Raw signal of the oxygen sensor

For trouble shooting purposes, for choosing an approbate gain setting, but also to understand how the oxygraph actually works, it is helpful to directly observe the raw signal, not only in (in the on-line mode) as a momentary value in the O2k signal line but also as a plot.

Observing the Raw Signal

Depending on the delivery date of your datlab version you will either find a predefined layout called „B Trouble shooting“ in your „layout“ menu, or you can easily set up this view yourself as a layout. To set up this view manually: from the main menu select “Layout” menu 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“ is a good idea) 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 ten volt. If this current is reached the signal will stay there and therefore the flux will appear to be zero. The raw signal can be modified by choosing an approbate gain setting. We recommendate a gain of 2 for experiments at or near air saturation, to avoid reaching the 10 volt without noticing. Typical recorded voltages at air saturation, in pure water should be

at gain 8: offsale ( “9.99V”)
at gain 4 : ca. 8- 10 V (don't 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 depends on the sensitivity of the individual sensor and the barometric pressure. Thus, labs situated at or near sea level will record higher values than labs situated in higher altitudes and gain 4 is to high for them at air saturation. The raw signal is converted into a oxygen concentration by the calibration. 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 calibration. Oxygen signal that have been obtained after proper calibration are independent of the gain setting. Therefore oxygen concentrations (and fluxes) that have been recorded at different gain settings can of course be used together.