Service and Calibration
The response of the Siemens analyser is non linear to CO2 mole fraction and so a quadratic calibration curve of the form: y = ax2 + bx + c is adopted. To define the parameters of the calibration curve (a, b and c) three calibration standards were used for the period 2007 – 2014 and four standards are used from mid 2014 onwards. A calibration is performed every 47 hours and the calibration cylinders’ concentrations are on the WMO defined scale for CO2: the NOAA X2007 scale.
A further calibration cylinder is run much more frequently than the multi point calibrations and is used to correct for baseline drift in the CO2 calibration curve. Baseline drift is typically much more variable than span or non-linearity drift, and is usually caused by external factors such as ambient temperature variability. The cylinder is run immediately after the three point calibration to establish an initial baseline response and it is subsequently run every 3-4 hours to redefine the baseline analyser response; any change from the initial baseline response is assumed to be owing to analyser drift and the magnitude of this change is applied as a correction to the c-term of the calibration curve for CO2 (the c-term or intercept is most susceptible to any drift in detector response).
The response of the Oxzilla analyser is highly linear to O2 mole fraction and so a calibration curve of the form: y = mx + c is adopted. Three calibration standards were used for the period 2007 – 2014 and four standards are used from mid 2014 onwards. A calibration is performed every 47 hours and the calibration cylinders’ concentrations are on the WMO defined scale for O2: the SIO S2 scale.
A target cylinder of known concentration is run every 8-9 hours to check the performance of the system.
All cylinders are stored horizontally in a thermally isolated box as this has been shown to minimise fractionation of gases within the cylinder that can adversely impact the precision of the measurements.