ASTM D5242-92(2001)
(Test Method)Standard Test Method for Open-Channel Flow Measurement of Water with Thin-Plate Weirs
Standard Test Method for Open-Channel Flow Measurement of Water with Thin-Plate Weirs
SCOPE
1.1 This test method covers measurement of the volumetric flowrate of water and wastewater in channels with thin-plate weirs. Information related to this test method can be found in Rantz (1) and Ackers (2).
1.2 The values stated in inch-pound units are to be regarded as the standard. The SI units given in parentheses are for information only.
1.3 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: D 5242 – 92 (Reapproved 2001)
Standard Test Method for
Open-Channel Flow Measurement of Water with Thin-Plate
Weirs
This standard is issued under the fixed designation D 5242; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3. Terminology
1.1 This test method covers measurement of the volumetric 3.1 Definitions:
flowrate of water and wastewater in channels with thin-plate 3.1.1 For definitions of terms used in this test method, refer
weirs. Information related to this test method can be found in to Terminology D 1129.
Rantz (1) and Ackers (2). 3.2 Definitions of Terms Specific to This Standard:
1.2 The values stated in inch-pound units are to be regarded 3.2.1 crest—the bottom of the overflow section or notch of
as the standard. The SI units given in parentheses are for a rectangular weir.
information only. 3.2.2 head—the height of a liquid above a specified point,
1.3 This standard does not purport to address all of the for example, the weir crest.
safety concerns, if any, associated with its use. It is the 3.2.3 hydraulic jump—an abrupt transition from supercriti-
responsibility of the user of this standard to establish appro- cal flow to subcritical or tranquil flow.
priate safety and health practices and determine the applica- 3.2.4 nappe—thecurvedsheetorjetofwateroverfallingthe
bility of regulatory limitations prior to use. weir.
3.2.5 notch—the overflow section of a triangular weir or of
2. Referenced Documents
a rectangular weir with side contractions.
2.1 ASTM Standards: 3.2.6 primary instrument—the device (in this case the weir)
D 1129 Terminology Relating to Water
thatcreatesahydrodynamicconditionthatcanbesensedbythe
D 2777 Practice for Determination of Precision and Bias of secondary instrument.
Applicable Methods of Committee D-19 on Water
3.2.7 scow float—an in-stream float for depth sensing,
D 3858 Practice for Open-Channel Flow Measurement of
usually mounted on a hinged cantilever.
Water by Velocity-Area Methods 3.2.8 secondary instrument—in this case, a device that
2.2 ISO Standards:
measures the depth of flow (referenced to the crest) at an
ISO 1438 Flow Measurement in Open Channels Using appropriate location upstream of the weir plate. The secondary
Weirs and Venturi Flumes—Part 1: Thin-Plate Weirs
instrument may also convert the measured depth to an indi-
ISO 555 Liquid Flow Measurement in Open Channels, cated flowrate.
Delusion Methods for Measurement of Steady Flow-
3.2.9 stilling well—a small free-surface reservoir connected
Constant Rate Injection Method through a constricted channel to the approach channel up-
stream of the weir so that a depth (head) measurement can be
made under quiescent conditions.
This test method is under the jurisdiction ofASTM Committee D-19 on Water
3.2.10 subcritical flow—open channel flow in which the
and is the direct responsibility of Subcommittee D19.07 on Sediments, Geomor-
average velocity is less than the square root of the product of
phology, and Open-Channel Flow.
the average depth and the acceleration due to gravity; some-
Current edition approved May 15, 1992. Published September 1992.
The boldface numbers in parentheses refer to a list of references at the end of
times called tranquil flow.
the text.
3.2.11 submergence—a condition where the water level on
Annual Book of ASTM Standards, Vol 11.01.
4 the downstream side of the weir is at the same or at a higher
Available from American National Standards Institute, 11 West 42nd Street,
NY, NY 10036. elevation than the weir crest; depending on the percent of
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 5242 – 92 (2001)
submergence the flow over the weir and hence the head-
discharge relation may be altered.
3.2.12 supercritical flow—open channel flow in which the
average velocity exceeds the square root of the product of the
average depth and the acceleration due to gravity.
3.2.13 tailwater—the water level immediately downstream
of the weir.
4. Summary of Test Method
4.1 Thin-plate weirs are overflow structures of specified
geometries for which the volumetric flowrate is a unique
functionofasinglemeasureddepth(head)abovetheweircrest
or vertex, the other factors in the head-discharge relation
having been experimentally or analytically determined as
functions of the shape of the overflow section and approach
channel geometry.
5. Significance and Use
5.1 Thin-plate weirs are reliable and simple devices that
havethepotentialforhighlyaccurateflowmeasurements.With
proper selection of the shape of the overflow section a wide
range of discharges can be covered; the recommendations in
this test method are based on experiments with flowrates from
3 3 3 3
about 0.008 ft /s (0.00023 m /s) to about 50 ft /s (1.4 m /s).
5.2 Thin-plate weirs are particularly suitable for use in
waterandwastewaterwithoutsignificantamountsofsolidsand
in locations where a head loss is affordable.
6. Interferences
6.1 Because of the reduced velocities in the backwater
upstream of the weir, solids normally transported by the flow
will tend to deposit and ultimately affect the approach condi-
tions.
6.2 Weirs are applicable only to open channel flow and
become inoperative under pressurized-conduit conditions.
7. Apparatus
7.1 A weir measuring system consists of the weir plate and
its immediate channel (the primary) and a depth (head)
measuring device (the secondary). The secondary device can
FIG. 1 Rectangular Weir
range from a simple scale for manual readings to an instrument
that continuously senses the depth, converts it to a flowrate,
and displays or transmits a readout or record of the instanta-
neous flowrate or totalized flow, or both.
7.2 Thin-Plate Weir:
7.2.1 Shapes—The thin-plate weir provides a precisely
shaped overflow section symmetrically located in a (usually)
rectangular approach section, as in Fig. 1 and Fig. 2.Although
information is available in the literature (3) on a variety of
overflow-section or notch shapes (for example, rectangular,
triangular, trapezoidal, circular) only the rectangular and trian-
gular shapes are considered to have a data base sufficient for
promulgation as a standard method.
FIG. 2 Crest-Length Adjustment,D
7.2.2 Weir Plate: L
7.2.2.1 The plate thickness in the direction of flow must be
from 0.03 in 0.08 in. (about 1 to 2 mm); the lower limit is fabricated of smooth metal or other material of equivalent
prescribed to minimize potential damage, and the upper limit is smoothness and sturdiness. Upstream corners of the overflow
required to help avoid nappe clinging. See 7.2.5.4 and 7.2.6.3 section must be sharp and burr-free, and the edges must be flat,
for plates thicker than 0.08 in. (2 mm). The plate must be smooth, and perpendicular to the weir face.
D 5242 – 92 (2001)
7.2.2.2 The plane of the weir plate must be vertical and the approach conditions in 7.3 is determined from the
perpendicular to the channel walls. The overflow section must Kindsvater-Carter equation (4):
be laterally symmetrical and its bisector must be vertical and
1 2 3 2
/ /
Q 5 ~2/3!~2g! C L ~H ! (1)
e e e
located at the lateral midpoint of the approach channel. If the
where g is the acceleration due to gravity in compatible
metal plate containing the overfall section does not form the
units, H and L are the effective head and effective crest length
e e
entire weir, it must be mounted on the remainder of the
respectively, and C is a discharge coefficient. The effective
e
bulkhead so that the upstream face of the weir is flush and
head, H , is related to the measured head, H, by:
e
smooth. (This requirement may be relaxed if the metal plate is
large enough in itself to form full contractions. See 7.2.3.) The H 5 H1dH
e
weir structure must be firmly mounted in the channel so that
where dH is an experimentally determined adjustment for
there is no leakage around it.
the effects of viscosity and surface tension valid for water at
7.2.2.3 Additional plate requirements specific to rectangular
ordinarytemperatures(about4to30°C);itsvalueisconstantat
and triangular weirs are given in 7.2.5.4 and 7.2.6.3.
0.003 ft (0.001 m). The effective crest length, L , is related to
e
7.2.3 Weir Contractions—When the sidewalls and bottom
the measured length, L, by:
of the approach channel are far enough from the edges of the
L 5 L1dL
e
notch for the contraction of the nappe to be unaffected by those
where the adjustment, dL, is a function of the crest length-
boundaries, the weir is termed “fully contracted”. With lesser
to-channel width ratio, L/B. Experimentally determined values
distances to the bottom or sidewalls, or both, the weir is
of dL for water at ordinary temperatures are given in Fig. 3.
“partially contracted”. Contraction requirements specific to
The discharge coefficient, C , is given in Fig. 4 as a function
e
rectangular and triangular weirs are given in 7.2.5.3, 7.2.5.6,
of L/B and the head-to-crest height ratio, H/P.
7.2.6.2, and 7.2.6.5.
7.2.5.6 Limits of Application—The discharge relations
7.2.4 Head Measurement Location—The head on the weir,
given in 7.2.5.5 are applicable for these conditions:
H, is measured as a depth above the elevation of the crest or
H/P# 2
vertex of the notch. This measurement should be made at a
H$ 0.1 ft (0.03 m)
distance upstream of the weir equal to 4H to 5H , where
max max
L$ 0.5 ft (0.15 m)
H is the maximum head on the weir. In some cases a stilling P$ 0.3 ft (0.1 m)
max
well may be desirable or necessary. See 7.5.
Although in principle Eq 1 could be applied to very large
7.2.5 Rectangular Weirs:
weirs, the experiments on which it is based included crest
7.2.5.1 Therectangularoverflowsectioncanhaveeitherfull
lengths up to about 4 ft (1.2 m) and heads up to about 2 ft (0.6
or partial contractions (7.2.3) or the side contractions may be
m); it is recommended that these values not be significantly
suppressed (7.2.5.2).
exceeded.
7.2.5.2 Suppressed Weirs—When there are no side contrac-
7.2.5.7 Aeration Requirements—In order to avoid nappe
tions and the weir crest extends across the channel, the weir is
clinging and maintain proper aeration of the nappe, the
termed “full width” or “suppressed”. In this case the approach
tailwater level should always be at least 0.2 ft (0.06 m) below
channel must be rectangular (see also 7.3.4) and the channel
the crest. In addition, in the case of suppressed weirs, aeration
walls must extend at least 0.3H downstream of the weir plate.
must be provided externally; this can be done with sidewall
7.2.5.3 Contracted Rectangular Weirs—The conditions for
vents, for example. The user must measure the pressure in the
full contraction are as follows:
airpockettoestablishthatitissufficientlyclosetoatmospheric
H/P# 0.5 for the flow to be unaffected (see 11.7.2).
H/L# 0.5
0.25 ft (0.08 m)# H# 2.0 ft (0.6 m)
L$ 1.0 ft (0.3 m)
P$ 1.0 ft (0.3 m)
( B − L )/2$2H
where Histhemeasuredhead, Pisthecrestheightabovethe
bottom of the channel, L is the crest length, and B is the
channel width. The partial contraction conditions covered by
this test method are given in 7.2.5.6.
7.2.5.4 Weir Plate—The requirements of this section are in
addition to those of 7.2.2. If the plate is thicker than 0.08 in. (2
mm) the downstream excess at the edges of the overflow
section must be beveled at an angle of at least 45° as shown in
Fig. 1. If there are side contractions, all of the edge require-
ments of this test method pertain to the sides as well as the
crest. The sides must be exactly perpendicular to the crest; and
the crest must be level, preferably to within a transverse slope
of 0.001.
7.2.5.5 Discharge Relations—The flowrate, Q, over a rect-
angular weir that conforms to all requirements of 7.2 as well as FIG. 3 Discharge Coefficient, C , for Rectangular Weirs
e
D 5242 – 92 (2001)
where C and H are the discharge coefficient and effective
e e
t t
head respectively. H is given by:
e
t
H 5 H1d
e Ht
t
where d is an adjustment for the combined effects of
H
t
viscosityandsurfacetensionforwateratordinarytemperatures
(4 to 30°C) and is given as a function of notch angle in Fig. 5.
The discharge coefficient is given in Fig. 6 as a function of the
notch angle for fully contracted weirs only. For partially
contracted weirs the data base is considered adequate for 90°
notches only and these discharge coefficients are shown in Fig.
7.
7.2.6.5 Limits of Application—For 90° notches only, the
discharge relations given in 7.2.6.4 are valid for these partially
contracted conditions:
H/P# 1.2
H/B# 0.4
P$ 0.3 ft (0.1 m)
B$ 2ft(0.6m)
0.15 ft (0.05 m)# H# 2ft(0.6m)
Forotheranglesbetween20and100°thedischargerelations
are valid only for full contractions (see 7.2.6.2).
7.2.6.6 Aeration Requirements—In order to avoid nappe
clinging and maintain proper aeration of the nappe, the
tailwater level should always be at least 0.2 ft (0.05 m) below
the vertex of the triangular notch.
7.3 Approach Channel:
7.3.1 Weirs can be sensitive to the quality of the approach
flow. Therefore this flow should be tranquil and uniformly
distributed across the channel in order to closely approximate
the conditions of the experiments from which the discharge
relations were developed. For this purpose, uniform velocity
distribution can be defined as that associated with fully
FIG. 4 Triangular Weirs
developed flow in a long, straight, moderately smooth channel.
Unfortunately there are no universally accepted quantitative
guidelines for implementing these recommendations. One
7.2.6 Triangular Weirs:
standard (5) recommends a straight approach length of ten
7.2.6.1 Shape—The overflow section of a triangular weir is
channel widths when the weir length is greater than half the
an isosceles triangle oriented with the vertex downward.
channel width. However, the presence of upstream channel
Experimental results are available for notch angles, u,of20to
bends or sudden enlargements would clearly lengthen this
100°. However, the most commonly used weirs are 90° (tan
approach requirement. Therefore the adequacy of the approach
u/2 = 1),53.13°(tan u/2 = 0.5)and28.07°(tan u/2 = 0.25).See
flow generally must be demonstrated on a case-by-case basis
Fig. 2.
using velocity traverses, experience with similar situations, or
7.2.6.2 Contractions—Theconditionsforfullcontractionof
analytical approximations.
triangular weirs are as follows:
7.3.2 In some cases baffles can be used to improve the
H/P# 0.4
velocity distributio
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.