ACCURACY
A common misconception about metering flumes is that one type of flume is more accurate than another. The differences between different types of flumes and different sizes of flumes of the same type, are really differences in resolution not accuracy.

Three things affect the accuracy of metering flumes, they are:

1. The accuracy of the rating tables developed for the flume in question.
2. Accurate fabrication to the standard dimensions.
3. The quality of the installation.

Flumes molded in fiberglass from one of the well-known manufacturers will have good dimensional integrity. Care should be taken to select a manufacturer that is knowledgeable about metering flumes and understands the importance of maintaining dimensional tolerances. Avoid manufacturers who have obtained their molds by merely taking a casting from another manufacturers product. This type of tooling work tends to magnify small deviations in the original mold. In the case of flumes that have been formed in concrete without the use of a fiberglass liner care should be taken to verify that the flume conforms to standard dimensions.

For fiberglass flumes installation conditions are going to be the most important variable affecting flume accuracy. Installation problems are of two kinds, dimensional distortion and site factors.

DIMENSIONS
Installation dimensional problems with fiberglass flumes are usually caused by deflecting the sidewalls of the flume during concrete placement. The larger the flume the less significant a given dimensional error is. For example a one-quarter inch deflection in the throat width of a one-inch Parshall flume may produce significant error in flow measurement. The same deviation in the throat of a 48-inch Parshall would probably not produce a detectable error in flow rate. Since it is often difficult to evaluate the effect of a given error it is best to properly brace the flume during concrete placement. The dimensions of the throat of the flume are more significant than those in the approach section. The discharge section dimensions are the least important. This makes sense when the functions of each section are evaluated.

The throat section produces a critical energy value for each flow rate. Under the sub-critical flow conditions that the flume is intended to operate under, the water has insufficient energy to force itself through the throat. In order to match the energy threshold the water will build up a head upstream of the throat. Since energy is conserved, this head will be a unique value for each flow rate. A change in throat dimensions will change the critical energy and thus change the discharge table.

The approach section functions to smooth the flow as it is constricted from the original channel into the narrow throat. As long as a uniform velocity profile is maintained the approach section has done its jobs.

The diverging section is primarily intended to prevent the turbulence created by passing through the throat from eroding the downstream channel. As long as the diverging section is not so distorted as to impede the flow through the throat it is not going to have an effect on the rating of the flume.

SITE CONDITIONS
Site factors are the most important factor in evaluating flume accuracy. Site factors include, crest (or sill) elevation, approach conditions and discharge conditions.

A LEVEL CREST
The crest of the flume is the floor of the flume at the highest point. The crest is called the sill when referring to Palmer Bowlus flumes. The crest may be in the throat, or in the approach section depending on the type of flume. In some flume types the floor is level throughout the length of the flume and the entire floor can be thought of as the crest.

The crest of the flume must be set level transverse to the direction of flow. If the crest has a high and low side there will be a corresponding high and low side at the head measuring point upstream of the throat. Since the crest is the zero reference point from which head is measured this will obviously create an undesirable situation.

In theory the crest should be set level in the direction of flow. Experience has shown that a slight slope in the direction of flow does not usually produce measurable errors.

An exception to the level crest recommendations is the sloping false floor sometimes seen on "H" type flumes. Here the floor is deliberately sloped to one side in order to concentrate the low flows. H flumes are probably the most sensitive to installation problems. H flumes must be level in the direction of flow and the head must be measured at exactly the recommended metering point. This is because of the relative shortness of the flume and the fact that the calibrated measuring point is actually in the drawn down zone created by the throat.

APPROACH CONDITIONS
When evaluating approach conditions we consider both energy state and velocity distribution.

ENERGY STATE
The throat of a flume is a constriction in the channel. As mentioned above at any given flow rate there is a critical energy threshold that the water must obtain in order to pass through the constriction. If the water has insufficient energy it will build a head upstream of the throat until the threshold is exactly matched. Obviously then the energy state of the water in the channel is an important approach consideration. If the water possesses too much energy it will not develop the predicted head. Both channels with gradients that are too steep (shallow fast flow) and gradients that are too flat (slow deep flow) will produce excess energy and should therefore be avoided.

VELOCITY DISTRIBUTION
The velocity must be evenly distributed before entering the flume. Flow should be laminar not turbulent. Flumes should be placed in straight uniform channel sections in order to avoid turbulence. Changes in direction, cross section, and grade should all be avoided in the approach to the flume.

DISCHARGE CONDITIONS
The water must be able to discharge from the flume freely. The ratio between surface elevation of the water at the head measuring point upstream of the flume's throat and the water surface elevation downstream is called the submergence ratio. The effect of excessive submergence is to retard the discharge. In other words the flow through the flume will be less for a given head in a submerged flume that through the same flume flowing free. Different types of flumes can tolerate different submergence ratios and still maintain a free discharge. H flumes tolerant almost no submergence. Parshall flumes tolerate 50-70 percent depending on the size of the flume. Palmer Bowlus flumes are the least sensitive at about 85%. Because of submergence the carrying capacity of the downstream channel can limit the maximum flow through a given flume. This effect is frequently seen when Parshall Flumes are selected to measure flows in small pipes.

RESOLUTION
Resolution is usually what people are concerned about when they ask about "accuracy". Resolution in the percent change in flow rate for a specified change in head at a specified flow rate. To evaluate the resolution of a given flume the first question to answer then is - To what resolution will the head be measured? This resolution of head change can then be applied to the flow table to see what the flow rate change will be at various flow rates. When flumes are monitored by "Open Channel Flow Meters" the usual resolution is ±0.01 foot.

By way of example the table compares live small flumes and shows their resolution as a percentage of flow rate at selected flows. The table assumes that the depth is resolved to ±0.01 foot.

This type of comparison is frequently used to arrive at the erroneous conclusion that Parshall flumes are more accurate than Palmer Bowlus flumes. What's missing from the above analysis is a consideration of the channel. Flumes do not function in isolation they are an integral part of a particular channel.

Due to the exponential nature of the flow tables the percent change in flow for a given change in depth will be greater at lower flow rates. Resolution is therefore of greatest concern when attempting to measure a low flow. In general when comparing two flumes the flume with the smallest cross section area in the throat will produce the best resolution. The trade off of course is the fact that the smaller throat creates more head loss and will therefore have a lower maximum flow rate.

Let's consider an eight-inch clay pipe at 0.5% slope in which the flow will not exceed 100 gpm and will normally be between 10 and 50 gpm. What flume should be used?

Referring to the above table (a similar table can be arranged for any flow and channel) we would first loot at simplicity of installation. In this case the eight-inch Palmer Bowlus and eight-inch Low Flow would most easily adapt to the channel. Since the Low Flow flume can handle the required maximum flow and offers the best resolution over the range it would be our first choice.

If the flow stream contains a lot of floating debris which could catch in the rectangular throat of the Low Flow flume we might consider the six inch Palmer Bowlus of the 60 degree "V" Trapezoidal. Either would require some channel modification, but the Trapezoidal shaped throats would then to allow floating material to rise and pass through.

SUMMARY
Accuracy is the degree to which an instrument can measure an absolute value.

Flume Accuracy depends on:

1. Accurate Rating Tables (generally a given)
2. Accurate fabrication (you can rely on Accura-flo flumes from CSI)
3. Accurate installation (pay attention to site conditions, place flumes carefully)

Installation - Site Conditions

1. Approach conditions: Flow velocity profile should be uniform, non-turbulent and have sub-critical energy.
2. Placement: Place flumes with care. They should be level and free from distortion. Particular attention should be paid to the throat.
3. Discharge Conditions: Flume should be free flowing. Avoid backup from too small or too flat discharge pipes.

Resolution is the ability to distinguish between two points. For flumes resolution of flow is dependent on resolution of depth. The exponential nature of the depth-flow relationship causes the percent change in flow to be greater at lower flows.

Selection - When selecting a flume for an application consider:

1. Flow rates: Minimum flow to be resolved, maximum expected flow rate.
2. Channel Shape: Select the flume whose cross section at the entrance most closely matches the channel.
3. Throat Shape: If solids are expected, select the throat shape that is least likely to trap them.