• Early warning of coral reef degradation
• Low-cost, local initiative design
• Year-round water temperature and visibility

The Rainbow Sensor® is a technological breakthrough in both water quality monitoring and diver information.

The traditional way scientists measure water quality is with a fluorometer that measures the organic content of seawater, chlorophyll and dissolved organic matter. This device costs upwards of $15,000 USD.  Worse, is an attractive target for thieves so a hardened, theft-proof underwater mooring is required.  The total cost of monitoring a single location winds up in the $30,000 USD range, well outside the budget of most third-world countries.

The Rainbow Sensor® costs about $1000-$1500 per location and no single component is expensive enough to attract theft. Although certainly not as accurate as a $15,000 fluorometer it does measure organic content continuously, every minute.

How the Rainbow Sensor® Works -- A layman’s explanation

The Rainbow Sensor® is an early warning system, warning when the ocean water’s organic content is increasing

How Seawater affects Sunlight
As every underwater photographer knows, seawater attenuates different colors of light at different rates. Water attenuates red light most quickly such that more than 95 percent of the red light has been absorbed at 10 meters of depth. Those fantastic reds one sees in underwater photographs are the result of artificial light from camera strobes or an underwater flashlight, not sunlight.

However, what most people don’t know is that seawater contains particles and dissolved matter in addition to the water itself that affect how the light is attenuated. If seawater has high concentrations of organic components it attenuates blue light faster than does pure water. The effect of this is very clean seawater appears to be deep blue and seawater with a lot of organic components may appear greener. This is the characteristic of light in seawater that the Rainbow Sensor® uses.

Construction of the Rainbow Sensor®
The Rainbow Sensor® is really three pairs of sensors, a blue pair, a green pair and a white pair. Standard Hobo® Pendant sensors that look like an elongated matchbook are modified to create a Rainbow Sensor®.

Hobo® Pendant sensors measure white light that spans all the colors. However a special filter that only allows light in a specific wavelength (color) to pass through is inserted into the Pendant’s case. A blue sensor is modified with a filter that only passes blue light while a green sensor only passes green light. The third sensor is unmodified and thus passes all wavelengths (white light).

How the Rainbow Sensor® Measures Water Clarity
Each blue pair and each green pair work in conjunction. The sensors at 12 meters measure blue and green wavelengths at that depth while the sensors at 20 meters measure blue and green at 20 meters depth.

Obviously, in the 8 meters between 12 meters depth and 20 meters depth the light has attenuated (become dimmer). The difference between the light levels at 12 meters and 20 meters is the attenuation (reduction in light intensity) that has occurred in the 8 meter water column between the sensors. Thus the Rainbow Sensor® measures light attenuation (light loss) in both the blue wavelength and the green wavelength over the same 8 meters of water.

Using some fancy scientific formula this reading is converted to numbers (attenuation coefficients) called Kblue12-20 and Kgreen 12-20. How the number is arrived at is not important except to scientists. Simply understand that Kblue12-20 is the attenuation (light loss) in the blue wavelength between 12 and 20 meters and Kgreen 12-20 is the attenuation in the green wavelength over the same span.

How the Rainbow Sensor® measures the organic content of Seawater
Kblue is a measurement of water clarity. Clear water has a very low Kblue while dirty water has a high Kblue. There are many things that affect water clarity include mechanical things like suspended particles and organic components such as dissolved organic material and chlorophyll that will increase the Kblue.

However, in the green range, light attenuation (increased Kgreen) is generally not affected by organic material but is affected by inorganic material. Therefore if the blue attenuation (Kblue) increases but the green attenuation (Kgreen) does not the difference is the result of organic material in the water.

Again, without going through the fancy scientific calculations, that is how the Organic Index (OI) is calculated. An increase of Kblue relative to Kgreen means increased organic content (OI)

Monitoring the organic content (OI) of coral reefs is important because dissolved organic material is essentially fertilizer for algae. Too much fertilizer and an algae bloom occurs. The result is that the algae covers the coral and smothers it.

Unfortunately, once an algae bloom occurs it is too late to stop the damage. Fortunately, water quality is an early warning system. If the organic content of the water is rising an algae bloom is in the future.

Temperature and Visibility

What every diver wants to know is what is the water temperature and is what is the visibility at a prospective dive location they want to visit.

The Rainbow Sensor® measures both at 1-minute intervals. A graph of the temperature and visibility can be displayed for as long as the sensor has been installed providing a historical seasonal record of temperature and visibility.

Visibility is a combination of water clarity and overall light. Using the Kblue (12-20) and the overall light intensity at 20 meters from the white sensor at that depth a visibility is calculated. Understanding that some claims on the Internet concerning visibility are optimistic, Rainbow Sensor® visibility is correlated with Rainbow Sensor® “apparent diver visibility” Two divers actually measure the maximum distance between them that they can see a dive buddy signaling trouble. The divers must be able to distinguish between a diver with his hand placed on top of his head (all-is-well) and a diver waving an arm frantically. (Distress)

The Rainbow Sensor® also reports temperature at 20 meters, a typical diving depth. The data shows the overnight low temperature at 20 meters and provides an excellent guide for a diver to select a wetsuit for a vacation.

From a marketing perspective, historical temperature and visibility graphs provide a powerful marketing advantages to locations where they are installed as well as providing an understandable relationship between water quality and the diving vacationer’s experience.