Verifying the accuracy of sewer flow meter data can be very important for applications such as billing or capital planning. Dye testing is one data QA/QC method to provide an independent check on the accuracy of a flow meter. Dye test results can even be used to correct systematic biases in flow meter data when used properly. This post provides an overview of dye testing, describes how it can be used to validate flow meter data, and discusses some of the limitations and challenges with using dye test results.
Dye Testing Overview
Dye testing is an accurate methodology of measuring instantaneous flows in a sewer. A fluorescent dye is injected into the system at a known flow rate and concentration. Then, a sample is withdrawn at a downstream point in the system where the dye is thoroughly mixed. The concentration of the dye in the downstream sample can be measured accurately, and the flow rate in the pipe can be determined from the dye injection rate and the dye dilution rate. Dye testing is generally considered to produce flow measurements that are within a few percentage points of the actual flows.
Dye testing is usually performed by a contractor that has the necessary dye injection and withdraw sample pump equipment and trained staff to collect the dye test samples in the field. Once the dye injection and withdraw samples are completed, the samples are sent to a lab for accurate determination of the dye concentration. The dye concentrations are then used to compute the dye dilution rate. Multiple samples are usually collected and each sample is used to estimate a dye flow, which is then compared to the metered flows to establish a dye adjustment factor (AF). The AF represents a factor that corrected the metered flow to match the dye dilution test flow.
Benefits of Dye Testing
Depending on the installation conditions, flow meters can produce very accurate flow measurements, or they can produce measurements that are off by as much as 10-50%. Dye testing provides an independent check of the meter accuracy to provide confidence in use of the data. When dye tests are consistent and repeatable, they can identify a systematic bias in the meter data and be used to develop adjustment factors to correct metering data. This can be extremely useful in meter-based billing systems, in which it is important to have high confidence in the metering data, especially during dry weather flows, which dominate billing volumes.
Adjusting Meter Data
Dye testing is a good method to check the accuracy of a flow meter and correct for repeatable, systematic biases in a flow measurement. For example, imagine a flume that is not level and that causes a consistent flow error, or an area-velocity meter with a repeatable systematic bias – these are the applications where a dye test is useful to correct the data.
A dye test factor can be used to adjust the meter flow in a mass flow balance. Most often, dye testing is used when meter data is being used for billing purposes. An example of adjusting metered flow with a a dye test factor is shown below. Because there are inaccuracies in dye tests as well, it is common to conduct at least two dye tests on different days to establish a correction factor for a billing meter. In many billing systems, meters are dye tested once a year, and a cumulative or rolling AF is computed to correct the meter data. By using many AFs developed over several years and averaging out the errors associated with dye testing, these rolling AFs can become a very accurate correction factor for the flow metering data.
There are some limitations to be aware of when considering the use of dye testing for your flow meters:
- Accuracy – Dye testing is not perfect either. There are several factors that can affect the accuracy of a dye test, just like any other measurement of a physical phenomenon. Dye test should be treated like any other physical measurement – be skeptical until the results are carefully reviewed and vetted before relying on them.
- Testing of dry weather flows – Dye tests are usually performed during dry weather and may represent a good correction factor for dry weather flows. The correction factors identified may not be applicable to wet weather flows when the flows and the meter are functioning under very different conditions. There are other processes that can be used to check wet weather flows.
- Period breaks – A period break is a point in time when there is a sudden shift in the performance of a meter. This may be caused by an object or debris interfering with the sensor, servicing or cleaning a sensor, or replacing a meter. The dye AF developed before a period break may not be applicable after the period break, necessitating a new round of dye tests to establish a new site AF.
- Costs – Dye testing is not cheap. It can cost $2,000 – $3,000 per test for a single metering site. Doing this once or twice a year at each metering site can be an expensive proposition. For billing meters in which millions of dollars in bills is being determined, dye testing costs may be money well spent.
Dye Testing and H2Ometrics
H2Ometrics has several tools built-in to use dye AF factors to edit and adjust the data. The H2Ometrics editing tools can be used to apply period AFs to the data to correct the data for dye test results. The data visualization tools and modeling tools can be used to create dry weather flow patterns and scrub the data searching for period breaks. Finally, the mass flow balance tool can be used to apply dye AF values to balance a metering network.