An open-channel flowmeter is calibrated by the use of local velocity and level characteristics existing at a given site, as measured by the flowmeter itself. Stored reference relationships that have been either theoretically or experimentally derived are compared to the velocity level relationships at any particular site so as to determine a correction factor which is valid at that site so as to modify the sensed velocity to be an accurate approximation of the average velocity under those site conditions. A local velocity signal is modified so as to make it an accurate approximation of the mean velocity for open-channel flowmeters of the level/velocity type.

SPECIFICATION 
1. Statement of the Invention 
The subject invention relates to a self-calibrating open-channel flowmeter 
including means for determining the flow coefficient for a particular site 
by examining the sensed velocity of the flowing fluid versus the level at 
one or more flow rates. The flow coefficient is then used to convert local 
velocity to average velocity over a wide range of fluid levels at that 
site. 
2. Brief Description of the Prior Art 
The need to monitor water and waste water over the last 20 years has 
increased significantly due to environmental laws and the cost of 
maintaining adequate water and waste water facilities. The measurement of 
open channel flow especially in sanitary and storm sewers is very 
difficult. 
For electronic flowmeters that are placed in a standard section of sewer 
pipe, both the level and velocity must be measured accurately because both 
parameters vary as flow varies in such a piping system under open channel 
flow conditions. The slope and roughness of the pipe control the 
relationship between velocity and level. The slope (or grade) of the pipe 
chosen for a particular site is the result of civil engineering 
considerations determined by such factors as the terrain, whereas the 
roughness factor is the result of the surface of the pipe and obstructions 
such as bends and elbows. 
Historically, flow in open channel piping has been measured through the use 
of the Manning equation or other empirical relationships where the pipe 
roughness and the slope of the water surface is used to yield a 
predictable relationship between the sensed level and flow through the 
pipe. Because it is very difficult to accurately measure either the 
roughness or the slope of the water surface directly, this technique has 
not been used where accurate flow measurements are required. 
More recently the preferred method of measuring flow in open channel sewers 
has been the velocity/area technique. This technique utilizes the 
continuity equation which states that the flow Q is equal to the product 
of the mean (average) velocity V and the area A of the partially filled 
pipe, both which are measured at a common cross-section. 
Instruments using the velocity/area technique typically cannot directly 
measure the average velocity accurately enough for commercial use, and 
hence they typically contain a modifier that acts on the sensed velocity 
(local velocity) to better approximate the average velocity. This is true 
when the sensed velocities are localized to the bottom of the conduit, the 
sensed velocity is at the surface itself, on any other location that is 
not a direct measure of the mean velocity. 
For any particular pipe size, the slope and roughness of the pipe 
establishes a velocity and level relationship under open channel flow 
conditions. As the slope of the pipe increases, the velocities tend to be 
higher for a given level and roughness, and as the roughness of the pipe 
increases the velocity tends to be lower for a given slope and level. 
Thus, in general, however, the relationship between velocity and level are 
controlled by the slope and roughness of the pipe and the pipe diameter. 
Because it is very difficult to measure the actual average velocity 
directly, a variety of techniques have been developed where local velocity 
is measured (i.e., a sensed velocity that is related but not equal to the 
average velocity), and then the local velocity is modified so as to 
provide an accurate estimate of average velocity. 
In the prior Marsh U.S. Pat. No. 4,083,246 (which is assigned to the same 
assignee as the present invention), a fluid flow measuring instrument is 
disclosed in which flow is determined by measuring both the level and the 
local velocity directly. The sensed local velocity is modified to 
approximate average velocity, and flow, F, is then calculated utilizing 
the continuity equation F=V.times.A, where V is the approximate average 
velocity of the flowing fluid and A is the partially filled area of the 
pipe. 
In the aforementioned Marsh patent, the local velocity is measured near the 
bottom of the pipe. An appropriate modifier is determined by profiling the 
site with a portable velocity meter or by other means where the average 
velocity is determined at one or more flow rates, and subsequently through 
empirical equations contained in computer software the sensed velocity is 
transformed to an accurate approximation of the average velocity V over a 
wide range of levels (i.e. flow rates). 
Others have proposed to measure velocities over a large portion of a 
cross-section of a flowing stream as an approximation of the true average 
velocity, together with the use of modifiers to improve the accuracy of 
the instrument. Refer for example, to the U.S. Pat. No. 5,371,686 to 
Nabity et al. In the Petroff U.S. Pat. No. 5,198,989, a velocity is 
measured that is the maximum observed in the cross-section, and in the 
Bailey U.S. Pat. No. 5,315,880, the velocity at the surface is measured. 
(See also Alan Petroff/ADS Technical Paper). 
It has been discovered that if one analyzes the relationship between the 
local (sensed velocity) and the level for any particular pipe size and 
site location and compares these on-site relationships with those 
relationships obtained under controlled conditions, the modifiers 
necessary for converting the sensed or local velocity to average velocity 
can be predetermined without performing a separate or independent 
determination of the actual on site flow rate or average velocity. 
It should be noted that a simple velocity and level relationship obtained 
under just one flow condition, while providing a reasonably accurate 
calibration coefficient, would not be as good as that obtained by looking 
at the relationship of velocity and level over a broader range of flow 
rates so as to better establish what true conditions exist due to the 
slope and roughness of that particular site. If, for instance, one looks 
at the velocity/level relationship over a period of time such as a week, 
the flow coefficient can be chosen based on the velocity/level 
relationship at a more narrow range of flow rates obtaining higher 
accuracy where most of the data is occurring for that particular site. 
It is important to note that such a self-calibrating device or process is 
capable of reacting to various conditions that could result from 
downstream blockages or from tidal effects. Such conditions could cause 
the water level to rise and the velocity to slow down creating a different 
velocity/level relationship for that period of time where the unusual 
conditions exists. Separating these data sets obtained from this changed 
condition allows for different coefficients to be applied to each of the 
sets of different flow regimes. 
SUMMARY OF THE INVENTION 
Accordingly, a primary object of the present invention is to eliminate the 
necessity of independent site profiling, which often must take place at an 
inconvenient time, at inappropriate water levels or under difficult, 
confined space condition thereby making it possibly less accurate and less 
representative of the flow condition where most of the important date is 
concentrated. 
A more specific object of the invention, is to provide an improved 
flowmeter in which any local velocity measurement--be it near the bottom, 
at the surface, or anywhere in between and utilizing different velocity 
sensing means--electromagnetic acoustic, microwave, optical or other is 
modified so as to cause it to better approximate the average or mean 
velocity. The modifying means utilizes velocity/level relationships for 
any particular velocity measuring techniques that have been gathered under 
controlled conditions and in comparison to local site characteristics 
provides information to determine what the multiplier factor relating the 
sensed velocity to the average velocity should be for a particular 
location (site). This multiplier factor (or flow coefficient) could either 
be a constant or vary with other parameters such as depth. 
A further object of the invention is to provide a self-calibrating 
flowmeter having a default flow coefficient. The self-calibrating 
flowmeter may be installed with the default flow coefficient, and once 
exposed to flow, the measured level and velocity data obtained on site is 
compared to a library of data stored in the memory to arrive at an 
improved flow coefficient using data stored in a computer or archived in 
tabular form. 
Other inputs needed to perform the self-calibration include the channel 
shape and channel dimensions. Direct knowledge of the conduit slope or 
roughness is not required.

DETAILED DESCRIPTION 
Referring first, more particularly, to FIGS. 1-6, flow in open channels 
will exhibit a pattern of velocities (a velocity profile) that is 
approximately logarithmic in shape. As flow increases, the liquid depth 
and the velocity at each point in the profile will increase, leading to a 
"family" of curves as shown in FIGS. 1, 3 and 5. For a given pipe 
characteristics (slope, roughness, etc.), as the flow (and depth) 
increase, the velocity profile is amplified exponentially, giving higher 
velocities at a given location in the flow. In FIG. 1, each curve 
represents a velocity profile that exists for different liquid depths. 
FIGS. 1, 3 and 5 illustrate how the family of profile curves varies with 
pipe characteristics. FIG. 1 represents a pipe with low slope/high 
roughness, FIG. 3 a pipe with moderate slope/moderate roughness, and FIG. 
5 represents a pipe with high slope/low roughness. 
As can be seen from FIGS. 1, 3 and 5, a low slope/high roughness pipe will 
exhibit lower velocities throughout the profile for a given depth of flow 
than a high slope/low roughness pipe will. For instance, at a flow depth 
of 10 inches, the velocity is only 2.5 ft/s at a position 7 inches above 
the channel bottom for the pipe characteristics represented in FIG. 1. But 
for the pipe represented in FIG. 3, under the same depth conditions, the 
velocity at 7 inches above the channel bottom is 6 ft/s because the pipe 
is steeper or smoother or both. Various combinations of pipe slope and 
roughness are found in most open channel piping systems. 
Referring now to FIG. 7, the relationship between pipe characteristics and 
scatter plot characteristics of the conduit conditions of FIGS. 1, 3 and 5 
are shown for corresponding electromagnetic velocity sensor locations one 
inch off the channel bottom, which corresponds to a typical location for 
an electromagnetic sensor positioned to measure a sensed local velocity in 
a sanitary sewer without collecting debris. 
A plot of the sensed velocity versus the flow depth (a scatter plot) for 
the family of profile curves shown in FIGS. 1, 2 and 5 is shown in FIG. 7 
depicting sensed velocity vs. depth. 
FIG. 7 demonstrates that the characteristics of a scatter plot (such as 
slope and offset) are related to the characteristics of the pipe (pipe 
slope, roughness) where a level/velocity sensor is installed. The scatter 
plot for the profiles in FIG. 1 depicts a smaller change in velocity for 
an incremental change in level (.DELTA.V/.DELTA.L) than the scatter plot 
of the profiles of FIG. 5 which have a greater velocity change for the 
same incremental change in level. This behavior is directly related to the 
nature of the family of profile curves, which, in turn, is directly 
related to the characteristics of the pipe size/slope/roughness etc.). 
Other characteristics of the scatter plot provide additional (but less 
dramatic) insight into the site characteristics, the curvature of a best 
fit curve through the scatter plot being one such characteristic. 
Referring now to FIG. 8, the pipe characteristics that influence scatter 
plot shapes also influence which flow coefficient is required to convert 
the sensed velocity (as measured by a level/velocity meter) to the average 
velocity. More particularly, the velocity multipliers (average velocity 
divided by sensed velocity) for the profile curves given in FIGS. 1, 3 and 
5 have been plotted versus the flow depth. Currently, use is made of a 
single "flow coefficient" to define a particular set of velocity 
multiplier values for various depths of flow. 
For the examples given, the pipe represented in FIG. 1 requires a flow 
coefficient to 1.5, while the FIG. 3 profiles require a flow coefficient 
of 1.35, while the FIG. 5 profiles require a flow coefficient of 1.2. The 
reason for the difference in flow coefficient is that the slope/roughness 
differences in the pipe affect velocity profile shapes. By examining the 
velocity/level scatter plot characteristics the flow coefficient can be 
surmised at a particular site because it is directly related to the pipe 
slope/roughness as well. 
Therefore, one can examine the characteristics of a scatter plot generated 
from a particular meter installation site and make a determination of what 
flow coefficient should be used to accurately site calibrate the meter. 
Referring now to the flow measuring system of the present invention 
illustrated in FIG. 9, a portable flowmeter installation includes a 
flowmeter 2 installed inside a manhole 4 so as to measure the flow 
velocity of liquid 6 in a conduit 8. Manholes are normally installed where 
there is a change in direction, slope, or pipe size, or otherwise where 
there is a need to have access to the flow. FIGS. 10-13 illustrate various 
examples of different sensors for measuring the velocity of liquid flow in 
a conduit. Velocity sensors are generally of the electromagnetic, acoustic 
Doppler, microwave doppler, laser Doppler, correlation or scintillation 
type. Level transducers normally include bubbler type pressure 
transducers, submerged pressure transducers, submerged acoustic level 
transducers, and look-down acoustic, laser or microwave level transducers. 
FIG. 10 shows an electromagnetic velocity sensor 10 mounted on the bottom 
of conduit 1. Such bottom-mounted transducers are often secured in place 
by mounting bands (not shown). This electromagnetic velocity sensor also 
contains in the same housing a submerged pressure transducer 12. These 
combinations of velocity and level transducers are connected to the 
electronic processing contained in flowmeter 2 unit via cable means 14. 
FIG. 11 illustrates a bottom-mounted Doppler velocity sensor 16 and a 
submerged pressure transducer 18. Again a cable 20 connects the sensors to 
the electronic processing unit. 
FIG. 12 shows a bottom-mounted Doppler surface velocity sensor 22 and a 
submerged pressure transducer 24. Again cable 26 connects the sensor to 
the flowmeter electronic processing means. 
FIG. 13 illustrates a look-down velocity sensor 28 and a look-down depth 
sensor 13 connected by cable 32 to the electronic processing module. 
Referring now to the block diagram of FIG. 14, the signal processing of the 
depth and velocity signals includes the improved processing means of the 
present invention. The outputs of depth sensor 46 and velocity sensor 48 
are converted by signal processors 50 and 52 to the site depth signal Ds 
and the site velocity signal Vs, respectively. By use of the 
microprocessor 54 with RAM memory, corresponding depth and velocity 
signals are stored as pairs of data. 
As will be described in greater detail below, contained in the more 
permanent read-only (ROM) memory means 56 are previously obtained pairs of 
velocity and level data that have been collected under controlled 
conditions in either a flow laboratory or in a carefully conducted field 
test. Data sets have been obtained for all pipe sizes expected to be seen 
by the device in the field under various slope and roughness combinations. 
Also contained in memory 56 are site flow coefficients that correspond to 
the level dependent multiplier and which serve to convert the sensed 
velocity to mean velocity once the particular flow condition at the field 
has been recognized. 
The flow condition at the field is recognized by comparison means 58 which 
includes a microprocessor that compares data contained in RAM 54 
(particularly the site velocity V.sub.S and the depth velocity D.sub.S). 
The pipe size signal P.sub.S from manual source 53 is supplied to 
comparison means 58 via encoder 57. By observing the scatter plot that 
exists between site velocity and site depth for that particular pipe size, 
comparison means 58 searches in memory means 56 to find an equivalent 
velocity/level relationship for the particular pipe size and that 
particular velocity measuring device (i.e., bottom-mounted 
electromagnetic, bottom-mounted Doppler, surface-mounted velocity sensor, 
and the like). For each type of velocity measuring device there must be 
stored the reference velocity and levels and corresponding site 
calibration coefficient C.sub.F. Once the corresponding curves have been 
matched, then the corresponding site calibration coefficient C.sub.F 
(which is generally depth dependent) is supplied to the site velocity 
modifying means 62 together with the depth signal D.sub.S. 
The site calibration coefficient C.sub.F and the depth signal D.sub.S are 
supplied to the site velocity modifying means 62, whereupon the 
depth-modified coefficient C.sub.F ' is supplied to first multiplier means 
64 together with the sensed velocity V.sub.S, thereby to produce the 
average velocity signal V which is supplied as one input to third 
multiplier means 66. To the other input of the third multiplier means 66 
is applied an area signal A.sub.S which is the product of pipe size signal 
P.sub.S produced by second multiplier means 67 from manually-set pipe size 
signal generator 53 and pipe size encoder 57, and the site depth signal 
D.sub.S. The desired flow signal Q.sub.S is produced at the output of the 
third multiplier means 66. 
Referring now to FIG. 15, in order to obtain the reference data stored in 
ROM data memory means 56, through a combination of mathematical modeling 
and actual testing of velocity and level sensors, under a variety of flow 
conditions a means of collecting these various relationships is achieved. 
In particular, inputs to a data acquisition memory consist of five 
different inputs (i.e., the average velocity obtained from the reference 
standard, the sensor type, the sensed velocity, the sensed depth, and the 
pipe size corresponding to each of the previous inputs). For each input 
the data is run on each pipe size, or on a range of pipe sizes, and are 
recorded on a ROM. The reference depth and reference velocity and the 
ratio R of average velocity to reference velocity at each depth are 
recorded in the ROM along with the corresponding pipe size and sensor type 
by means of which the measurements were obtained. Of course, if only one 
sensor type is used, then the sensor type factor can be eliminated. 
Typically, not every condition is tested, but through mathematical 
modeling and interpolation a good estimation for the intermediate sizes 
can be achieved and the data either stored directly in ROM or, alternately 
an equation depicting the velocity and depth relationships stored. 
Referring now to FIG. 16, the processing of the site depth signal D.sub.S 
and the site velocity signal V.sub.S are processed together with pipe size 
signal P.sub.S, and the sensor type signal produced by manually-set sensor 
type signal generator means 59 and sensor type encoder 61. These signals 
are processed by microprocessor 54 and a corresponding RAM (Randon Access 
Memory) and ROM. The local sensed velocity and depth signals are digitized 
and stored in RAM 54 along with information regarding the pipe size and 
the sensor type for that particular site. Inputted to the comparison means 
58 is the sensed velocity, sensed depth pipe size conduit size and sensor 
type information. The comparison means searches the ROM 56 so as to find 
that stored data that is pertinent for this sensor type. The next step is 
that from the stored data for the sensor type the stored data 
corresponding to the site conduit size is located. From the stored 
information pertinent to both the pipe size and the sensor type, the 
V.sub.S and D.sub.S of the site is compared to the V.sub.R and D.sub.R in 
the appropriate memory section so as to find the best fit V.sub.R and 
D.sub.R that match the data pair V.sub.S and D.sub.S. For each matching 
pair of data points, the corresponding flow coefficient C.sub.F is chosen 
and subsequently provided to the site velocity modifying means for 
multiplying by the sensed local velocity V.sub.S. 
The average velocity, the level and the subsequent flow rates can either be 
stored for later use, or be channeled to an indicator for instant read 
out. It is important to note that the set of stored relationships in 
memory 56, 156 can be a computer program that compares the velocity/level 
data that is recorded over some period of time, or just one velocity/level 
part itself so as to provide the correction factor. In any event, the 
stored general relationship of memory 56, 156, which have been 
theoretically or experimentally derived, are compared with the local 
velocity and depth relationship at any particular site so as to determine 
a correction factor which is tailored to that site so as to modify the 
sensed velocity to be an approximation of average velocity under those 
site conditions. 
An alternate means of modifying the sensed velocity utilizing 
velocity/level relationships is indicated in FIG. 17. In this alternate 
embodiment the depth signal D.sub.S is divided by the site velocity signal 
V.sub.S via division module 153 producing at its output the ratio Ds/Vs. 
The level/velocity relationship represents the level/velocity at a 
particular flow condition. Comparison means 158 is a microprocessor that 
combines this ratio of depth to velocity D.sub.S /V.sub.S and the depth 
D.sub.S and pipe size P.sub.S to search for an equivalent pair of 
reference ratios DR/VR. Once the set of reference ratios have been found 
in memory 156, the calibration coefficient C.sub.F for this particular 
site condition is processed similar to that of FIG. 14. 
Because of expense, quite often users do not wish to install a 
velocity/level flowmeter at all sites. In particular a "level only" type 
flowmeter will work as long as the velocity and level relationship are 
stable and repeatable for a particular site. In such cases the user may 
use a weir or flume which have predictable level-to-flow rate 
relationships or alternatively, by performing extensive site profiling to 
determine the level-to-flow rate relationship of a standard piece of 
conduit under various flow conditions, and if such conditions are stable 
one can utilize this information to obtain flow from just a level reading 
itself. However, such site profiling at different levels (flow rates) is 
both time consuming and possibly dangerous due to confined space entry 
conditions. The present invention lends itself to provide a much easier 
and more accurate means of establishing the level to flow relationship in 
a standard conduit. 
Referring now to FIG. 18, the flow rate Q.sub.S1 at a particular site has 
been obtained by the instant invention and stored along with the depth 
D.sub.S1 at the site for each flow rate. 
In other words pairs of data points Q.sub.S1 and D.sub.S1 that were 
measured with the instant invention are stored in an electrical erasable 
prom EEPROM 260. The information in this EEPROM can either be copied 
electrically, or the EEPROM physically removed and installed in a second 
flowmeter. This second flowmeter does not have a velocity sensor but only 
has a level or depth sensor. The depth sensor does not necessarily need to 
be of the same type as was used to collect the original depth and flow 
rate data. This depth signal D.sub.S2 is analyzed by a microprocessor in 
this level-only flowmeter. For every depth signal D.sub.S2, the memory 
means 260 is searched for an equivalent depth signal D.sub.S1, and once 
this is found, the flow corresponding to D.sub.S1 is read out of memory 
260 and is outputted as Q.sub.S2.