle of operation for a mechanical meter, or rotating element current meter, is based on the proportionality between the velocity of the water and the resulting angular velocity of the meter rotor. By placing a mechanical current meter at a point in a stream and counting the number of revolutions of the rotor during a measured interval of time, the velocity of water at that point can be determined from the meter rating. The operational requirements, construction, calibration, and maintenance of rating element current meters is described in ISO 2537 (2005, in publication).
An electromagnetic current meter is based on the principle that a conductor (water) moving through a magnetic field will produce an electrical current directly proportional to the speed of movement (Faraday’s law). By measuring this current and the resultant distortion in the magnetic field the instrument can be calibrated to determine point velocities of flowing water.
The acoustic meter uses the Doppler principle to determine point velocities of flowing water as well as complete vertical velocity profiles. Acoustic Doppler velocimeters (ADVs) are a class of acoustic meter that measures a point velocity and can thus be used to make measurements with a wading rod.
The Acoustic Doppler Current Profiler (ADCP) has been adapted for use with the moving boat method of measuring discharge, as described in a later section of this Manual.
The optical current meter uses a stroboscopic device which is calibrated to measure surface velocities of flowing water. This meter cannot be used to determine sub-surface velocities.
The following sections of this Manual describe the various types of current meters in more detail, give advantages and disadvantages of each, and provide guidance on care and maintenance.
5.3.2 Current meters, mechanical, vertical axis
The meter most commonly used by the USGS is the vertical axis, mechanical current meter. The original prototype for this kind of current meter was designed and built in 1882 by W.G. Price while working with the Mississippi River Commission.
The Price current meter has evolved through a number of different models and refinements since 1882, but the basic theory and concepts remain the same. The Price AA meter is currently used for most discharge measurements made by the USGS, however there are other variations of this meter:
such as the Price AA slow velocity, the Price pygmy, and the Price AA winter. The following sections of this Manual describe the various Price meters in more detail, and Table I.5.1 summarizes the various configurations and recommendations for the Price current meter.
Price AA
The basic components of the Price AA meter include the shaft and rotor (bucket wheel) assembly, the contact chamber, the yoke and the tailpiece. The rotor, or bucket wheel, is 5 inches in diameter and 2 inches high with six cone-shaped cups mounted on a stainless-steel shaft. A vertical pivot supports the vertical shaft of the rotor, hence the name verti-cal-axis current meter. The contact chamber houses the upper part of the shaft and provides a method of counting the number of revolutions the rotor makes. Contact chambers that can be used on the Price AA meter are described in a later section of this Manual. The yoke is the framework that holds the other components of the meter. A tailpiece is used for balance and keeps the meter pointing into the current. See Figure I.5.4 for a detailed drawing of the Price AA current meter. A photograph of the Price AA current and Price pygmy meters are shown in Figure I.5.5.
When placed in flowing water the rotor of the Price current meter turns at a speed proportional to the speed of the water. For practical purposes these current meters are considered non- directional because they register the maximum velocity of the water even though they may be placed at an angle to the direction of flow.
Advantages of the vertical axis current meter are:
(a) They operate in lower velocities than do horizontal-axis meters;
(b) Bearings are well-protected from silt-laden water;
(c) The rotor is easily repairable in the field without adversely affecting the rating;
Table I.5.1. Price current meter configurations, usage’s, and recommended ranges of depth and velocity
Price AA Standard, cat-whisker and penta gear
Headphones,
CMD2 or EFN3 Standard 0.2 to 12
1.5 or greater
The Price AA meter can be used as a low velocity meter if equipped with an optic contact chamber.
low velocity Optic Cat-whisker
CMD2, or EFN3 Individual 0.1 to 12 1.5 or greater This is the traditional Price AA low velocity meter. An individual rating is recommended, however a standard rating can be used if less accuracy is acceptable.
Price pygmy Cat whisker Headphones,
CMD2 or EFN3 Standard or calibration limits. It is not recommended that the meters be used for velocities less than the lower limit. The velocity rating for the Price meter may allow additional extrapolation in the upper range to about 20 fps. The upper range of the Price pygmy meter rating may be extrapolated to about 5 fps. Standard errors within the meter calibration limits are less than +/– 5% in all cases. Standard errors in the extrapolated range of velocities are unknown, but are probably within +/– 5%.
2 Current Meter Digitizer. Observe cautions for low velocities. See text.
3 Electronic Field Notebook such as Aquacalc or DMX. Observe cautions for low velocities. See text.
4 Water Survey of Canada
1 Cap for contact chamber 2 Contact chamber 3 Insulating bushing for contact
binding post
4 Single-contact binding post 5 Penta-contact binding post 6 Penta gear
7 Set screws
8 Yoke
9 Hole for hanger screw 10 Tailpiece 11 Balance weight 12 Shaft 13 Bucket-wheel hub 14 Bucket-wheel hub nut 15 Raising nut
16 Pivot bearing 17 Pivot 18 Pivot adjusting nut 19 Keeper screw for pivot
adjusting nut 20 Bearing lug 21 Bucket wheel 21
Figure I.5.4. Assembly drawing of the Price AA current meter
(d) Standard ratings apply to the Price AA and Price pygmy meters and
(e) A single rotor serves for the entire range of velocities.
Price AA, slow velocity
In addition to the Price AA meter described above, there is a Price AA meter modified slightly for use in measuring low velocities. In this meter the penta gear has been removed, which reduces friction.
Also, the shaft has two eccentrics making two contacts with the cat’s whisker per revolution. The low-velocity meter normally is rated from 0.06 to 0.75 m/s and is recommended when the mean velocity at a cross section is less than 0.30 m/s.
Price pygmy
A miniature version of the Price AA meter is the Price pygmy meter, also shown in Figure I.5.5, which is used for measuring velocities in shallow depths. The Price pygmy meter is scaled two-fifths as large as the standard meter and has neither a tailpiece nor a pentagear. The contact chamber is an integral part of the yoke of the meter. The Price pygmy meter makes one contact for each revolution and is used only with rod suspension.
Price AA winter
In streams where slush ice is present, a modified Price AA meter is recommended, as shown in Figure I.5.6. This meter is built with a Water Survey of Canada (WSC) winter-style yoke, and uses a polymer bucket wheel in place of the standard metal-cup rotor. The solid polymer rotor has the advantage that it does not fill with slush ice during a measurement, and the slush ice does not easily adhere to it. If slush ice is not present it is recommended that measurements be made with metal cup rotors in place of the polymer rotors.
Regular Price AA meters with metal-cup rotors are also acceptable for slush-free conditions if cutting the required larger holes through the ice is not a problem.
5.3.3 Current meters, mechanical, horizontal axis
A number of mechanical current meters are available that have a propeller, or vane, type of rotor mounted on a horizontal shaft. These meters are used extensively in Europe and some eastern countries, but very little in the United States Horizontal axis current meters include the Ott (Germany), Neyrpic (France), Haskell (United States of America), Hoff (United States of America), Braystoke (United Kingdom of Great Britain and Northern Ireland), and Valeport (basically the same as the Braystoke).
Various models of each of these are also available.
As a group, horizontal axis current meters have the following advantages:
(a) The rotor, or propeller, disturbs flow less than do vertical-axis rotors because of axial symmetry with flow direction;
(b) The rotor is less likely to be entangled by debris than are vertical-axis rotors;
(c) Bearing friction is less than for vertical-axis rotors because bending moments on the rotor are eliminated;
Figure I.5.5. Price AA (right) and Price pygmy (left) current meters
Figure I.5.6. Price AA meter with winter-style yoke and polymer rotor
(d) In oblique currents, some of these meters (such as the Ott meter) measure the velocity normal to the measuring section when the meter is held normal to the measuring section;
(e) Rotors with propellers of different pitches are available for some of the meters, allowing measurement of a considerable range of velocity.
See Figures I.5.7, I.5.8 and I.5.9 for examples of the Ott, Hoff, and Valeport current meters.
The makers of the Ott meter have developed a component propeller which in oblique currents automatically registers the velocity projection at right angles to the measuring section for angles as much as 45° and velocities as much as 3 m/s. For example, if this component propeller were held in the position AB in Figure I.5.10 it would register V cos _ rather than V max, which the Price meter would register. The Neyrpic and Ott meters are rarely used in the United States.
The Haskell meter is very suitable for use in streams that are deep, swift and clear. By using propellers with a variety of screw pitches, a considerable
Figure I.5.7.
Ott current meter
Figure I.5.8.
Hoff current meter
Figure I.5.9.
Valeport current meter
range of velocity can be measured. The Haskell meter is more durable than most other horizontal axis current meters.
The Hoff meter has a lightweight propeller with either three or four vanes of hard rubber. The meter is suited to the measurement of low velocities, but is not suitable for rugged use.
5.3.4 Comparison of performance of vertical axis and horizontal axis current meters
Comparative tests of the performance of vertical axis and horizontal axis current meters, under favourable measuring conditions, indicate virtually identical results from use of the two types of meter.
This was the conclusion reached in 1958 by the United States. Lake Survey, Corps of Engineers, after tests made with the Price, Ott, and Neyrpic current meters. The results of one of their tests are shown in Figure I.5.11.
Between the years 1958 and 1960, the USGS made 19 simultaneous discharge measurements on the Mississippi River using Price and Ott meters. The average difference in discharge between results from the two meters was – 0.15 per cent, using the measurements made with the Price meter as the standard for comparison. The maximum differences in discharge measured by the two meters was – 2.76 and + 1.53 per cent.
5.3.5 Contact heads for Price, vertical axis current meter
The Price current meter is normally fitted with a contact chamber having a cat whisker type of circuitry used for counting the number of revolu-tions of the rotor. Two other types of contact chambers, the magnetic switch type and the optical type, can be fitted to the Price AA meter.
Figure I.5.10. Velocity components measured by Ott (V cos _) and Price current meters (V max)
V cos
V max
Measurem
ent se ction A
B
Cat whisker
When placed in flowing water the rotor of the current meter turns at a speed proportional to the speed of the water. The number of revolutions of the rotor is obtained by counting electrical impulses generated in the contact chamber. An eccentric contact on the upper end of the rotor shaft wipes a slender bronze wire (cat’s whisker) attached to the binding post which closes an electrical circuit. This electrical impulse produces an audible click in a headphone or registers a unit on a counting device.
Contact points in the chamber are designed to complete the electrical circuit at selected frequencies of revolution, such as twice per revolution, once per revolution, or once per five revolutions. A separate reduction gear (penta gear), wire, and binding post provide a contact each time the rotor makes five revolutions. Figure I.5.12 shows the contact chamber and shaft for the cat whisker type chamber, with the dual binding posts.
Two types of cat-whisker wires have been used, one is the simple bronze wire, and the other is the old type wire with a small solder bead on the end of it. It is recommended that the beaded wire not be used, and that it be replaced with the simple bronze wire. Also, the cat whisker for the penta gear should always be adjusted to touch the penta eccentric, even when the penta counter is not in use. Otherwise the meter rating may be affected.
Magnetic switch
A contact chamber housing a magnetic type switch, as shown in Figure I.5.12, is available to replace the cat-whisker contact chamber. The magnetic switch
is glass enclosed in a hydrogen atmosphere and hermetically sealed. The switch assembly is rigidly fixed in the top of the meter head just above the tip of the shaft. The switch is operated by a small permanent magnet rigidly fastened to the shaft.
Two types of magnets are in use: (a) a bar magnet and (b) a circular magnet. If the contact chamber uses the bar magnet it should be identified with an
“A” stamped on the top surface of the chamber to indicate it has been modified. Older, unmodified, contact chambers with the bar magnet were found to under-register for velocities greater than about 2 fps. The chambers that utilize the circular magnets fit the standard rating throughout its range.
1.4 1.3 1.2 1.1 1 0.9 0.8
0.7
0.8 0.9 1 1.1 1.2 1.3 0.8 0.9 1 1.1 1.2 1.3 0.8 0.9 1 1.1 1.2 1.3
(a) PRICE vs. OTT (b) PRICE vs. NEYRPIC (c) NEYRPIC vs. OTT
Velocity (m/s)
Velocity (m/s)
Figure I.5.11. Comparison of mean velocities measured simultaneously by various current meters during 2-minute periods, Stella Niagara section, panel point 5
Figure I.5.12. Contact chambers for cat whisker heads and magnetic heads
The magnetic switch quickly closes when the magnet is alined with it and then promptly opens when the magnet moves away. The magnet is properly balanced on the shaft. Any type AA meter can have a magnetic switch added by replacing the shaft and the contact chamber. The magnetic switch is placed in the contact chamber through the tapped hole for the binding post. The rating of the meter is not altered by the change.
An automatic counter as described in a later section of this Manual is used with the magnetic-switch contact chamber. A headphone should not be used with the magnetic head because arcing can weld the contacts.
Optical
A contact head utilizing fibre-optics technology is available for reading the pulse rate of the Price AA current meter. A special rotor, containing two fibre-optics bundles, is attached to the upper end of the bucket-wheel shaft. The rotation of these fibre-optics bundles gates infrared light from a photo-diode to a photo-transistor creating a pulse rate that is proportional to the rotor RPM. The pulses are counted, stored and then compared to a quartz crystal oscillator. This information is processed to display stream velocity on a liquid crystal readout. The display has three averaging periods selected by a rotary switch. The averaging periods range from a minimum of about 5 seconds to a maximum of about 90 seconds. The unit is powered by a 9-volt battery.
Output of pulses from the optical sensing unit can be counted by the current meter digitizer and the electronic field notebooks described in subsequent sections of this Manual. A standard current meter rating table is used to convert pulse rate to stream velocity.
A special tail-fin assembly is required for the optical meter so it will balance properly when submerged.
The vertical section of this tail-fin is marked with the letters OAA, and the horizontal section is marked PAA.
5.3.6 Current meter timers and counters used with Price current meter
The determination of velocity using a mechanical current meter requires that the number of revolutions of the rotor be counted during a specified time interval, usually 40 to 70 seconds.
Several methods are available for timing and counting the revolutions for the Price current meter as described in the following paragraphs.
Stopwatch and headset
For current-meters having a cat-whisker type contact chamber an electrical circuit is closed each time the contact wire touches the single or penta eccentric of the current meter. A battery and headphone, as shown in Figure I.5.13, are parts of the electrical circuit and an audible click can be heard in the headphone at each electrical closure.
Some hydrographers have adapted compact, comfortable, hearing-aid type phones to replace headphones. Beepers that can be heard without the headset are also sometimes used. A headset, or similar device, should not be used with the magnetic contact chamber because arcing can weld the contacts.
The time interval is measured to the nearest second with a stopwatch. Figure I.5.13 shows the standard analog stopwatch that is frequently used, however, a digital wrist watch is also acceptable.
Current-Meter Digitizer
An automatic electronic counter, or Current-Meter Digitizer (CMD), as shown in Figure I.5.14, has been developed for use with the cat-whisker, optic and magnetic contact chambers. It can be used with any of the mechanical, vertical-axis, current meters, but care should be taken to avoid false counts when using it for low velocities when the cat-whisker contact chamber is used. The CMD automatically counts and displays the number of revolutions of the current-meter rotor and the elapsed time. A buzzer produces an audible signal
An automatic electronic counter, or Current-Meter Digitizer (CMD), as shown in Figure I.5.14, has been developed for use with the cat-whisker, optic and magnetic contact chambers. It can be used with any of the mechanical, vertical-axis, current meters, but care should be taken to avoid false counts when using it for low velocities when the cat-whisker contact chamber is used. The CMD automatically counts and displays the number of revolutions of the current-meter rotor and the elapsed time. A buzzer produces an audible signal