Maintenance

Some routine attention will ensure that the LI-720 provides dependable performance over long time periods, and will help you get more complete, continuous datasets.

Basic sensor checkup

You can conduct a basic check just by inspecting the LI-720. Look for the following:

  • Inspect the vertical sonic transducers to be sure they are not covered in dirt, bird droppings, or other deposits. Clean them with a soft, damp cloth. Use a mild detergent if needed.
  • Inspect the optics for dirt, bird droppings, or other deposits. Check the RSSI (see Checking and adjusting the signal strength). If the optics are dirty or the RSSI is low, clean the lenses with water and a soft, lint-free cloth. Use a mild detergent if needed.
  • Inspect the horizontal sonic anemometer components. Look for insect colonies or other foreign objects. Clear anything that might obstruct the airflow or ultrasonic signals.
  • Inspect the radiation shield that covers the air temperature and RH sensor. Clear any obstructions.
  • Wipe the quantum sensor with a soft lint-free cloth. Use mild detergent if needed. Do not use alcohols on the quantum sensor; alcohols can damage the diffuser.

Updating the firmware

Note: If going to a remote field site that lacks internet access, download the latest version of the CarbonWare application before going to the site. LI-720 firmware is included with the CarbonWare app.

Manual firmware updates are carried out with the USB cable (part number 99512-062). To update the firmware:

  1. Download the CarbonWare app to your computer.

    Firmware is included with the CarbonWare application here.

  2. Connect the USB cable between the LI-720 and computer.

    Connect the USB cable directly to the LI-720 and computer. Do not use a USB extension cable or device extension cable when operating the LI-720 from a 5 V USB power supply. Extension cables may result in voltage drops that affect both power and digital communication. Do not deploy the LI-720 while powered from 5 V USB power. The LI-720 requires 9 to 33 VDC for field operation.

    LI-720 to PC with the USB cable.

  3. In CarbonWare, connect with the LI-720 and click the LI-720 Firmware tab.

    The firmware running on the LI-720 and the latest version are both displayed. If an update is needed, click Update LI-720 to apply it.

  4. Keep the sensor powered and connected to the computer during the update.

Calibrations

User calibrations adjust the output from the sensor to ensure accurate measurements. Check the calibration in a controlled environment such as a lab or office. Do not attempt to calibrate while installed on a tower or tripod.

Before adjusting the zero and span, check the RSSI and clean the optics (see Checking and adjusting the signal strength). Wipe each lens with a soft lint-free cloth. Use mild dish soap and water to remove any stubborn deposits from the lenses. Wipe the lenses dry after cleaning.

Calibration gases

A user calibration is only as good as the gas. For the zero gas, use a tank of CO2-free dry air or a user-supplied pump and chemicals such as soda lime and magnesium perchlorate or Drierite® to scrub CO2 and H2O from the air. For the CO2 span, use a tank with a known CO2 concentration that is above the typical concentrations you will measure. For most applications, 450 to 500 ppm CO2 balanced in air is good enough. For the H2O span, use a dewpoint generator such as the LI-610 or Sable Systems DG-4 to create an air stream with a known, stable dewpoint.

Setting the H2O and CO2 zeros

Start with the zeros. A zero adjustment changes every measurement by a fixed amount.

  1. Clean the lenses with a soft, lint-free cloth.

    Use a mild detergent, if needed. Dry the lenses. Check the RSSI. If it is less than 95, clean the optics again.

  2. Insert the calibration fixture into the optical path, being cautious to not apply pressure to the upper and lower spheres - do not deform the structure.

    The hose barb on the calibration fixture is the inlet; install the fixture so the inlet is next to the bottom sphere. Loosen the set screw and insert the shroud into the optical path. Extend it so that each end surrounds the lens and is in gentle contact with the sphere. Tighten the set screw. The calibration fixture will not seal. It should have outward leaks from the top, bottom, and the vent hole.

  3. Connect the zero gas to the hosebarb on the LI-720 calibration shroud.

    The zero gas can be from a tank (Figure 8‑1) or created with scrubbers and a user-supplied pump (Figure 8‑2).

    Figure 8‑1. Calibration gas can be from a tank.
    Figure 8‑2. Zero gas can be created with scrub chemicals, a pump, and filter.

  4. Set the flow rate to approximately 1 liter per minute.

    Allow the gas to flow through the cell while observing the readings in the CarbonWare application. The application reports the slope and provides guidance about the stability for a zero or span using a 20-second running average.

    • Poor: Slope >1×10-4 (20 second running average).
    • Fair: Slope <1×10-4
    • Good: Slope <1×10-5

  5. When the interface reports that the reading is Good (allow up to 20 minutes), click Zero for each gas.

  6. Verify that the readings have changed to match the zero gases.

Setting the CO2 span

  1. Connect a tank of high-quality span gas that has a known CO2 concentration.

  2. Set the flow rate to about 1 liter per minute.

  3. Allow the gas to flow through the cell while observing the readings in the CarbonWare application.

  4. Enter the concentration of the span gas.

  5. When the interface reports that the stability is Good, click CO2 Span.

  6. Verify that the reading has changed to match the span gas.

Setting the H2O span

  1. Connect an air stream with a known dew point.

  2. Set the flow rate to about 1 liter per minute.

  3. Allow the air to flow through the cell while observing the readings in the CarbonWare application.

  4. Enter the Dewpoint Temperature (°C) of the air stream.

  5. When the stability indicator shows Good, click H2O Span.

Under the Manual page, verify that all intended calibrations have a new value and updated date.

Checking and adjusting the signal strength

Signal strength (RSSI for Residual Signal Strength Indicator) indicates the relative strength of optical system. RSSI decreases when the optics are dirty. You can adjust the RSSI scale so it has a maximum near 100 to simplify interpretation of the measurement. When RSSI is scaled to a maximum of 100, clean optics are indicated by an RSSI >95.

  1. Clean the optics.

    It is undesirable to rescale the RSSI while the optics are dirty. If low, clean the optics with mild detergent and a soft, lint-free cloth.

  2. When the lenses are clean, check the RSSI in the CarbonWare app.

    adjust RSSI.

  3. To rescale RSSI, enter a maximum and click Reset.

    For simplicity, RSSI should have a maximum near 100.

Replacing desiccant and scrub chemicals

A small packet of desiccant and CO2 scrub chemical is in each of the upper and lower gas analyzer housings. The packets keep the housing interior free of water vapor and CO2 gas. We recommend replacing them if the RH in the lower housing is >4%, as indicated in the variable RH_src (if using CarbonWare, see Adding variables to the display list). At an RH_src >4% for 10% of samples in a 30-minute reporting period, the LI-720 will report an issue in the diagnostics for that 30-minute flux result (see Table 7‑1).

Warning: Scrub packs enclose small quantities of Ascarite II and Magnesium Perchlorate. These chemicals may be hazardous. Wear goggles and skin protection (e.g., latex gloves). Do not breath dust or allow the chemicals to come into contact with your skin. Check local regulations before discarding the scrub packs. For MSDS, go to licor.com/support and search for MSDS.

To ensure freshness, scrub packs should be purchased when needed and installed promptly after delivery. Contact LI-COR or your sales representative for replacement scrub packs.

To replace the scrub packs:

  1. Power off the sensor, remove it from the tower and set it on a workbench.

  2. Remove the four screws (PH1 Phillips driver) and washers that secure upper the scrub packet cap.

    Collect all four screws and washers.

  3. Lift out the pack with a small flat screwdriver.

    Be sure the O-ring comes out with the scrub pack.

    use a screwdriver to remvoe the cap.

  4. Open the scrub pack only when you are ready to install it.

    Do not open scrub packs early, as doing so will reduce the scrubbing efficacy. Open them once you are ready to install them.

  5. Press the new scrub pack into place.

    The flat side should be toward the bottom of the sensor.

  6. Insert each screw and washer and tighten until snug.

  7. Repeat this with the lower scrub pack.

Replacing the air temperature and RH sensor

The external temperature and RH sensor is protected by the solar radiation shield. If the readings are implausible or -9999, replace the sensor. One spare is included with the spares kit (part number 99512-007). Additional sensors are available for purchase.

To replace the temperature and RH sensor:

  1. Power off the sensor, remove it from the tower and set it on a workbench.

  2. Remove the four screws (PH1 Phillips) and washers that secure the radiation shield.

  3. Remove the radiation shield and collect the O-ring.

  4. Grip the sensor and pull it free from the connector.

  5. Insert the new sensor.

    Handle it carefully. Avoid bending the component. Grip it by the end. Do not touch the element with bare fingers. The sensor element faces the front.

  6. Be sure the O-ring is on the white radiation shield, and align the slit in the foam pad with the sensor and gently maneuver the radiation shield into position.

  7. Tighten the screws to snug.

  8. Power on the LI-720 and check the temperature and RH values.

    If the readings are plausible, the replacement was successful.

Shipping the LI-720

Keep the foam and cardboard packaging materials so you can ship and store the LI-720 safely. Always use the box and both foam pads to protect the optics.

The LI-720 in the shipping box and foam.