Check of Equilibrator Temperature Sensors

In marine applications, fCO₂ results are generally reported at in situ seawater temperature (T_{in situ}). As the seawater temperature in the equilibrator during measurement (T_eq) usually deviates from T_{in situ}, a temperature correction needs to be applied. The size of this correction obviously depends on the choice of the parameterization and the size of the temperature deviation itself. As the fCO₂ strongly varies with temperature, measurements of T_{in situ} and T_eq have to be made rather accurately. An error of 0.1°C in the resulting temperature deviation (T_eq-T_{in situ}) is equivalent to an error of about 0.4 % or 1.5 µatm in fCO₂ (at 350 µatm).

During the exercise, T_{in situ} was measured with a CTD installed at the seawater intake in the bottom plate of the "moon pool". The CTD had been calibrated just before the exercise to an accuracy of ±0.05°C. These in situ temperature readings were used in the calculation of all fCO₂ data. In order to exclude possible errors contributed by inaccurate measurements of T_eq, all groups had their equilibrator temperature probes referenced against a recently calibrated platinum resistance thermometer (Pt-100) provided by WHOI. For every comparison, equilibrator probe and reference probe were kept together in a water bath until the readings had stabilized. In most cases this was done at three temperatures between 0°C and up to 30°C. Based on the deviation from the reference, an individual linear correction was calculated for every system and applied to its measurements of T_eq. In one case (lab "F"), the temperature probe could not be removed easily from the water bath surrounding the equilibrator and the reference probe had to be installed next to it in the bath thus yielding only a single measurement which was then treated as a uniform offset.

Figure 5 shows measured deviations and the resulting correction lines of only those temperature sensors which were used in the calculation of final fCO₂ values (some systems feature up to three equilibrator temperature sensors). The observed deviations are roughly between -0.5°C and +0.1°C with a clear tendency towards negative values and a negative slope of the linear correction line. If this inconsistency of the temperature measurements is not accounted for, differences of up to 2% or about 7 µatm (at 350 µatm) in the final fCO₂ values are caused as an artifact entirely the result of inaccurate temperature measurements.

Even though the CTD as well as the WHOI reference thermometer may themselves have been affected by some degree of miscalibration, the present procedure of referencing all measurements to these two temperature sources removes the incompatibility of all temperature readings to better than 0.1°C or 1.5 µatm. It should be pointed out that the observed deviations of up to 0.5°C are clearly above a tolerable level. Temperature readings have to be carried out with an accuracy of at least 0.1°C. Ideally they should be checked for consistency with the temperature probe used to measure in situ temperature.