Production sensors: The disadvantage nobody talks about

 

Production is the biggest factor that determines the cost of a dredging project.

So naturally you would want to monitor the production level during the project. There are several ways to do that such as hand soundings, dry surveying for a reclamation project and wet-surveying usable for all projects. But today I want to talk to you about another option available and that is the radio-active production sensor. Specifically about a disadvantage of the sensor that nobody talks about.

Firstly let me shortly outline how this sensor works, before I go into the advantages and disadvantages. This type of sensor is installed on board and measures the production in real-time. To calculate the production both the velocity and the density of the mixture are required. In modern sensors these two are integrated into one unit, but in the end they are two separate measurements.

The velocity is measured by a magnetic field which gets distorted depending on the velocity of the mixture. The more distorted it is, the higher the velocity is.

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The density is measured by using a radioactive source. Radioactive particles are send to the other side of the pipe, where they are collected by a receiver. If the pipe is only filled with water a lot of the radioactive parts reach the opposite side and hence the density is low. If there is a lot of sand in the pipe, more and more radioactive parts are stopped by the sand and don’t reach the receiver. This indicates a high density.

So both sensors don’t measure the velocity or density directly. Instead they measure a physical phenomenon which is then calculated to either the velocity or the density. In turn the velocity and density are used together with some parameters to calculate the production in cubic meters (or cubic feet, or Tons Dry Solid).

So now we have a basic understanding of the sensor, let’s move on to the pro’s and con’s.

Advantages

  • Real-time measurement. All other methods only measure the production after the fact. With a real-time measurement the operator can use it to optimize the dredging process and hence achieve higher productions.
  • Usable for automation. Because it is a real-time electronic measurement it can be fed into a dredge computer which in turn automates part of the dredging process. The automation generally achieves higher production over longer time periods and provides the operator with more time to fine tune the dredging process.

Disadvantages

  • Expensive. It is generally known that the production meter can be viewed as an expensive item when buying it. The lifetime can be several years, but still the initial investment can be substantial.
  • Accuracy. The disadvantage nobody talks about! Although the manufacturers of these type of sensors won’t agree I view these sensors as inaccurate, compared to hydrographic surveys. There are two main reasons.
    The first is that the sensor requires the In-Situ density of the material dredged as an input, to be able to calculate the cubic meters. But there is no reliable In-Situ density measurement, so the input is an educated guess at best.The second reason is inherent to the way the density sensor works, with a radioactive source. Any radioactive material becomes less radioactive over time. In the case of the sensor this means that over time it will indicate a higher and higher density. This is a process that takes several months to have a measurable influence and it can be rectified by re-calibrating the sensor, so all seems fine.
    But in practice this re-calibration is neglected, so the density is measured wrong. Even at one of the big-four dredging companies the first thing I did when getting on board for a production visit was to check the accuracy of the density meters and I have found deviations as much as 10%. And that is the result at one of the most sophisticated dredging companies.
Source: royalihc.com
Source: www.royalihc.com

Conclusion, is the sensor worth the money?

The answer is, as with most things, it depends.

For smaller dredgers, no.

The increase of production provided by the feedback of the sensor is not enough to earn the investment back. In addition the crew of these dredger are usually less knowledgeable of the dredging process and hence aren’t the best suited for optimizing the production, even with real-time feedback. So for smaller dredgers the answer is no.

For larger dredgers it is a resounding Yes.

The crew tends to have the knowledge and the drive to achieve a higher production and subsequently the cost of the sensor is easily earned back by the higher production. But I would strongly suggest to periodically check and re-calibrate the sensors, to get the best results.

So although this sensor is not at the top of the sensor wish list (that would be the vacuum sensor), I’m always very happy when I go on board for optimization purposes and it is available.

I’m curious to hear what experience you have with production meters in the field, so please share your experiences or questions in the comments box below.

To long term great results,
Timon Vinke

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