Raw signal of the oxygen sensor

For trouble shooting purposes of the polarographic oxygen sensor (POS), for choosing an appropriate gain setting (default is Gain 2), but also to understand how the O2k actually works, it is helpful to observe the raw signal (on-line or off-line).

Observing the Raw Signal

Depending on your DatLab version, a predefined layout called „B Trouble shooting“ is in the „Graph layout“ menu. If not, you can easily generate this Graph layout. To do so, take the following steps in DatLab [MiPNet12.07]:

• From the main menu select “Layout” menu and choose „1 Calibration Exp. G3-Temp“
• From the main menu select „Graph“ / „Select Plots“, by default you will see the „plots“ tab from the drop down menus for the oxygen signals. Select „O2 raw signal“ instead of „O2 concentration“ for both chambers.
• Select the „scaling“ tabs: adjust the scaling for the oxygen signal to 0 – 10 Volt (Start at 0, Range 10)
• At the bottom of the window insert a name for the new layout („B Trouble Shooting“); press „Save Graph Layout“.

Remember to select the period of the experiment your are interested in or select “Graph” / “Autoscale Time Axis” to view the entire experiment. The maximum signal is 10 volt. Above 9.99 V, the amplifier is in the saturation range, the apparent signal will remain constant at this value, and the flux will appear to be zero. The raw signal can be modified by choosing an appropriate gain setting. We recommend a gain of 2 as a default for experiments at and below air saturation, to avoid reaching the upper 10 volt limit without noticing. Typical recorded voltages at air saturation, in pure water or experimental media, should be

• at Gain 8: offscale ( “9.99V”)
• at Gain 4 : ca. 8 - 10 V (do not use at air saturation, especially if your lab is situated at or near sea level)
• at Gain 2: ca. 4 - 5 V
• at Gain 1: ca. 2 V

The actual value of the raw signal at air saturation (during calibration) depends on experimental temperature, barometric pressure, and the sensitivity of the individual POS. At or near sea level higher values are recorded than at higher altitudes. The raw signal is converted to oxygen concentration [µM or nmol/ml] or partial oxygen pressure [kPa] by calibration of the POS [MiPNet12.08]. Each gain requires a separate calibration of the oxygen signal. Therefore, after changing the gain you should not only perform a calibration at air saturation (as always before an experiment), but also a new zero oxygen calibration.