The LI-7700 is the only open path instrument suitable for field measurements of CH4 Eddy Covariance. Open path instruments offer several advantages, including in situ measurement of signal without any alteration, minimized signal attenuation, and low power consumption. The exposed optical path, however, has its own set of challenges, including signal decay as a result of the deposition of foreign objects in the optical path and on the associated optical components. These may include dust, dirt, debris, pollen, salt spray, hard water deposits and insects. In this tech tip we highlight the techniques incorporated in the LI-7700 specifically aimed at preventing and/or correcting optical signal degradation.
The LI-7700 reports Residual Signal Strength Indicator (RSSI), a value that indicates the available optical power for measurements. The RSSI ranges from 0% to 100%, where a value higher than 90% RSSI indicates a signal with exceptional quality. The CH4 measurements made by the LI-7700 are consistent for a wide range of RSSI values. However, as the RSSI value decreases due to environmental factors, the signal-to-noise ratio of the measurement also decreases. The LI-7700 design features three methods of ensuring good RSSI values; the automated washer for the bottom mirror, configurable heaters for both top and bottom mirrors, and scratch-resistant durable mirrors for manual cleaning. Each of these components should be used with the aim of preventing signal degradation rather than recovering from a degraded signal.
The LI-7700 automated washer accessory uses a cleaning solution to spray and spin the bottom mirror at specified times when the laser signal strength drops below a user specified threshold. Cleaning frequency and spray duration need to be set depending on local conditions and the availability of power to run the motor and pump. However, there are some recommendations that can help reduce data loss. We recommend setting a schedule to spray at least twice a day, especially during the early morning hours, with spray duration of at least 15 seconds and a signal threshold of higher than 80%. We recommend pure cleaning solutions such as common windshield washer fluid or a water/alcohol mixture. If water is used instead of a cleaning solution, it should be de-ionized to avoid hard water deposits on the mirrors. For remote sites a larger reservoir can be connected to reduce the frequency of refilling.
Figure 1. (Left) Abundant power washer and spin motor setup; (Right) limited power washer and spin motor setup for early morning hours only.
Both top and bottom mirrors of the LI-7700 can be heated to prevent condensation on the mirrors. A total of 8 Watts of power can be allocated for the mirror heaters; however, that level of power is rarely needed. The bottom mirror heating can be scheduled to run when RSSI falls below a user specified threshold during certain times of the day. As a general minimum guideline the bottom mirror should be provided with at least 0.5 Watts of heating (6.25% of the allocated power) to prevent condensation. If total power consumption is not a concern, the heating should be held constant. If power is limited, schedule the heating during the early morning hours with a signal threshold of higher than 80%.
The heating of the top mirror can be scheduled and configured to keep the mirror above ambient temperature for a user specified offset. For the top mirror approximately 2.5 Watts of heating is sufficient to prevent condensation in most environments. These settings translate to 31.5% of the available power setting. The offset above ambient temperature is different from site to site but a minimum of 1 °C should be considered. Like the bottom mirror, if total power consumption is not a big concern, the heating should be held constant. If power is limited, schedule the heating during the early morning hours.
Figure 2. (Left) Abundant power constant heating; (Right) limited power heating for early morning hours.
For difficult-to-remove deposits, manual cleaning may be necessary for both top and bottom mirrors. The LI-7700 mirrors are very scratch-resistant, but it is best to treat them with care when cleaning. Wipe off dust and dirt with a damp cloth, but be sure to swipe them with a clean cloth each time; minerals that are removed can act as abrasives and damage the glass if they are swiped across the mirrors a second time. If a stronger solution is required, apply it to surrounding materials first to make sure it will not damage the mirror. Lay a paper towel on the mirror and soak it in solution for a few minutes before wiping away hard deposits. A variety of cleaning solutions may be applied, depending on the type of deposits; examples include vinegar, mild soap, commercial glass cleaner (such as Windex®), dilute hydrochloric acid, or commercial water spot remover (such as LIME-A-WAY®). Avoid hydrofluoric acid, which etches glass. Check the (RSSI) before and after cleaning to verify that the mirrors are sufficiently clean.