Participating Underway fCO2 Systems
Throughout this report we present technical details as well as the results of the participating systems in a semi-anonymous fashion. The main reason for this is the fact that the results of the exercise cannot easily be extrapolated to the performance of any participating system in general. Strictly they are only representative for this single cruise. To avoid the erroneous association in the scientific community of the performance of a particular system during this exercise with the general performance of this system, we choose to report in this semi-anonymous fashion.
Seven underway fCO2 systems, all of which are based on NDIR detection of CO2, participated in this exercise. Most of these systems have received detailed descriptions in the literature, which can therefore be omitted here. Where such publications are available, they are reprinted at the end of hard copy report (Appendix B). For two systems, however, this is not the case. One is the fCO2 system of CSIRO which features a slightly smaller Weiss-type equilibrator and is otherwise quite similar to the other systems. The second one is a system that is manufactured commercially by a U.K. company (Challenger Oceanic, Haslemere, Surrey, U.K.). For details about the latter system, further information is available through the company's internet site http://www1.btwebworld.com/challengeroceanic/index.html
Whereas most of the underway fCO2 systems are similar in the general design and principle of measurement, they are considerably different in detail. For quick reference, the main features of all underway fCO2 systems are summarized in Table 2. All different equilibrator design principles (i.e., showerhead, bubbler, and thin film type) were represented by at least one system, with the majority being of the showerhead type. In most systems (except "D" and "F") these equilibrators are vented to the atmosphere and thus operated at ambient pressure. The volumes of water and air in the equilibrators cover a wide range from a few milliliters to 15 liters. This is also true for the flow rates of water (0-15 L/min) and air (0.17-0.8 L/min) through the equilibrators.
Table 2. Summary of main features of the underway fCO2 systems "A" through "G" that participated in the exercise
| Parameter | "A" | "B" | "C" | "D" | "E" | "F" | "G" |
|---|---|---|---|---|---|---|---|
| Equilibrator | |||||||
| Design | Showerhead | Bubbler | Showerhead | Thin film* | Showerhead | Bubbler | Showerhead |
| Total volume | 1000 mL | 1400 mL | 13.1 L | 119 mL | 11.0 L | 36 mL | 1200 mL |
| Water volume | 500 mL | 1000 mL | 2.3 L | 21 mL | 10.0 L | 18 mL | ~75 mL |
| Air volume | 500 mL | 400 mL | 10.8 L | 98 mL | 1.0 L | 18 mL | 500 mL |
| Water flow rate | 4-6 L/min | 2.0 L/min | 8.0 L/min | 2.0 L/min | 10-15 L/min | 0 L/min** | 1.2 L/min |
| Air flow rate | 0.2 L/min | 0.8 L/min | 0.5 L/min | 2.0 L/min | 0.5 L/min | 0.17 L/min | 0.18 L/min |
| Vented? | Yes | Yes | Yes | No*** | Yes | No | Yes |
| CO2 measurement | |||||||
| Method | NDIR | NDIR | NDIR | NDIR | NDIR | NDIR | NDIR |
| Wet/dry? | Wet | Wet | Dry | Dry | Dry | Dry | Wet |
| Analyzer calibration | |||||||
| No. of st. gases | 2 | 2 | 2 | 2 | 4 | 2**** | 2 |
| Zero gas? | No | Yes | No | No | No | Yes | No |
| Measurement cycle | |||||||
| Calibr. freq. | 6-8 h | 6 h | 6 h | 4-6 h | 1.5 h | 15 min | 2 h |
| Air meas. freq. | 6-8 h | 1 h | 6 h | 4-6 h | 0.5 hr | n/a | 7 min |
| Interrogat. interv. | 6 sec | 6 sec | 1 sec | 10 sec | 0.1 sec | 15 min | 0.33 sec |
| Averaging interv. | 1; 3 min | 1 min | 4 min | 5 min | 1 min | n/a | 1 sec |
| Data points/interv. | 10; 30 | 10 | 240 | 33 | 600 | 1 | 3 |
**Semicontinuous approach.
***Vented only every 20 min.
****Standard gas generator is initially calibrated using all six calibration gases; linearity checks are carried out for every sample with only two calibration gases.
A further distinction can be made in whether the sample gas is measured dry or wet. The traditional procedure is based on NDIR measurement of the dried sample gas ("D," "E," and "F"). However, in four systems ("A," "B," "C," and "G") the sample gas is not dried prior to NDIR measurement. This is feasible on the basis of the LI-6262 CO2/H2O gas analyzer (Li-Cor Inc., Lincoln, Nebraska, U.S.A.) which is a dual-channel instrument that simultaneously measures CO2 and H2O mole fractions of the sample gas and provides internal algorithms for correction of the diluting and pressure-broadening effects of water vapor on the CO2 measurement (McDermitt et al. 1993).
All NDIR instruments were calibrated with the NOAA/CMDL CO2 standards provided by the organizer (Table 1). Because of the individual calibration procedures, different numbers of gases (2 to 4) were required. Some systems also required a zero gas (nitrogen, purity 99.999%) for calibration purposes or as a reference gas.
Whereas underway fCO2 systems "A" through "E" and "G" are similar in that fCO2 is calculated from the CO2 mixing ratio in a gas phase that is in equilibrium with a constantly renewed seawater phase, system "F" is of a principally different design. Here, for every fCO2 measurement, five aliquots of a discrete seawater sample (semicontinuous mode) are equilibrated with five different standard gases bracketing the observed range of seawater fCO2. For each equilibration run, changes with time in the standard gas CO2 concentration as a result of CO2 exchange with the sample aliquot are recorded in terms of positive or negative deviations from the standard's initial CO2 concentration. If flow conditions during these five equilibration runs are kept identical, the heights of the resulting deviation peaks are proportional to the concentration difference between the carrier gas and a gas that is in equilibrium with the sample. If peak heights are plotted versus the initial xCO2 of the standard gases, the equilibrium xCO2 can be found where a linear regression to the five data points intersects the x-axis.
Participating groups were asked to operate their systems according to their typical operation profile (i.e., frequency of calibration and air measurements, interrogation, and averaging intervals, etc.). This strategy was chosen to ensure that all systems were operated in modes to which they have been optimized in the field and in which their operators have gained the highest confidence. The consequence, however, was quite different averaging and/or reporting intervals for the different groups. In particular, the averaging intervals between 1 and 5 minutes have certain implications that need to be taken into account when the data are being compared. This inherent discrepancy of the whole data set represents a certain limitation for the temporal resolution to which the interpretation can be extended. This is discussed in more detail in the results section.
