SediMeter™ model SM4
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 and avery second one as oblique backscatter, and reported in formazin turbidity units (FTU) with no decimals. The level is estimated with 0.01 mm resolution.
About 11 cm higher up there are two turbidity sensors, one ISO style with NIR light, and one EPA style 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 have 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 (only 8-bit) 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 have a built-in memory that holds 32,786 measurements. The time interval between measurements is from 1 seconds to 24 hours, although the recommended minimum is 3 s. 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 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.
The instrument 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. It is powered by a size AA 900 mAh rechargeable Lithium battery with solder tabs, located in a separate compartment. It can be replaced by qualified personnel using a soldering iron. The SM4 can also take a primary battery, such as a AA 2700 mAh 3.6 V Lithium battery. Furthermore, it can be equipped with both a primary and a rechargeable battery at the same time. In such a case, the primary battery acts as a backup to the rechargeable battery, i.e., the instrument prefers to take power from the rechargeable battery but if the voltage drops sufficiently, it starts drawing power from the primary battery. If event more power is desired a custom battery compartment can be made to order.