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UDC 532.57: 627.133
IS0
I N T ERN AT1 O N AL O R G AN I Z AT1 O N FOR S TA N DARD I Z AT I O N
IS0 RECOMMENDATION
R 555
LIQUID FLOW MEASUREMENT IN OPEN CHANNELS
DILUTION METHODS FOR MEASUREMENT OF STEADY FLOW
PART I
CONSTANT RATE INJECTION METHOD
1st EDITION
December 1966
COPYRIGHT RESERVED
The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized therefore only
by the national standards organization of that country, being
a member of ISO.
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
The IS0 Recommendation R 555, Liquid Flow Measurement in Open Channels - Dilution
Methods for Measurement of Steady Flow - Part 1 - Constant Rate Injection Method, was drawn
up by Sub-Committee ISO/TC 30/SC 1, Measurement of Liquid Flow in Open Channels, under
the direction of Technical Committee ISO/TC 30, Measurement of Fluid Flow in Closed Conduits,
the Secretariat of which is held by the Association Française de Normalisation (AFNOR). In
November 1964, the Sub-Committee was raised to the status of Technical Committee ISO/TC 113,
with the Indian Standards Institution (ISI) responsible for the Secretariat.
Work on this question by Technical Committee ISO/TC 30 began in 1958 and led, in 1963,
to the adoption of a Draft IS0 Recommendation.
L-
In July 1963, this Draft IS0 Recommendation (No. 592) was circulated to all the IS0
Member Bodies for enquiry. It was approved, subject to a few modifications of an editorial nature,
by the following Member Bodies:
Belgium Hungary Portugal
Chile India Romania
Co 1 o m b i a Israel Switzerland
Czechoslovakia Italy United Kingdom
France Korea, Rep. of U.S.A.
Germany Netherlands U.S.S.R.
Greece New Zealand
One Member Body opposed the approval of the Draft:
Japan.
The Draft IS0 Recommendation was then submitted by correspondence to the IS0
Council, which decided, in December 1966, to accept it as an IS0 RECOMMENDATION.
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ISOIR 555 - 1968 (E)
CONTENTS
Page
1. Scope . 5
2. Terminology . 5
3. Units of measurement . 5
4. Principle of the constant rate injection method . 5
4.1 Method of direct measurement of concentration. . 5
4.2 Method of comparative dilutions . 6
5. Required characteristics of injected solutions . 6
6. Choice of the measuring reach. . 6
6.1 General considerations for selection of site . 6
6.2 Preliminary tests . 7
7. Procedure . 9
7.1 Preparation of the concentrated solution . 9
7.2 Injection of the concentrated solution. . 9
7.3 Measurement of rate of injection. . 12
7.4 Sampling . 13
8. Principal chemical substances and methods of analysis in present day use . 14
8.1 Method of colorirnetric analysis . 14
8.2 Conductivity method . 15
8.3 Method of volumetric chemical analysis . 17
9. Errors in flow measurements. . 18
ANNEX A. - Example of the calculation of flow rate and estimation of tolerances in the
.................................. 20
case of volumetric chemical analysis
ANNEX B. - Example of the calculation of flow rate and estimation of tolerances in the
case of conductivimetric analysis . 22
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ISOlR 555 - 1966 (E)
IS0 Recommendation R 555
December 1966
LIQUID FLOW MEASUREMENT IN OPEN CHANNELS
DILUTION METHODS FOR MEASUREMENT OF STEADY FLOW
PART I*
CONSTANT RATE INJECTION METHOD
1. SCOPE
This IS0 Recommendation deals with the measurement of flow in channels under steady flow
conditions by the dilution method using constant rate iiijection. The method is applicable to the
measurement of the flow in channels where the degree of turbulence is sufficiently high to ensure
efficient mixing of the injected solution throughout the whole flow. This is generally the case in
hill streams and torrents.
The apparatus and the chemicals to be used, the specifications of the techniques of injection and
sampling, and also the methods of analysis, are detailed.
2. TERMINOLOGY
For the purposes of this IS0 Recommendation, the definitions given in IS0 Recommend-
ation R . . . ** apply.
3. UNITS OF MEASUREMENT
The units of measurement used in this IS0 Recommendation are metres (or feet) and seconds.
4. PRINCIPLE OF THE CONSTANT RATE INJECTION METHOD
A solution of a suitably selected salt should be injected at a constant rate, at a cross-section, at
entry of the measuring reach of the channel in which the rate of flow is constant over the period
of test.
In a downstream cross-section of this reach, far enough from the first for the injected solution to be
uniformly diluted throughout this cross-section, samples should be taken at regular intervals of
time. Provided that the concentration of the added chemical has attained a steady value, the rate
of flow Q in the channel may be calculated from the equation:
Q=qN
where = rate of injection of the chemical salt solution,
q
N = ratio of the concentration of the salt in the injected solution to that at the
downstream cross-section of the channel; however, if traces of the chemical
salt are already present in the stream prior to the injection, N is defined by
the equation given at the end of clause 4.1.
The ratio N, which is called the dilution ratio, may be determined by one of the following methods.
4.1 Method of direct measurement of concentration
The concentration of the salt should be determined:
(U) upstream of the point of injection where the water is in its natural state (concentra-
tion CO). See however clause 7.4 (U)
(b) in the injected solution (concentration Ci)
(c) in samples obtained from the downstream section (where the concentration of the
chemical has attained a steady value CZ).
Equating the mass of the salt passing the injection section (COQ + Ciq) and that passing the
sampling section CZ (Q + q) gives:
Ci - c2
Q= 4
c2 - Co
and if C2 is small compared with Cl
Ci
_- e -N=
4 cz - Co
* Further parts of this IS0 Recommendation will deal in particular with the sudden injection method and the method
using radio-active isotopes, which are at present under study.
*. Vocabulary of Terms and Symbols Used in Connection with the Measurement of Liquid Flow with a Free Surface, at
present Draft IS0 Recommendation No. 954
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ISOlR 555 - 1966 (E)
4.2 Method of comparative dilutions
A set of standard solutions should be made by diluting samples from the injected solution
with different amounts of water from the natural stream, taken from a point upstream of the
injection station. These known dilution ratios NI, N2 . . . should be approximately the same as
those expected of the samples from the downstream section.
The standard solutions and the samples taken downstream should be compared by a similar
technique to establish the dilution ratio N of the unknown samples.
5. REQUIRED CHARACTERISTICS OF INJECTED SOLUTIONS
The chemical substance to be used for the injected solution should comply with the following
requirements :
(a) it should not give any reaction with the natural water of the river concerned or with any
matter (organic matter in particular) which this may contain in solution or suspension,
or with the material which forms the river bed,
(b) it should be stable to light,
(c) it should not be toxic to fish in the dilutions used,
Y
(d) it should be capable of being accurately determined at the concentration level of the diluted
sample,
(e) it should be used only when the concentration of the chemical in solution in the natural
water is relatively low.
The following substances, which may not be suitable for all waters, are given as examples, together
with final minimum concentrations at which they can be used after dilution :
- sodium dichromate (Na2 Cr2 07, 2H20) : O. 2mg/l
- sodium chloride (NaC1) : 2 mg/l
Other salts have been used, and in particular:
- sodium nitrite (Na NO2) and
- manganese sulphate (Mn SOI, 4H20)
A chemical substance such as fluorescein (CZO Hl2 05) having sufficient colouring power to allow
the passage of the cloud in the river to be checked visually is useful particularly in preliminary
tests to check the delay period before sampling (see clause 6.2.1).
6. CHOICE OF THE MEASURING REACH
6.1 General considerations for selection of site
There should be no loss or gain of water in the measuring reach (for example, from a tributary
joining or a distributary leaving the main flow, or overflow from or to the banks of the stream)
and its length should be such that, allowing for the natural mixing action of the stream, the
solution injected at its beginning should be uniformly diluted throughout the sampling
cross-section.
The distance between the injection and sampling cross-sections should be as short as practi-
cable and the dead water zones should be as small as possible, to reduce the duration of
injection and the quantity of salt injected.
For rivers, this condition is easier to satisfy in relatively narrow channels; mixing is also
improved by disturbances such as bends, narrows, shelves, falls, etc. In particular, very wide
channels should be avoided, and reaches in which the stream divides into a number of
branches should not be used.
Great care is required to make a satisfactory choice of a reach suitable for the measurement
of discharge by the dilution method using constant rate injection. This choice is facilitated by
carrying out the preliminary tests described in clause 6.2.
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ISOlR 555 - 1966 (E)
6.2 Preliminary tests
It is essential to the success of the method that the injected material should have a constant
of the cross-section at the downstream sampling station during
concentration over the whole
the whole of the sampling period.
Before selecting the measuring reach, it is advisable to make preliminary tests to ensure that
efficient mixing in the river will be achieved, to choose the injection and sampling cross-
sections and, once these have been chosen, to determine the duration of the injection period.
6.2.1 Determination of length of measuring reach. * A first test can be made by using a strong
dye such as fluorescein. A concentrated solution of this dye can be injected for a relatively
short time at a point on a cross-section of the upstream portion of the measuring reach.
Study of the diffusion of the solution will make it possible to determine whether there
are any dead zones or similar side-tracking of the chemical and be a rough guide on the
minimum distance between the injection point and a suitable sampling cross-section.
6.2.2 Duration of injection and of the steady regime condition. Duration of injection should be
such that a steady regime of the concentration may be achieved for an adequate duration,
generally 10 to 15 minutes, in the sampling section.
The duration of the injection, which is generally related to the degree of turbulence, will
vary directly according to the length of the measuring reach and the extent of the dead
water zones, and inversely according to the mean velocity of the water.
When the measuring reach has been selected, preferably where the river bed is stable, the
duration of injection may be determined from preliminary tests at different values of the
rate of flow. These tests consist of determining the variation, as a function of time, of the
concentrations of an injected chemical substance at a number of sampling cross-sections.
* The length of the measuring reach I, between the injection cross-section and a cross-section where mixing may be expected
to be within 1 % of complete homogeneity, cannot be predicted with certainty; but, as a guide, an empirical formula
has been evolved, though not widely established on experimental data, as follows:
I = 0.13 K- (in SI units)
d
where 6 = average width of the water surface in the measuring reach (in SI units)
d = average depth of the water in this same reach (in SI units)
K = coefficient defined below:
C (0.7 C + 2
K=
g
where C = Chezy coefficient for the measuring reach, and 15 g = acceleration due to gravity (in Si units).
It is emphasized that the length obtained from the above equation may only be used as a preliminary guide, because the
equation may not at all apply in certain instances due to very low turbulence, and the actual length required should be
established by practical tests. For example, since the equation was based on injection at a single point in a straight reach,
the length will be shortened if multiple injections across the cross-section are used. In addition, some tests seem to show
that the equation under-estimates the length for small streams of the order of 5 m in width and over-estimates the length
for rivers of the order of 50 m in width.
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ISOIR 555 - 1966 (E)
An instantaneous injection of fluorescein may also be used for this purpose. For a given
flow, observation of the time of appearance and disappearance of colouration in each
1 and 2 of Figure 1 to be plotted. If it is desired to obtain,
cross-section allows curves
at a selected sampling cross-section, a steady regime condition of duration d t, it is only
necessary to add the time d t to the time of disappearance of colouration at this point
(Fig. 2), and to trace through the point obtained in this way a curve (1’) parallel to the
curve (1) of appearance of the colouration. The ordinate at the origin of this curve gives
the minimum duration of injection to be made.
The same figure directly gives the time lag between the beginning of the injection and the
beginning of the steady regime (A tl).
MEASURING REACH
(schematic)
avoided)
distance from injection
cross section
FIG. 1. - Curve of times of appearance and disappearance of coloured cloud as a function of distance
from point of injection
Time
Sampling cross-section
2
/
I
I
Minimum mixing
Zone of steady regime
length
// UI concentration
Duration of steady regime
Minimum duration
at sampling cross-section
of injection
-Interval of time between
Distance from injection point
FIG. 2. - Determination of duration of injection
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ISOlR 555 - 1966 (E)
6.2.3 Non-absorption of chemical. It is particularly necessary to check that there is no absorption
of the injected chemical in the measuring reach, either by matter in suspension or by the
material of the river bed. For this purpose, samples should be taken from the sampling
cross-section and at least one other cross-section further downstream to check that there
is no systematic difference in the mean concentration from one sampling section to
another.
7. PROCEDURE
7.1 Preparation of the concentrated solution
It is essential that the concentration of the injected solution should be as homogeneous as
possible.
Homogeneity of the solution is achieved by vigorous mixing performed with a mechanical
stirrer or with a closed-circuit pump.
It is recommended that the injection solution should be prepared in a separate tank from the
supply tank (Fig. 3). Water taken from the open channel should be used for preparing the
concentrated solution. If, however, the mixing is effected in the supply tank, this tank should
be of sufficient capacity to avoid the need for the addition of water or salt during an injection.
The solution should be drawn from a level above the bottom of the tank and provision should
be made to prevent particles of undissolved salts passing out with the injected solution.
Liquid level indicator
7
Supply tank
I P
Pump
-
Constant level
Regulating weir
Screen
-7
WI III __- Il
Head on
orifice
L=l Ill- -Ill
1
Orifice
FIG. 3. - Arrangement for injecting concentrated solution by means of a constant level tank,
a regulating weir
with
7.2 Injection of the concentrated solution
The concentrated solution should be introduced into the stream of which the discharge is to be
measured at the chosen injection cross-section. The concentrated solution should be injected
at a constant rate of flow which may be controlled by one of the following devices:
(a) a constant level tank,
(b) a volumetric pump driven by a constant speed motor,
(c) a Mariotte vessel,
(d) a floating siphon.
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ISOlR 555 - 1966 (E)
(a) A constant level tank. A constant level tank (for example as shown in Fig. 3) should be of
such dimensions and volume as will permit suitable arrangements to be made internally
to disperse effectively the flow emanating from the inlet pipe and to avoid any short
circuiting of flow between the inlet and outlet pipes with the consequent creation of “dead”
zones within the tank. The crest of the weir controlling the water level in the tank should
be sharp, horizontal and of sufficient length to control the head on the outlet pipe within
limits of 0.25 % of this head regardless of minor fluctuations in the rate of inflow.
The back of the weir plate should be smooth. The weir should extend completely across
one side of the tank to avoid “contractions” and preferably should be situated immediately
above the outlet pipe. Arrangements should be included for the collection of any solution
which overflows from the tank during a test. A range of injection flow rates may be
obtained by using a set of orifice plates or nozzles of appropriate dimensions.
(b) Volumetric pump driven by a constant speed motor. If a volumetric pump (for example as
shown in Fig. 4) is used this should be of the rotary type and great care should be taken
to ensure that the speed of this pump is constant. The speed can either be determined
directly or, if driven by a synchronous electric motor, from a knowledge of the supply
frequency.
FIG. 4. - Arrangement for injecting concentrated solution by rotary volumetric pump
(c)&(d) Other methods. Illustrations of the Mariotte vessel and the floating siphon are given in
Figures 5 and 6, page 11.
7.2.1 General recommendations. It is generally desirable, even when the injection rate can be
calculated from the pump dimensions or from the details of one of the other devices, that
a second measurement method should be incorporated in the injection apparatus, to
serve as a check on the accuracy and repeatability of the methods.
Where the natural turbulence of the stream is not such as will assure uniform and homo-
geneous mixing of the two liquids within the predetermined measuring reach, the
distribution of the concentrated solution may be assisted by the use of perforated pipes
spanning the injection cross-section. Care should be taken to ensure that the whole flow
of concentrated solution reaches the stream, i.e. that none is deposited on the banks or
on rocks above the water level.
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ISOlR §55 - 1966 (E)
Filling plug Gland
,-
-\
s
Hermetic see
Gauge glass
/-
Adjustable air
--
inlet tube
-
Detail of air
inlet tube
Datum (Atmospheric
pressure)
Ori
7
t
Head on
lsolating valve
orifice plate
!z!?---
FIG. 5. - Mariotte vessel for injecting concentrated solution
Priming valves
' '
bting funnel for constant flow
FIG. 6. - Floating siphon for injecting concentrated solution
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ISOlR 555 - 1966 (E)
7.3 Measurement of rate of injection
The rate of injection of the concentrated solution may be measured by direct observation if
the regulating device used has been previously calibrated. Where the rate of injection is
controlled so that it is constant but is not exactly determined, it may be ascertained indirectly
by measuring the total volume of solution discharged over a measured period of time.
The degree of accuracy with which the rate of injection can be measured depends on the
particular measuring device used. Suitable devices are described in clauses 7.3.1 and 7.3.2, and
the estimates of accuracy associated with the particular device should be taken into considera-
tion when estimating the overall accuracy of the flow measurement in the channel.
7.3.1 Direct measurement of rate of injection. The rate of injection should be determined from
readings, taken during the injection period, of one of the following characteristics de-
pending on the device used:
(a) level of constant level tank in relation to terminal orifice,
(b) speed of pump,
(c) level of orifice in relation to the atmospheric pressure datum in the Mariotte vessel,
(d) height between solution surface and orifice of floating siphon arrangement,
(e) reading of flow meter.
For procedures (a), (c) and (d) described above, the rate of injection of the concentrated
solution can then be determined from the calibration curves for these devices. The devices
should be calibrated both before and after the tests, and if the two calibrations differ by
more than, 1 % the tests should be repeated. If a volumetric pump is used as in procedure
(b), it is advisable that a calibration check be carried out before and after the tests to
check that there is no leakage, or alternatively, to determine the leakage if this exists,
that is to calibrate the pump under the same conditions as those experienced during the
test. There should not be a difference of more than 1 % between the two calibrations.
A flow meter may be installed in the injection apparatus for the specific purpose of
measuring the rate of injection. It should be calibrated before and after the test and these
calibrations should be carried out with the meter installed in the apparatus.
Calibration is not necessary:
(a) if the meter has been manufactured and installed in conformity with the specifi-
cations laid down by IS0 Recommendation R 541," in particular if sufficient
lengths of straight pipe have been installed on either side of the meter,
(b) if it can be shown that an adequate straightening device has been incorporated to
ensure that the available meter calibration data is unaffected by any flow distur-
bances caused by the injection apparatus.
The flow meter may conveniently be an orifice plate placed between the flanges of two
lengths of straight pipe connecting the injection apparatus to the injection point. Its parts
should not be affected by the chemical solution being used.
The average of the two calibrations of the device or meter, carried out before and after
the test, should be used in the calculation of the rate of injection.
7.3.2 Indirect measurement of rate of injection (volumetric method). Indirect measurement of
the rate of injection involves the determination of two factors: the duration of the dis-
charge and the total volume of solution delivered:
(a) the time which elapses between the commencement of injection and its termina-
f O. l % of the period of
tion should be measured to an accuracy of within
injection,
(b) the volume may be measured by observing the drop in level in a previously
calibrated feed tank or by the use of a positive displacement meter.
* Measurement of Fluid Flow by Means of Orifice Plates and Nozzles.
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ISOIR 555 - 1966(E)
7.3.2.1 Calibrated feed tank. Where the volume of concentrated solution is to be measured in
a calibrated feed tank, the tank which has to be calibrated should be erected so that
the vertical axis of the tank is truly vertical. If calibration is made by mensuration of
the tank it should be ensured that no distortion takesplace when the tank is subsequent-
ly filled.
Horizontal cross-sections should be measured to an accuracy of 0.1 %
at intervals sufficient to enable the height/volume relationship to be determined with
an accuracy of & 0.25 %. Bolt heads or bolt shanks and nuts should be measured
for each type and counted and the appropriate deductions made at the requisite levels.
Internal flanges, tie-rods or other structural members should be measured and also
deducted. These measurements should be carried out immediately before the tank is
used.
Alternatively, the height/volume relationship may be determined by the use of a
smaller calibrated vessel, the contents of which should be added to the main tank, and
height measured in successive steps. The smaller vessel should be calibrated to an
accuracy of f 0.1 % by weighing the contents.
Where the rate of injection is controlled by a constant head tank, the spillage there-
from should be collected and measured in a vessel, calibrated to an accuracy depending
on the ratio of the amount of spillage to the amount injected. If this ratio is a the
spillage should be measured to an accuracy of
1
&- %
IO a
Alternatively, the spillage may be collected and returned to the main tank before the
final reading is taken.
7.3.2.2 Positive displacement meter. Where the volume of concentrated solution is measured
by means of a volumetric meter, the meter should be of a positive displacement type
and should have been calibrated recently to give an accuracy of f 1 % or better.
The meter should be connected in the pipelines between the constant head tank and
the injection point and should be of such size and so placed, in relation to the constant
head tank, that sufficient operating head is available to sustain flow through the meter
to the injection point at the greatest injection rate required.
Precautions should be taken to remove all suspended impurities from the concentrated
solution before it is delivered to the constant head tank.
The volume of solution delivered should be ascertained from the difference between
the readings on the meter index before and after the injection.
7.4 Sampling
Samples should be taken as follows:
(a) Upstream from the injection cross-section, generally two or three samples before and
after the injection.
However, if any variation of the background concentration is anticipated along the
measuring reach, the samples of the natural water should be taken at the downstream
sampling station before and after the passage of the salt solution.
(b) At the outlet of the injection apparatus three to five samples are recommended, either
just before and just after the injection period or alternatively during the injection
period. It should be noted that sampling during the period of injection will affect the
total amount of the salt injected and the constancy of the injectionrate. However,
this source of error is generally negligible.
(c) At two or three poiiits in the sampling cross-section, at regular intervals during the
steady regime, five to ten samples at each point.
These samples should be taken by means of immersing bottles or by pumping. With
a view to avoiding the influence of more or less rapid chance variations of the concen-
tration in the sampling section, it is recommended that the time taken between suc-
cessive samples should be as short as possible.
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ISOIR 555 - 1966(E)
8. PRINCIPAL CHEMICAL SUBSTANCES AND METHODS OF ANALYSIS IN PRESENT DAY USE
8.1 Method of colorimetric analysis
8.1.1 Choice of chemical substance and minimum concentration that can be measured. The salt
at present favoured for applying the constant rate injection dilution method with colori-
metric analysis is sodium dichromate. Its solubility in water is relatively high (concen-
trations up to approximately 600 g per litre of water may be used in practice) and the salt
satisfies most of the requirements of section 5.
Colorirnetric analysis permits the measurement of very low concentrations of sodium
dichromate. With final concentrations between 0.2 and 2 mg/l, the accuracy of the analy-
sis will depend upon the concentration used and the sensitivity and accuracy of the
colorimetric apparatus.
Natural waters do not generally contain chromium ions in solution. The presence of such
ions would lead to errors in the measurement. The nature and quantity of any matter
in suspension in the natural water can seriously affect the accuracy of analysis.
8.1.2 Method of analysis. The principle of colorimetric analysis is to compare, by means of a
colorimeter, the dilutions of sodium dichromate (the natural dilution of the samples from
the channel and the standard dilutions), by their absorption of light after a reagent has
v
been added to each solution.
The recommended reagent has the following composition :
- Diphenylcarbazide (crystallized) [(CsHsNH. NH)2 CO] : 0.25 g
- Phthalic anhydride (crystallized)
...