Fig 1.
Spherical falling film equilibrator diagram.
Water is pumped through the roof of the chamber and directly onto the topmost point of sphere. The water forms a falling film that coats the sphere and creates a gas exchange surface. Water releases from the sphere at the water line below, where it drains out of the chamber. Air enters the lower part of the chamber, flows upward, counter-current to the water flow, and exits out the roof of the chamber. Air is circulated in a closed loop from the EQL to a sensor and back into the bottom of the chamber.
Table 1.
Dimensions of four spherical equilibrators tested.
Fig 2.
Performance comparison of a single 10-in diameter spherical equilibrator with an equilibrator consisting of two stacked 10-in diameter spheres in a large chamber.
See Table 2 for EQL specifications. Note: see Supporting Information (S1–S11 Tables) for all data relevant to Figs 2–12.
Fig 3.
Performance comparison of a single 10-in diameter spherical equilibrator with a two stacked 10-in diameter spheres in a large chamber.
Water hose and sample air lines to air pump and IRGA circuit were rapidly switched manually between EQLs for comparison. Water tank was enriched with compressed CO2 at min 20. The single 10-in EQL was swapped for the double 10-in EQL (containing ambient room air) at min 94 –note 2-min dead time prior to sensor response. See Table 2 for EQL specifications.
Fig 4.
Performance comparison of 10-in and 8-in diameter spherical equilibrators.
Common water was pumped continuously to both EQLs throughout the 6-day period and valves connecting sample lines to air pump and IRGA circuit were switched periodically for instantaneous comparisons. Changes in slope are due to increased bubbling of test tank with CO2-stripped air. Tank was enriched with compressed CO2 at minute 4457. See Table 2 for EQL specifications.
Fig 5.
Performance comparison of 8-in, 3.7-in, and 6-in diameter spherical equilibrators.
Common water was pumped continuously to selected pairs of EQLs throughout the 4-day period and valves connecting sample lines to air pump and IRGA circuit were switched periodically for instantaneous comparisons. Changes in slope are due to increased bubbling of test tank with CO2-stripped air. Tank was enriched with compressed CO2 at minute 1500. Note: 3.7-in EQL was replaced with 6-in EQL at minute 1304, as indicated by small spike caused by brief non-equilibrium starting condition of 6-in EQL. See Table 2 for EQL specifications.
Fig 6.
Accuracy test of 3.7-in EQL using 7579 ppmv ± 1% (nominal CO2 concentration).
Test #1 failed to reach the 7579 ppmv target, reaching an asymptote Accuracy test of EQL using 7579 ppmv ± 1% (nominal CO2 concentration). Once equilibration initially reached within 1% of standard gas concentration, the average xCO2 = 7578 ± 12.2 (mean ± 1 SD, n = 29).
Fig 7.
Accuracy test of EQL using 1047 ppmv ± 1% standard (nominal CO2 concentration).
Once equilibration initially reached within 1% of standard, the average xCO2 = 1034 ± 13.6 (mean ± 1 SD, n = 21).
Fig 8.
Results from three low to high concentration equilibration trials.
Multiples of τ (mins) correspond to 63.2%, 86.5%, 95.0%, 99.3% of maximum values measured. Arrows indicate mean τ values for all three trials (see Table 1 for summary of mean and standard deviation values). Concentrations are plotted on a log scale to better visualize low and high ranges.
Fig 9.
Results from five high to low concentration equilibration trials.
Multiples of τ (mins) correspond to 36.8%, 13.5%, 5.0%, 0.7% of maximum values measured. Arrows indicate mean τ values for all five trials (see Table 1 for summary of mean and standard deviation values). Concentrations are plotted on a log scale to better visualize low and high ranges.
Table 2.
Summary of time constant, τ, for rising and falling equilibration.
Table 3.
Response times for measuring pCO2 in water by active air-water equilibration (after Webb et al. [13]).
Fig 10.
Side-by-side field measurements of multi-chamber thin film open loop and 8-in spherical falling film closed loop EQLs.
Measurements were made autonomously at 1-min intervals February 14–16, 2017 in the Rhode River from the SERC pier, Edgewater, MD. The multi-chamber thin film EQL was programmed to sample atmospheric air for 15 mins at 6-hr intervals. The spherical falling film EQL consistently measured lower values than the multi-chamber thin film EQL throughout the time period (mean difference ± SD = -20.3 ± 20.2 ppmv, n = 2707, when readings are excluded during atmospheric readings).
Fig 11.
Side-by-side field measurements of multi-chamber thin film open loop and 8-in spherical falling film closed loop EQLs.
Measurements were made autonomously at 1-min intervals February 27–28, 2017 in the tidal creek that drains the Kirkpatrick tidal marsh in the Rhode River, Edgewater, MD. This system is tidally driven, and CO2-rich water is drained during low tides. The spherical falling film EQL consistently measured substantially higher xCO2 values than the multi-chamber thin film EQL during low tides, and slightly lower values during high tide (mean difference ± SD = 662.7 ± 427.7 ppmv, n = 1106).
Fig 12.
Time series (1-min intervals) of xCO2 from the SERC pier on the Rhode River, MD.
The EQL was connected to a Senseair K30 non-dispersive infrared sensor and detects the strong biologically-driven diel cycling that is typical in warm months at this location. Gray vertical bars indicate nighttime.