The SUNA V2 UV nitrate sensor is the ultimate chemical-free solution for autonomous monitoring of nitrogen-based nutrient concentrations in ocean, estuarine and high turbidity freshwater environments.

How does absorption from CDOM affect the SUNA V2 nitrate measurement?

The SUNA V2 determines nitrate concentrations from the shape of the UV absorption curve. The least squares curve fitting algorithm uses calibrated extinction coefficients for nitrate and bromide (strong absorbing species in salt water) to calculate the concentration of nitrate from the UV absorption curve. The algorithm also employs a linear baseline correction that accounts for absorption that is not associated with either nitrate or bromide. The linear baseline correction successfully compensates for CDOM absorption in cases where the CDOM absorption is close to linear in the low UV. The composition of CDOM is dependent on the type of drainage area around a particular watershed and is therefore highly variable. As a result, the shape of CDOM absorption curve can vary from region to region. For this reason, the baseline correction does not always successfully compensate for CDOM absorption. In cases where the CDOM absorption curve mimics the shape of the nitrate absorption curve, a positive bias can occur.

The most common approach for correcting a positive bias caused by CDOM absorption is to correlate the continuous in situ nitrate data provided by the SUNA V2 with nitrate concentrations from discrete water quality samples measured in a laboratory. The bias may then be calculated either as an absolute offset or as a factor. In order to provide the most robust correction possible, the discrete sample size should be sufficiently large to allow for comparisons and the relationship between the in situ and discrete concentrations should be highly correlated. 

How do I tell if my SUNA is a V1 or V2?

You can identify which model of SUNA you have by the firmware revision it is running. The firmware information can be viewed by either starting the SUNA in a terminal emulator (information is in the start-up banner), or connecting the instrument to SUNACom (information is in top left corner, next to serial number). The first digit of the firmware revision identifies the version of the sensor, ie. firmware v1.9.0 indicates a SUNA V1.

When running my SUNA V2 on the bench, the data shows that the lamp is turning on and off. Why is this happening?

This is a safety feature of the instrument, whereby the lamp is turned off when its temperature reaches 35°C (95°F) to prevent damage from overheating. The SUNA continues to output dark frames while monitoring the temperature, and will restart the lamp once it has dropped back below 35°C. If extended in-lab use is required, immersing the SUNA in a cool bath can  prevent this behavior.

My SUNA V2 fails the self-test function when it tries to report the internal voltage measurements?

Although most functions of the SUNA V2 can be accomplished though the USB connection, parts of the self-test require that external power be applied. In this case, connect external power and repeat the test. If the problem persists, please contact Satlantic Customer Support.

How does the free calibration differ from the fresh water calibration?

The included class-based calibration coefficients are average coefficients obtained from our library of historical calibrations. The historical coefficients that are averaged to create the class based coefficients vary by about 10%.

Is an instrument with the class based calibration still within the published specification for accuracy? If not, what is the accuracy?

Instrument specific calibrations are more accurate and correspond to the published accuracy specifications. 

The accuracy of the class based calibration is estimated to be 2.2 uM +/- 20%.

Why is the diameter larger?

The diameter is larger because the pressure case wall thickness was increased to improve the robustness of the instrument for more rugged deployments. As a result, the depth rating also increased to be 500 m from the previous 100 m.

What is "adaptive sampling" and how does it work?

The SUNA V2 contains a 256 channel spectrometer that is programmed to integrate for a specific length of time (usually 300 - 500 ms) while sampling to maximize signal. That is, when the SUNA V2 takes a sample, the spectrograph collects UV light for the length of the integration period. In optically dense waters (e.g. high turbidity or CDOM), very little UV light is transmitted through the water and therefore the spectrometer "sees" a much lower signal. The new SUNA V2 is programmed to automatically increase the integration period to compensate for the low light levels. This enables the instrument to collect a strong signal in extreme environmental conditions.