Configuring a SUNA V2 for Real-Time Analog Output

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Where

• C_nitrate is the final nitrate concentration, as calculated from the analog voltage output.
• DAC_min is the nitrate concentration at the minimum voltage.
• DAC_max is the nitrate concentration at the maximum voltage.
• V_min is the minimum voltage. Nominally 0.095 V.
• V_max is the maximum voltage. Nominally 4.095 V.
• V is the generated voltage, output by the SUNA V2 and received by the CTD.

The V_min and V_max coefficients are dependent on the particular SUNA V2, as well as the cable and CTD to a lesser extent. For optimal accuracy, calculate these coefficients prior to deployment, especially when integrating the SUNA V2 with a new CTD. When these coefficients are available, nitrate concentration (C_nitrate) scales directly with V, the voltage output from the SUNA V2 and logged by the CTD.

Calculating Coefficients

Calculating two sets of coefficients are crucial for optimal accuracy and resolution:

1. Obtaining an accurate voltage for V_>min and _Vmax
2. Fine tuning the DAC min/max settings to optimize resolution

These coefficients are specific to the system (SUNA V2, cable, and CTD) and the deployment environment. Proper calculation of these coefficients is crucial for obtaining accuracy and resolution from the analog output.

Required Materials:

• SUNA V2
• Deployment CTD
• Deployment Cable
• DI Water
• Nitrate Standard (higher than 15 μM)
• Parafilm or plastic wrap

Establishing the Voltage Range (V_min and V_max)

The voltage output is generated using a precision 12-bit digital-to-analog converter (DAC) with a nominal range of 0.095 to 4.095 Volts. In a deployed system, voltage and current differ slightly due to transmission losses. To optimize accuracy, obtain a more accurate voltage range for the specific SUNA V2, cable, and CTD.

1. 1. 1. Connect the SUNA V2 to UCI
      2. Navigate to SUNA Settings → “Advanced” tab
      3. Set the Nitrate DAC range to 0 μM → 15 μM. This will provide the zero voltage at 0 μM and the highest voltage at 15 μM
      4. Upload the settings
      5. Connect the SUNA V2 to the CTD. Use the cable that will be used for the deployment.
      6. Ensure the SUNA V2’s windows are clean and fill the optical path with DI water as though performing a Reference Update (see the SUNA V2 manual for more information).
      7. Power the system and command the CTD to read the voltage from the SUNA V2.
         • 19plusV2: Send the “TV” command to read the voltage on every analog voltage channel. Note the voltage in the column that corresponds with the SUNA V2’s voltage channel.
         • 25plus: Send the “TSV” command to read the voltage on every analog voltage channel. Note the voltage in the column that corresponds with the SUNA V2’s voltage channel. Alternatively, you can use the “GetVAux#” command, where # corresponds with the connector number that the SUNA is connected to (GetVAux2 will measure voltage on J2, which contains voltage channels 4 and 5)
         • 9plus: Connect the system to SeaSave and add the voltage channel connected to the SUNA V2 to a “Scrolled Display window” (Display→Add New Scrolled Display Window). The voltage from the SUNA V2 will appear in this display once real-time acquisition starts.

1. 1. 8. Record this as the low voltage, “V_min”
      9. Remove the DI water, and fill the flow path with a high-nitrate standard. This must be higher than 15 μM to max out the signal.
      10. Repeat step 7 to capture the voltage again. Log the voltage as “V_max”.

Adjusting the DAC Range For the Expected Nitrate Range

The SUNA V2 automatically scales the voltage output to the nitrate range encompassed by DAC_min and DAC_max. After obtaining the V_min and V_max, adjust the DAC_min and DAC_max to the expected nitrate range of the deployment environment for optimal resolution.

If the maximum range of nitrate in the deployment environment is completely unknown and cannot be estimated, use the 2000 μM maximum detection range of the SUNA V2 until the nitrate range can be established. This reduces resolution but ensures the full range of nitrate is sampled. For SUNA V2s with the optional 2 GB memory, users can upload high-resolution data after the deployment.

1. 1. 1. Reconnect the SUNA V2 to UCI.
      2. Navigate to SUNA Settings → Advanced
      3. Set the DAC minimum value to 0, and the DAC maximum value to your expected maximum.
      4. Upload the settings to the SUNA V2. The maximum concentration in μM will now correspond with the maximum voltage V_max.

WARNING: Scaling DAC_max too low will prevent the SUNA V2 from sampling the full nitrate range. An extremely low DAC_max may cause the SUNA V2 to output maximum voltage upon entering seawater.

Processing Data

Once the Voltage and DAC ranges are established, the voltage output from the SUNA V2 to the CTD should reflect accurate nitrate concentrations. The user can then convert these voltages to nitrate in real-time with the formula below:

25plus Dual Serial/Analog cable

Sea-Bird Scientific offers a Y-cable (PN 173402) designed to simultaneously connect the SUNA V2 analog output to a voltage channel and the RS-232 output to one of the 25plus serial input channels.

This cable is useful for applications that benefit from lower resolution real-time data, but need to retain the high resolution data for post-processing. For instance, the CTD operator can determine areas of interest for water sampling with the real-time analog data, then compare the water samples to the high-resolution data from the serial output after the profile.