SediMeter™ SM4 & SM4P

The SediMeter™ is a patented instrument for measuring siltation, sedimentation, erosion, and bedload sediment transport. It measures a vertical turbidity profile through the sediment – water interface, and estimates the exact level of the bottom. It has been used for many kinds of projects.

The SediMeter™ sensor invented by Dr. Ulf Erlingsson in 1985 for detecting incipient sediment motion on the seafloor. It consists of a vertical array of optical backscatter detectors (OBS), or reflex detectors, operating in the near infrared band of the spectrum so as to become virtually immune to daylight under water. The data is plotted in the software as a “time scan,” with time on the X axis and the Z level on the Y axis. Algorithms both in the instrument and in the software search for the bed level using an edge detection algorithm on this data. Already from the first functional prototype (which used a Z80 NMOS CPU) it became clear that the sensor was able to detect incipient sediment motion: It reacted to the addition to a single grain of sand adjacent to the sensor. The 1st generation had only 16 OBS detectors, but since the 2nd generation, launched in 2007, all SediMeters have had 36 OBSes.

The 4th generation SediMeter™, launched in 2018, adds a range of new sensors and features:

  • ISO 7027-conforming nephelometric turbidimeter
  • EPA 180.1-compliant nephelometric turbidimeter
  • Nephelometric fluorescence
  • Backscatter fluorescence
  • Luxmeter
  • Accelerometer
  • Conditions based monitoring (CBM)
  • UV anti-fouling with automatic optimization
  • Vibrator for shedding particles and bubbles

The sensor consists of 36 near infrared optical backscatter detectors (NIR OBS) spaced 1 cm apart. The active measurement length is 35 cm. Turbidity is measured at 71 levels, every second one as straight backscatter (OBS) and every second one as oblique backscatter, and reported in formazin turbidity units (FTU) with no decimals. The level is estimated with 0.01 mm resolution.

SediMeter™ SM4. The copper tape is optional and user applied.

About 11 cm higher up there are two turbidity sensors, one ISO style turbidity meter with NIR light, and one EPA style turbidity meter with white light, both reflected 90 degrees. There is also a fluorescence meter emitting UV light and measuring the returned white light. The window of the turbidimeters are protected against fouling by UV LEDs.

The SediMeter SM4 also has a 3D accelerometer for measuring tilt and vibrations. Every measurement it stores either 20 measurements with 12-bit resolution, or 30 measurements with 8-bit resolution. The default is +-2 g range and 10 Hz frequency, although it is capable of up to +-16 g range and from 1 Hz to over 5 kHz frequency. Apart from giving information on tilt and vibration, the instrument offers Conditions Based Monitoring (CBM): When the acceleration surpasses a pre-defined level, an extra measurement is taken the next whole second. This means that the data before and after the triggering event is saved. To avoid being triggered all the time there is a 1-second hold-off, plus the threshold is raised after every extra measurement. This means that the largest accelerometer event of every deployment is expected to be captured. It is possible to adjust high-pass filter frequency and minimum duration of the acceleration event so as to target specific types of events (e.g. vibrations caused by strong currents, or earthquakes). A possible application is for capturing the head of a turbidity current.

The SediMeter™ SM4 has a 15 mm diameter sensor intended to go into a 20 mm holder tube, included in the kit.

The SediMeter SM4 has a built-in memory that holds 32,786 measurements. The time interval between measurements can be configured from 1 seconds to 24 hours using the included software. It can also be used in real-time monitoring, alone or in a network. When used in stand-alone mode it is powered by a built-in battery that is recharged through the same USB cable used to set up the instrument and to download data.

The SM4 instrument can be mounted in sediment using the standard anchor and holder tube included if purchasing a kit. This holder is screwed down in mud and sand with the help of a handle. This model is available separately or as a kit in a convenient case, complete with all necessary accessories for stand-alone use in the field (handle, anchor, holder tube, cable, blind plug).

The SM4 model has a 15 mm diameter level sensor that goes into the holder tube, and a 20 mm diameter turbidimeter region that stays free of the holder tube. The battery house is designed to avoid galvanic corrosion even in shallow, tropical seas, and to have a natural antifouling property.

The SM4P model, where P stands for professional, has 20 mm diameter throughout. It is intended to be used without a holder tube. Instead, the user would typically mount it to a platform or tripod. It is physically stronger than the SM4. It sits in a straight tube, threaded in each end, and this tube can be easily replaced at factory. The SM4P with firmware rev. 2 also works as a SediSond.