Calibration of YSI EXO2 Water Quality Sonde

PROTOCOL FOR CALIBRATING YSI EXO2 WATER QUALITY SONDE

INTRODUCTION

This protocol outlines basic calibration methods for a YSI EXO2 multiparameter water quality sonde, specifically dissolved oxygen, conductivity, and pH calibration. Additional calibration of other sensors (e.g., Chlorophyll RFU, fDOM, Turbidity, etc.) is not detailed in this protocol.

SAFETY

Standard operating procedures are available for pH buffer 4, pH buffer 7, pH buffer 10, and conductivity standard.

MATERIALS

  • YSI EXO2 Multiparameter Water Quality Sonde
  • Optional: YSI Handheld unit
  • pH Buffer solutions (pH = 4, 7, 10)
  • Conductivity calibration solution (Cond = 50,000 ms/cm)
  • Bucket of filtered water (collected from Lake Washington or other natural body of water) allowed to come to atmospheric equilibrium (See protocol).
  • Tap H2O – lab sink is okay.

PRE-CALIBRATION

  • Connect YSI to computer (via Bluetooth) or handheld device (via Bluetooth or connecting cord).
  • If computer, open KOR-EXO software. Click on the “Scan for Bluetooth Devices” button in the bottom left side of the home page. If handheld device, turn on the device, and search for either Bluetooth or wired devices.
  • Once sonde is connected to the KOR-EXO software, unscrew calibration cup, exposing sensors. The protective cage surrounding the sensors should always stay in place, unless replacing a specific sensor probe.

NOTE: pH and Conductivity should be calibrated weekly during regular field use. Dissolved Oxygen should be calibrated daily during regular field use.

DISSOLVED OXYGEN CALIBRATION 

Note: Detailed below are two methods of calibrating the Optical Dissolved Oxygen sensor. You need only choose one method for calibration.

METHOD 1: Air Saturated with Water

(Preferred method if you’re in the field)

  1. Rinse sensor and cup thoroughly with clean H2
  2. Fill calibration cup with ~1 inch of water. Make sure sensors are NOT submerged. Do NOT tighten down calibration cup so that the pressure inside the cup matches the atmosphere.
  3. Let sensor sit for ~10 minutes to allow air inside chamber to become completely saturated with water vapor.
  4. Click on “Calibrations” and select “Optical DO” and “ODO sat.”
  5. Choose 1-point calibration and air-saturated water.
  6. Next, you will need to enter the current barometric pressure.
  7. Go to the NOAA weather page (noaa.gov) for Seattle, WA. NOTE: NOAA reports pressure standardized to sea-level, which means that unless you will be sampling at sea-level, their reported value is likely higher than the absolute pressure at your field site.
  8. Convert NOAA’s standardized pressure reading, reported in millibars (mb), to inHg.  1inHg = 33.8mb
  9. You will need to subtract 0.22inHg from their value to compensate for the elevation in Seattle (200ft). If you are sampling at a different elevation, you will need to subtract by a different value, relative to the elevation of your field site. You can estimate that value at your field site’s elevation using the table found on pages 6-7 at https://novalynx.com/manuals/bp-elevation-correction-tables.pdf.
  10. Since the KOR-EXO software requires Absolute Pressure (mmHg), convert your standardized pressure (from NOAA) to absolute pressure using the equation below. Here, 0.22 is the correction value for Seattle’s elevation, and 25.4 is used to convert between inHg and mmHg:  Absolute Pressure (mmHg) = (inHg – 0.22) * 25.4
  11. Enter the absolute pressure into KOR-EXO. This value will likely be 750-760 mmHg for Seattle but will be lower at a higher elevation.
  12. Allow sensor to stabilize and click “Apply.”
  13. Click “OK” or “Exit” after calibration is complete. You may dump the 1-inch of water down the drain.

METHOD 2: Water Saturated with Air

(Preferred method if you’re in the lab)

  1. Prior to calibration with this method, you will need to allow a bucket of water to come to atmospheric equilibrium. For instructions on how to prepare this bucket of water, please see the “Bucket to Atmospheric Equilibrium” protocol. The water in the bucket does notneed to be filtered lake water if it is only used for sonde DO calibration; clean tap H2O will suffice.
  2. Rinse sensor and cup thoroughly with clean H2
  3. Click on “Calibrations” and select “Optical DO” and “Percent Saturation.”
  4. Choose 1-point calibration and air-saturated water.
  5. Gently place sonde in bucket of water and begin calibration.
  6. Allow sensor to stabilize and click “Apply.” The complete saturation value should be around 100%, assuming the bucket achieved true atmospheric equilibrium.
  7. Click “OK” or “Exit” after calibration is complete. You may dump the bucket of water down the drain.

 CONDUCTIVITY CALIBRATION 

  1. Rinse sensor and cup thoroughly with clean H2
  2. Gently shake sensor and cup to dry out.
  3. Fill calibration cup to the first line with calibration solution (50,000 ms/cm). Concentration of conductivity standard may be different depending on your intended field site.
  4. Click on “Calibrations” and select “Conductivity.”
  5. Select a single point calibration and enter the correct calibration solution concentration.
  6. Click “Start Cal.”
  7. Allow the sensor to stabilize. You may agitate the cup the make sure no air bubbles remain on the sensor.
  8. Once reading is stable, a green label will appear on screen. Click “Apply.” Click “Complete.”
  9. Pour conductivity solution back into storage container.

pH CALIBRATION

  1. Rinse sensor and cup thoroughly with clean H2
  2. Select pH buffer solutions (two- or three-point calibration) that bracket the expected pH range of your field site: pH 7 and 10 for mesotrophic to eutrophic lakes, rivers, and streams; pH 4 and 7 for dystrophic or acidic lakes, rivers, and streams. If performing a two-point calibration, always begin with the pH 7 buffer solution.
  3. Fill calibration cup to the first line with the first pH buffer solution.
  4. Click on “Calibrations” and select “pH.” Select two- or three-point calibration and enter the correct pH buffers.
  5. Click “Start Cal.”
  6. Allow sensor to stabilize in first buffer. You may agitate the cup to make sure no air bubbles remain on the sensor.
  7. Once reading is stable, a green label will appear on screen. Click “Apply.” Click “Next.”
  8. The software will tell you to move to the second buffer. Pour the first buffer back into the storage container. Rinse sensors and calibration cup thoroughly with clean H2
  9. Pour second buffer in the calibration cup. Allow sensor to stabilize and click “Apply.”
  10. Repeat for third buffer solution if necessary. Pour pH buffer solutions back into respective storage containers.

Notes about Using EXO2 Sonde

  • When sonde is not in active use:
    • For short-term storage: Store sonde with calibration cup full of tap water (filled to first line on calibration cup). Tighten calibration cup to prevent spills.
    • For long-term storage: Remove pH probe, and store in pH=4 buffer solution. Store stoned with calibration cup full of tap water (filled to first line on calibration cup). Tighten calibratoinc up to prevent spills.
  • ALWAYS lay sonde down on its side when it is not in active use. Standing on its end (vertically), the sonde is likely to fall over, which may cause irreparable damage to the sensors.

Author: Julia Hart

Date: 25 July 2018

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