ASTM D5242-92(2007)
(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
SIGNIFICANCE AND USE
Thin-plate weirs are reliable and simple devices that have the potential for highly accurate flow measurements. 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 about 0.008 ft 3/s (0.00023 m 3/s) to about 50 ft 3/s (1.4 m 3/s).
Thin-plate weirs are particularly suitable for use in water and wastewater without significant amounts of solids and in locations where a head loss is affordable.
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.
This standard does not purport to address all of the safety concerns, 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.
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: D5242 − 92 (Reapproved2007)
Standard Test Method for
Open-Channel Flow Measurement of Water with Thin-Plate
Weirs
This standard is issued under the fixed designation D5242; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3.1.1 For definitions of terms used in this test method, refer
to Terminology D1129.
1.1 This test method covers measurement of the volumetric
3.2 Definitions of Terms Specific to This Standard:
flowrate of water and wastewater in channels with thin-plate
3.2.1 crest—the bottom of the overflow section or notch of
weirs. Information related to this test method can be found in
a rectangular weir.
Rantz (1) and Ackers (2).
3.2.2 head—the height of a liquid above a specified point,
1.2 The values stated in inch-pound units are to be regarded
for example, the weir crest.
as the standard. The SI units given in parentheses are for
information only.
3.2.3 hydraulic jump—an abrupt transition from supercriti-
cal flow to subcritical or tranquil flow.
1.3 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
3.2.4 nappe—the curved sheet or jet of water overfalling the
responsibility of the user of this standard to establish appro-
weir.
priate safety and health practices and determine the applica-
3.2.5 notch—the overflow section of a triangular weir or of
bility of regulatory limitations prior to use.
a rectangular weir with side contractions.
2. Referenced Documents
3.2.6 primary instrument—the device (in this case the weir)
2.1 ASTM Standards: thatcreatesahydrodynamicconditionthatcanbesensedbythe
D1129 Terminology Relating to Water secondary instrument.
D2777 Practice for Determination of Precision and Bias of
3.2.7 scow float—an in-stream float for depth sensing,
Applicable Test Methods of Committee D19 on Water
usually mounted on a hinged cantilever.
D3858 Test Method for Open-Channel Flow Measurement
3.2.8 secondary instrument—in this case, a device that
of Water by Velocity-Area Method
measures the depth of flow (referenced to the crest) at an
2.2 ISO Standards:
appropriate location upstream of the weir plate. The secondary
ISO1438 FlowMeasurementinOpenChannelsUsingWeirs
instrument may also convert the measured depth to an indi-
and Venturi Flumes—Part 1: Thin-Plate Weirs
cated flowrate.
ISO 555 Liquid Flow Measurement in Open Channels,
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
3. Terminology
made under quiescent conditions.
3.1 Definitions:
3.2.10 subcritical flow—open channel flow in which the
average velocity is less than the square root of the product of
This test method is under the jurisdiction of ASTM Committee D19 on Water
the average depth and the acceleration due to gravity; some-
and is the direct responsibility of Subcommittee D19.07 on Sediments,
times called tranquil flow.
Geomorphology, and Open-Channel Flow.
Current edition approved June 15, 2007. Published July 2007. Originally
3.2.11 submergence—a condition where the water level on
approved in 1992. Last previous edition approved in 2001 as D5242 – 92 (2001).
the downstream side of the weir is at the same or at a higher
DOI: 10.1520/D5242-92R07.
The boldface numbers in parentheses refer to a list of references at the end of
elevation than the weir crest; depending on the percent of
the text.
submergence the flow over the weir and hence the head-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
discharge relation may be altered.
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
3.2.12 supercritical flow—open channel flow in which the
the ASTM website.
4 average velocity exceeds the square root of the product of the
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036, http://www.ansi.org. average depth and the acceleration due to gravity.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D5242 − 92 (2007)
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
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.
FIG. 1 Rectangular Weir
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.
7.2.2 Weir Plate:
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
prescribedtominimizepotentialdamage,andtheupperlimitis
FIG. 2 Crest-Length Adjustment, ∆
required to help avoid nappe clinging. See 7.2.5.4 and 7.2.6.3 L
for plates thicker than 0.08 in. (2 mm). The plate must be
fabricated of smooth metal or other material of equivalent
smoothness and sturdiness. Upstream corners of the overflow 7.2.2.2 The plane of the weir plate must be vertical and
section must be sharp and burr-free, and the edges must be flat, perpendicular to the channel walls. The overflow section must
smooth, and perpendicular to the weir face. be laterally symmetrical and its bisector must be vertical and
D5242 − 92 (2007)
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 aretheeffectiveheadandeffectivecrestlength
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
H 5 H1δH
e
large enough in itself to form full contractions. See 7.2.3.) The
where δH is an experimentally determined adjustment for
weir structure must be firmly mounted in the channel so that
the effects of viscosity and surface tension valid for water at
there is no leakage around it.
ordinarytemperatures(about4to30°C);itsvalueisconstantat
7.2.2.3 Additional plate requirements specific to rectangular
0.003 ft (0.001 m). The effective crest length, L , is related to
and triangular weirs are given in 7.2.5.4 and 7.2.6.3. e
the measured length, L, by:
7.2.3 WeirContractions—Whenthesidewallsandbottomof
the approach channel are far enough from the edges of the L 5 L1δL
e
notch for the contraction of the nappe to be unaffected by those
where the adjustment, δL, 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 δL 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 ofApplication—Thedischargerelationsgiven
7.2.4 Head Measurement Location—The head on the weir,
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 4 H to 5H , where
max max L$ 0.5 ft (0.15 m)
P$ 0.3 ft (0.1 m)
H is the maximum head on the weir. In some cases a stilling
max
well may be desirable or necessary. See 7.5. Although in principle Eq 1 could be applied to very large
weirs, the experiments on which it is based included crest
7.2.5 Rectangular Weirs:
lengths up to about 4 ft (1.2 m) and heads up to about 2 ft (0.6
7.2.5.1 Therectangularoverflowsectioncanhaveeitherfull
m); it is recommended that these values not be significantly
or partial contractions (7.2.3) or the side contractions may be
exceeded.
suppressed (7.2.5.2).
7.2.5.7 Aeration Requirements—In order to avoid nappe
7.2.5.2 Suppressed Weirs— When there are no side contrac-
clinging and maintain proper aeration of the nappe, the
tions and the weir crest extends across the channel, the weir is
tailwater level should always be at least 0.2 ft (0.06 m) below
termed “full width” or “suppressed.” In this case the approach
the crest. In addition, in the case of suppressed weirs, aeration
channel must be rectangular (see also 7.3.4) and the channel
must be provided externally; this can be done with sidewall
walls must extend at least 0.3H downstream of the weir plate.
vents, for example. The user must measure the pressure in the
7.2.5.3 Contracted Rectangular Weirs —The conditions for
airpockettoestablishthatitissufficientlyclosetoatmospheric
full contraction are as follows:
for the flow to be unaffected (see 11.7.2).
H/P# 0.5
7.2.6 Triangular Weirs:
H/L# 0.5
0.25 ft (0.08 m)# H# 2.0 ft (0.6 m)
7.2.6.1 Shape—The overflow section of a triangular weir is
L$ 1.0 ft (0.3 m)
an isosceles triangle oriented with the vertex downward.
P$ 1.0 ft (0.3 m)
( B − L )/2$2H
where Histhemeasuredhead, Pisthecrestheightabovethe
bottom of the channel, L is the crest length, and B is the
channel width. The partial contraction conditions covered by
this test method are given in 7.2.5.6.
7.2.5.4 Weir Plate—The requirements of this section are in
addition to those of 7.2.2. If the plate is thicker than 0.08 in. (2
mm) the downstream excess at the edges of the overflow
section must be beveled at an angle of at least 45° as shown in
Fig. 1. If there are side contractions, all of the edge require-
ments of this test method pertain to the sides as well as the
crest. The sides must be exactly perpendicular to the crest; and
the crest must be level, preferably to within a transverse slope
of 0.001.
7.2.5.5 Discharge Relations—The flowrate, Q, over a rect-
angular weir that conforms to all requirements of 7.2 as well as
the approach conditions in 7.3 is determined from the
Kindsvater-Carter equation (4):
1/2 3/2
Q 5 2/3 2g C L H (1) FIG. 3 Discharge Coefficient, C , for Rectangular Weirs
~ !~ ! ~ !
e e e e
D5242 − 92 (2007)
where δ is an adjustment for the combined effects of
H
t
viscosityandsurfacetensionforwateratordinarytemperatures
(4 to 30°C) and is given as a function of notch angle in Fig. 5.
The discharge coefficient is given in Fig. 6 as a function of the
notch angle for fully contracted weirs only. For partially
contracted weirs the data base is considered adequate for 90°
notches only and these discharge coefficients are shown in Fig.
7.
7.2.6.5 Limits of Application—For 90° notches only, the
discharge relations given in 7.2.6.4 are valid for these partially
contracted conditions:
H/P# 1.2
H/B# 0.4
P$ 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
developed flow in a long, straight, moderately smooth channel.
Unfortunately there are no universally accepted quantitative
guidelines for implementing these recommendations. One
standard (5) recommends a straight approach length of ten
FIG. 4 Triangular Weirs
channel widths when the weir length is greater than half the
channel width. However, the presence of upstream channel
Experimental results are available for notch angles, θ,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
θ/2 = 1),53.13°(tanθ/2 = 0.5)and28.07°(tanθ/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
...
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