EQUIPMENT AND SUPPLIES--Current Meters
In Reply Refer To: May 13, 1985
WGS-Mail Stop 415
SURFACE WATER BRANCH TECHNICAL MEMORANDUM 85.07
SUBJECT: EQUIPMENT AND SUPPLIES--Current Meters
Surface Water Branch technical reviewers have recently noted many instances
where field personnel were using the wrong current meter to make discharge
measurements for given field conditions.
The purpose of this memorandum is to provide guidance in choosing which type
of meter to use for various field conditions and to consolidate and update
information on current meters contained in several Techniques of Water-
Resources Investigations (TWRI). With the development of the polymer bucket
wheel and optic contact chamber, there are now seven possible configurations
of the Price AA and Pygmy meters. Additionally, there are ice, low-velocity,
and vertical-vane meters available for special purposes.
Table 1 (attachment 1) shows the availability, recommended depth range, and
recommended velocity range of the various meters. Meters should be used
with caution outside these ranges. Any deviation from these recommendations
should be noted and the measurement accuracy downgraded accordingly. The
meters described in this table are as follows:
1. Price AA - The standard U.S. Geological Survey current meter.
2. Price-Pygmy - A small version of the Price AA meter.
3. Low velocity - A Price AA meter modified as described below to
measure low velocities.
4. Ice meter - A vertical vane meter designed to be used with a rod for
measuring under ice cover.
5. F-Combination - A modified version of the Price-Pygmy meter designed
to be used with rod, cable, or ice suspensions.
Table 2 (attachment 2) shows the standard error as a percentage of mean
velocity using a standard meter rating for various meters. Note that
generally meters are less accurate at lower velocities. Investigation of a
limited number of meters has shown a Price AA meter has a standard error of
about 10 percent or greater at a velocity of 0.08 foot per second (ft/s).
SURFACE WATER BRANCH TECHNICAL MEMORANDUM 85.07 2
Also note that the Price AA meter has less variance than the Pygmy meter.
This indicates that when depth is adequate, the Price AA meter is more
accurate than the Pygmy.
The maximum observable velocity for some current meters has been limited in
the past by the ability to count the audible signal. With the development
of the Current Meter Digitizer (CMD), this is no longer a limitation.
Theoretically the CMD (Model 1.3) can count a cat-whisker type Pygmy meter
to the equivalent of 12 ft/s and cat-whisker type Price AA to 32 ft/s.
Meters are now generally rated to 12 ft/s. However, investigations have
shown that these ratings may be extrapolated upward to greater than 15 ft/s
with little or no loss in accuracy.
The low-velocity meter is simply a Price (or type AA) current meter with the
penta-counter assembly and the cat whisker removed. This meter also has a
special binding post with two knife-edge contacts for the cat whisker to
generate a double count for each bucket-wheel revolution. The low-velocity
meter was individually rated for use in velocities (0.08 to 3.0 ft/s) that
are less than the velocities recommended for the type AA current meter (0.25
to 12.0 ft/s).
A rating comparison was made between standard Price AA meters (with penta-
count gear and counter in place) and low-velocity meters. The results indicate
the following:
1. For low-velocity meters there is little practical difference between
the individual rating equation and the standard Price AA rating
equation.
2. A standard Price AA meter in good condition and adjustment can be
used to accurately measure velocities to about 0.10 ft/s. However,
note the meter has a standard error of 10 percent or greater at
0.08 ft/s.
Reviewers also have noted that some field personnel remove the penta gear
and/or lift the penta-gear cat whisker from the type AA current meter to
obtain better low-velocity performance. This practice may alter the rating
of the meter at low velocities. Always use a current meter in "as-rated"
condition. This means that a meter should be clean, oiled, properly adjusted,
and have all contacts in place when making velocity observations.
All meters should be maintained in the best possible condition. In regard to
meter condition, it was shown by Smoot and Carter in a 1968 investigation
that "The effect (on meter ratings) of slightly dented cups is more pronounced
than the effect of either a worn pivot or worn upper bearing. The rating is
affected throughout the range of velocities." Therefore, it is recommended
that Price meters be converted to P-AA as new bucket wheels are required and
as soon as the new polymer bucket wheels and standard ratings are available.
SURFACE WATER BRANCH TECHNICAL MEMORANDUM 85.07 3
Recent investigations of the effect of boundary proximity on meter accuracy
have not shown that any changes should be made to previous recommendations of
minimum depths for meter use shown in TWRI Book 3, Chapter A8, "Discharge
Measurements at Gaging Stations."
Verne R. Schneider
Acting Chief, Surface Water Branch
2 Attachments
WRD Distribution: A, B, S, FO, PO
This memorandum does not supersede any previous memorandum.
Table 1.--Configuration, availability, recommended velocity, and depth ranges for U.S. Geological Survey Current meters
____________________________________________________________________________________________________________________________
|_____Bucket______|_________Contact___| | Velocity | Depth |
Meter | | | | | | range in | range |
type__________|Metal__|Polymer|Catwhisker_|Optic|Availability|___ft/s___|_inft____|Remarks____________________
| | | | | | | |
Price AA | x | | x | | Available | 0.10-12 | 1.5 and |
| | | | | | | greater |
| | | | | | | |
Price AA | x | | | x | Not planned | | do. |
| | | | | | | |
P-AA | | x | x | | Available | 0.15-12 | do. |
| | | | | | | |
F-AA | | x | | x | Available | 0.15-12 | do. |
| | | | | | | |
Price-Pygmy | x | | x | | Available | 0.50-12 | 0.3-1.5***| Low velocities shown on
| | | | | | | | standard rating table are a
| | | | | | | | questionable extrapolation
| | | | | | | |
Price-Pygmy | x | | | x | Not planned | | do. |
| | | | | | | |
P-Pygmy | | x | x | | FY 1987 | | do. |
| | | | | | | |
F-Pygmy | | x | | x | FY 1987** | | do. |
| | | | | | | |
Low velocity | x | | | | Available | | --- | No difference in rating
| | | | | | | | from Price AA meter
| | | | | | | |
Ice |Vertical-| | | | Available | 0.5-8* | 1.5 and | Rod use - not to be used
| vane | | | | | | greater | closer than 0.5 foot to
| | | | | | | | surface or bed
| | | | | | | |
F-Combination | | | | | FY 1987 | | --- | This meter will be usable
| | | | | | | | with rod, cable, or ice
______________|_________|_________|_____________|_______|______________|__________|___________|suspension________________
*Meters are individually rod rated. Those rated prior to December 1, 1981, should only be used within this range.
**Specially rated versions of this meter can be requested from the Hydrologic Instrumentation Facility. Meter is
recommended for high velocities and shallow depths.
***Meter may be used in depths as low as 0.3 foot by using the 0.6-depth method. It should be recognized, however, that
there is an unknown underregistration error when the meter is used in depths less than 0.75 foot.
Table 2.--Standard error as percent of mean velocity using
a standard rating for various meter configurations
___________________________________________________________
|__Velocityin_ft/s______|
| | | | Range in rating
Meter___________|_>0.5__|__0.25__|__0.15__|____inft/s____
| | | |
Price AA | <1.0 | 1.9 | --- | 0.25 to 8.0
| | | |
F-AA | <1.2 | 3.0 | 5.2 | 0.15 to 12.0
| | | |
P-AA | <1.7 | 2.9 | 4.7 | 0.15 to 12.0
| | | |
Price Pygmy | <1.8 | <4.5 | --- | 0.25 to 3.0
| | | |
F-Pygmy | --- | --- | --- |
| | | |
P-Pygmy | --- | --- | --- |
| | | |
F-Combination___|__---__|__---___|__---___|________________