The differences between the dotted and straight lines is what a 2013 US government report calls “random errors”. In reality it is a non-linearity error, a systematic error that is repeated cyclically every centimeter. By labeling it a “random” error the accuracy estimate of the instrument was downgraded by more than on order of magnitude, which was then used to justify not to use it to monitor sediment spill at the dredging of the Port of Miami—an operation that killed some 200 acres of coral reef due to siltation that was not monitored in real time.
Of course, this non-linearity error does not in any way prevent the instrument from measuring siltation with sub-mm accuracy, since the error is proportional to the distance measured: The shorter the distance, the smaller the error. The report also, for good measure, doubled the error in the conclusions, without any valid justification whatsoever (they claimed that difference data were level data and therefore doubled the error to account for difference data—which it already was from the start).
Furthermore, what matters in siltation monitoring is actually not the accuracy in determining the bottom level, but rather if it’s possible to detect siltation. This self-evident truth is easy to forget. The SediMeter measures the turbidity at 37 different levels. It’s like a scanner that delivers a vertical profile through the bottom, like a “photo” with elevation up and time to the right. It’s much more information than just a bottom level, all of which makes the SediMeter a tremendous asset for monitoring siltation in real time.
If you have something to hide don’t deploy a SediMeter, since it was designed specifically for siltation monitoring with utmost transparency.
When Ulf Erlingsson invented the SediMeter™ for his doctoral dissertation in 1985, the goal was to detect incipient sediment motion on the bottom of the sea, so as to compare that with wave and current data to see what combination of processes led to the initiation of sediment transport for different grainsizes, waves, and currents. The question was how to define sediment transport, but once the SediMeter was invented, it became a non-issue: The instrument is capable of detecting the difference that a single grain of sand makes in front of the sensor, and it is stable enough to give the same value when nothing changes. Thus, the definition became “what the instrument can detect,” and that was pretty much anything that happened to the sediments.
Fast forward to the 1990’s, and now Dr. Erlingsson was hired as an expert in sediment spill monitoring by the Swedish government, during the building of the Öresund bridge and tunnel between Sweden and Denmark, and the dredging of a new navigation channel to the Baltic Sea. Seeing this ambitious project from the front seat, from the regulator’s perspective with full insight into the executor’s monitoring and analysis, he became convinced that it would be more cost-effective, and wise, to use a monitoring system of stationary SediMeters™ in a real-time network, monitoring the sediment accumulation and near-bed turbidity directly, and to connect permit conditions to the sensitivity of each biotope.
When Erlingsson in 2006 got an opportunity to manufacture the SediMeter™ instrument himself, he decided to create “the best siltation monitoring system in the world,” based on his experience from the Öresund project. Since he by then lived in Miami, he designed it with the purpose of monitoring hard bottoms—including coral reefs—when there were dredging operations going on nearby. His new version of the SediMeter™ that came out in 2007 was designed specifically for the requirements identified in the siltation monitoring white-paper.
Since the only transparent anti-fouling paint on the market was banned a few years back, he next had to develop a new method for keeping the sensor clean from biofouling. In 2013 he released the third generation SediMeter™, with exactly the same proven sensor, but with a mechanical cleaner integrated in the instrument from the outset (it is also offered without cleaner). It has no logger house at all, since everything has been made to fit on the half inch wide sensor PCB.
Next Dr. Erlingsson turned his attention to wireless networking. All SediMeters™ made in Miami can be networked using RS485, which allows for mile long cables, but cables cost money. After several semi-custom solutions, in 2015 he developed the SediLink™ radio modem with a built-in small solar panel that can sit on a buoy over a single or a few SediMeters™. This allows for mixed networks with radio links and cables. The radio modem has a socket for a radio that the customer himself can mount, meaning that wherever in the world the client is, there is a license-free radio available.
The SediMeter™, its program and network abilities were developed to fit the role of a siltation monitoring system, which was formulated based on experiences from the most ambitious sedimentation monitoring project in the world. That is why Dr. Erlingsson does not hesitate to say that in his opinion, his design is the best siltation monitoring system in the world.
On February 1st, 2016, a public demonstration of the SediMeter™ precision was held in Miami, Florida. During the course of the day, people were given a chance to put sand in the tank and see in real time on a connected computer the sediment level the instrument was measuring with a resolution of 0.001 cm (10 µm, equivalent to the thickness of a typical human hair). The instrument never failed to detect the addition of the 2.2 g of sediment, which corresponds to 0.1 mm in the 162 cm2 tank (given a bulk density of the top few centimeters of 1360 kg·m-3). The event was summed up in this video.
Recently we found out that the US government decided to not even consider using the SediMeter™ for monitoring siltation on the coral reefs while dredging the harbor of Miami, because a US government study had concluded that it was not suitable for the purpose, since it allegedly did not have enough accuracy in distance measurements. If they had used the SediMeter™ to monitor siltation during the dredging, it might have been possible to avoid some or most of the costly damage to the coral reef.
It should be noted that the evaluation of the SediMeter™ was not conducted in order to evaluate its usefulness for the Miami dredging project, but for using it in a research project (cf. page 8). However, it was left out of consideration in the Miami project exactly because of the previous study by the same organization. Since the US government study landed two orders of magnitude wrong, we feel we need to put the record straight.
The US government measured and calculated the error in distance measurements (differential level) of various lengths from 1 mm and up; calculated the standard deviation; assumed normal distribution (thus justifying multiplying the standard deviation by 3 to estimate the 99.7% confidence interval); and in the conclusion stated that the calculated error value referred to a level (rather than a distance as they had said earlier in the report) why they doubled the error (in the conclusions!) in order to get the uncertainty of distance measurements. Based on such flawed arguments they concluded that the SediMeter™ is only suitable for cm-scale measurements, two orders of magnitude more than the detection threshold guaranteed by the manufacturer.
Dr. Ulf Erlingsson of Lindorm, Inc. has recently made a study of the accuracy by adding a small amount of sand 68 times to a tank with a SediMeter, once every 2 minutes. In contrast, the US government moved the SediMeter rather than adding sand, which is known to introduce small variations in the measurements. On the other hand, adding sand suffers from several sources of errors in the “true” value: The sand falls randomly and may not fall equally each time; consolidation may start after deposition; and a fine fraction may stay in suspension only to be deposited later, after measurements were made. These effects were minimized by depositing sand at an approximately even rate, and deposit several batches before starting to measure so that the bottom adjusts to the spatial pattern of deposition (so that the new layers become conformal). Also, values that were visibly affected by consolidation (the bottom level falls over time) were excluded from the final analysis. The sand was weighed with a 0.1 g resolution electronic scale.
Most of the errors detected were found to be systematic with a period of 1 cm, and not random (Figure 2). It is due to the non-linearity in the interpolation between the detectors, and since they are spaced 1 cm apart, the error is cyclical. The US government report assumed that the errors were random, which they are not. Since the errors are not random, one cannot use a multiple of the standard deviation to calculate the confidence interval. Rather, the analysis of the confidence level must be made based on an understanding of the nature of the error.
Figure 3 shows that the maximum absolute error in distance measurements varies with the same distance as the spacing between the optical backscatter detectors, again revealing that the error is not random. The maximum error was under 1 mm, and the occurred at a distance of between 4 and 6 mm. Looking instead at the relative error it increases towards shorter distances. The largest calculated relative error was 44% (the SediMeter had recorded 1.66 mm when the calculated sediment accumulation based on added sand was only 1.15 mm).
It may be noted that the level determination in the SediMeter was originally only made in software, and the process allowed for user input and interactivity. While it’s still possible for the user to adjust the correction of non-linearity errors, new SediMeters output a level directly. They also allow for burst sampling, with 20 burst samples in a single measurement. Taking the average of those measurements the noise is further reduced. It’s perfectly possible to get many measurements in a row with the same level value to 1/100th of a millimeter.
The only way to determine the detection threshold is to measure very small differences, and calculate if the change in the measurement data is statistically significant. The SM3 instrument resolution is 10 µm, and the detection threshold is advertised as 100 µm. Previous measurements have shown the detection threshold to be 50 g/m2 at 95% confidence level and 100 g/m2 at 99% confidence level, which corresponds to about 37 µm and 75 µm, respectively. For the SM2 instruments tested by the US government, the only sensitivity value given in advertisement was the resolution of 0.1 mm.
The new study confirms that the instrument indeed is capable of detecting 0.1 mm, and that the conclusion from the US government study simply is wrong. Surely many are asking by now, how could they come to a result that is off by two orders of magnitude? We can identify four contributing factors:
First, they measured by moving the SediMeter which introduced additional errors and appears to have increased the standard deviation of the data by about a factor 2.
Second, not realizing the distance-dependence of the error they assumed random data, why they calculated the standard deviation without taking into account the distance. This exaggerates the short-distance error by about a factor 25.
Third, again assuming that their measured error was random they doubled the erroneous standard deviation to calculate the 95% confidence level. This also doubled the error, but since the same doubling is made using the lower and true random error values for the detection threshold study made by Lindorm, the factor will be entered as 1 in the equation.
Fourth, they forgot that the measured standard deviation referred to a distance, and thinking it referred to a level they erroneously doubled it in order to make it apply to a distance (differential level). This increased the error by exactly a factor 2.
Multiplying 2 x 20 x 1 x 2 = 80, two orders of magnitude. Thus, instead of the US government value of 3.4 mm error at the 95% confidence level, the Lindorm estimate of the 95% uncertainty level in detection of siltation is 37 µm (50 g/m2), and the data shows that sedimentation of less than 1 mm can be measured well enough (±20%) for the requirements for siltation monitoring, QED.
On February 1st, 2016, 10 AM, a public demonstration will be made of the SediMeter accuracy. Anybody who has doubts about its ability to detect the sedimentation of 0.1 mm of sand (corresponding to 2.2 g in the 162 cm2 tank) is welcome to come and see with his or her own eyes. UPDATE: The demonstration was carried out and a video made from it. See later posts.
We have conducted experiments to determine the SediMeter sensor response using a method that as closely as possible mirrors an actual measurement situation. The instrument was mounted in a tank and sediment was added in a controlled manner. The analysis showed that as expected most of the error is not random, but a cyclical nonlinearity error. It also proved that the detection limit is ≤0.1 mm, just as advertised.
How sensitive is the SediMeter? How little siltation can it detect? Will it detect siltation that can kill corals? To find out we did experiments in which we added small amounts of sand to a tank, while measuring the level with a SediMeter. This video shows an experiment in which 100 g/m2 was added, and how this caused a clear signal on the SediMeter: SediMeter Detection Limit
A couple of years ago Lindorm, Inc. introduced the third generation SediMeter™. While the second generation had featured a display, an input device, USB connector, SD card, and RS485 communication, and relied on primary batteries, the third generation only features RS485 communication in a permanently sealed unit with a built-in rechargeable battery. The goal was simplicity and durability for monitoring sediment accumulation, erosion, and siltation around dredging sites, in shipping channels, and so on.
Unfortunately, the battery management was not ideal for research users who tend to store instruments most of the time, with maybe a year between deployments. The battery would run empty and require special reset procedures to start the instrument up again, and sometimes even a battery replacement at the factory.
To overcome this, Lindorm is now, from January 1st, equipping all SediMeter™ units with a backup battery. When the rechargeable battery runs empty, the backup battery prevents it from getting over-discharged for up to 10 years. When charging again, a new automatic reset is designed to turn the instrument on automatically. This is expected to completely eliminate the potential for hassle when taking instruments out of storage.
Several other changes have also been made: A stronger handle to eliminate bent bayonets during anchor retrieval; thicker military-standard internal cables to avoid malfunction due to vibration; a much improved line for the cleaner shuttle to avoid snags; and numerous process improvements during assembly to increase chemical compatibility and strength in the long run.
To reflect this increase in quality, Lindorm increases the warranty for SediMeter™ instruments from 1 to 2 years. Furthermore, the warranty now also covers the batteries since there is no longer a requirement to periodically charge the battery to keep it from getting ruined.
Version 3.4 of the SediMeter™ software is now available on the Lindorm web site. It features a new Export function that allows the user to export all data from a SediMeter™ in a text file, suitable for import to a spreadsheet.
The generated file includes the UTC time, temperature, mean bottom level, and mean turbidity from the 37th optical backscatter center. Furthermore it contains the individual turbidity data from each of the 36 optical backscatter detectors in the SediMeter™ probe. Finally it contains the burst data. These are up to 20 measurements of level and turbidity that are used to calculate the mean level and mean turbidity.
The previous update, version 3.3, provided e-mail alerts. The software can be used for real-time monitoring of a single or a network of instruments. The user can define thresholds for erosion, accumulation, and turbidity. If any of these thresholds is exceeded, for any SediMeter™ in the network, an e-mail alert is sent. This is useful in a dredging operation, or a beach replenishment, where there are environmental concerns. The e-mail can go to the dredge operator, to the supervisor, to the regulatory agency, or to all three.
Version 3.2 provided burst sampling of the bottom level and turbidity from the 37th optical backscatter detector. The user can take from 1 to 20 samples in each measurement, with an interval from 1 s to 15 s. This can be a huge benefit if the SediMeter™ is deployed in an environment with wave action. Without burst sampling, the short-term variability within a single wave period can be larger than the variability in a tidal cycle, for instance. The burst sampling allows for this “nugget effect” to be quantified, and also to get a much more reliable average value in each measurement.
Version 3 of the sedimeter.exe software is the one that corresponds to the third generation SediMeter™. The SediMeter™ was invented in the 1980’s and patented in Sweden. SediMeter™ is a trade name of Lindorm, Inc.
The SediLink™ radio modem is intended to be mounted on a buoy and connected by cable to a SediMeter™ underneath. The solar cells keep the modem and the SediMeter™ charged, and a blinking white LED in each cardinal direction alerts seafarers at night. An XBee radio provides license-free radio communication. There are different frequency and range options available, and many of them also offer meshing. This means that the radios forward traffic, so that the total distance from base station to the farthest radio can be much larger than the range. This allows a dredging operator to monitor siltation, turbidity, or even erosion from the work area in real time.