SM4 Final Design

The new generation SediMeters have finally been born, two months delayed due to a high order volume so we are not complaining. The tentative designs have been adjusted so that the SM4A and SM4C now are identical in all but one aspect: The C model has a much larger battery capacity.

SM4A and SM4C final designs. Click for full size.

Shape: Instead of making the A version 15 mm diameter and the C version 20 mm, we have decided to make both 15 mm in the lower end over the OBS array, and 20 mm in the upper end over the turbidimeters. The OBS array is visible on the drawing as 36 black dots in the narrow portion of the instrument. This allows both models to be mounted in the traditional holder tube, which allows the instrument to be replaced at regular intervals (like every week or fortnight) while the holder tube stays stationary, thus giving a continuous measurement series. This is standard operating procedure in many organizations; they have two instruments for each measurement site, one is deployed and the other is serviced, battery recharged, data extracted, and recalibrated if needed.

Turbidimeters: SM4 has two turbidimeters mounted side by side, so as to measure the same water volume, with the light exiting and entering in the same part of the sensor. One is an ISO style (International standard) with a NIR laser light source, the other is an EPA type (U.S. standard) with a white LED light source. The location of the light sources and the photodetectors is indicated with two pairs of black dots in the drawing.

Ambient light: The visible light photodetector has a spectral sensitivity similar to that of the eye, so it will be calibrated to show light in lux. The caveat is that the sensor is mounted horizontally, so it is not measuring incident light from above but from the side. Still, it gives an idea of the ambient light level. The NIR photodetector is similarly used to measure the ambient level of near infra-red light. The NIR light is absorbed very quickly by water, so if this value is high, the instrument was not submerged.

Accelerometer: The SM4 has an accelerometer that is programmed to continuously measure at a rate of 10 Hz and store the last 32 measurements in a FIFO memory. When a SediMeter measurement is made, most of those data are fetched and stored in the data record. In the PC software the tilt of the instrument is calculated. This is important for users who deploy the instrument attached to a platform lowered from the surface, since it will give them certainty regarding the position of the platform on the bottom. The software also calculates vibration (RMS) and peak acceleration. When a strong current is present the instrument may start vibrating due to vortex formation, why the vibration level indicates the presence and strength of the current (or waves).

Conditions Based Monitoring: The accelerometer measures independently from the processor, and is programmed to set a flag if exposed to certain acceleration events. The thresholds and conditions can be changed from software in expert mode. The purpose of this is to trigger extra measurements based on detected conditions, which could be indicative of an earthquake, a turbidity current, an object hitting the instrument, a ship grounding nearby, etc. To avoid getting too many extra measurements in an ongoing condition, the trig level is increased after each time it has triggered an event. The objective is to capture the largest events during each mission.

UV LEDs: The instruments have two UV LEDs for anti-fouling of the turbidimeter sensors, one where light goes out, one where it comes back in. These are programmed to blink with very strong light at regular intervals, the user decides how often. The 365 nm UV light disrupts organic molecules in organisms, and that has an antimicrobial effect (it also causes cataracts so don’t look at this blinking light for extended times, or at short distance, without UV-filtering eye protection). Remember, you see them as blinking but you only see a few percent of the light; most of the light is invisible and harmful to your eyes! The UV LEDs consume battery when used a lot, which is why the SM3C version has a much larger battery, for longer stand-alone deployments. It has up to 10 times more power available for the UV LEDs.

Fluorescence: One of the UV LEDs can be used together with the visible light photodetector to measure fluorescence. We have not programmed it to do that, but the possibility exists if there is interest from our customers.

SediMeter: Not to forget, the OBS array for measuring sediment level and vertical turbidity profile through the water/sediment interface remains identical to past versions. The sensor measures 36 levels of straight backscatter, and 35 levels of oblique backscatter in between the 36 levels, thus creating a vertical profile 35 cm long with 5 mm resolution. The software can present the straight levels, the oblique levels, both types combined, and lastly, a false color image with 5 mm resolution by combining the two. In false color air becomes blue, sediment becomes beige, and water becomes black or gray depending on turbidity.

Finally, you may notice that the B version is missing in the lineup. The SM4B version has a mechanical cleaner, but based on field tests in Miami the past summer we are inclined to believe that the SM4C is a better choice in fouling environments. The combination of copper tape away from the optical surfaces, and UV LEDs at the turbidimeters, seems enough to keep fouling at bay sufficiently for a successful mission. Thus, the failure risks and complications of a mechanical cleaner can be avoided. We also have another card up the sleeve, a transparent antifouling paint that we are testing. For the time being we are therefore working on alternatives to the mechanical cleaner.

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