Oxygen sensor test
The Sensor test is The Main Tool for Troubleshooting. It should be done
- whenever a problem is encountered
- when the performance of the instrument is doubtful
- from time to time for Quality Assurance
- after a sensor service or after applying new membranes
- during the troubleshooting procedure by switching components a sensor service is done after each switching step.
After a sensor service or after changing membranes the oxygraph should run (i.e. the oxygraph, stirring, etc is switched on) over night (chambers with EtOH/water or just water) before a sensor test is performed or other experiments are started. This ensures proper stabilization of the signal.
It is of advantage to follow the sensor test by observing the [[Raw Signal] = Layout "Z Troubleshooting".
Alternatively "Layout01" can be used.
How to do a Sensor Test:
- fill O2k-Chamber with water
- set T = 37 °C
- set gain = 2
- set stirring = 750 rpm
- wait for thermal equilibration (use Layout01, watch for Peltier power % to get stable)
- with open chamber wait for stable signal
- air calibration (open chamber, stable signal = zero flux)
- closed chamber: stable, non-zero flux
- stirring test (switch off stirrer, wait, switch on stirrer, wait)
- change to Layout "Z-Troubleshooting" (if available); alternatively remain in Layout01
- zero calibration (dissolve a few crumbs of dithionite (= zero solution powder) in a drop of water, inject immediately 50 µl into the closed chamber, wait until most of the drop in oxygen concentration or raw voltage has occurred and then inject another portion to make sure that zero oxygen has really been reached. No stepwise drop should occur after the second injection otherwise oxygen has not been zero after the first injection. A second important criterion is to observe the (red) flux signal. At zero oxygen plus an excess of dithionite that consumes any oxygen diffusing into the solution the flux should be zero. However, especially with a rather slow sensor it may take some time to reach zero flux. The flux will in this case be positive and decreasing. It is not necessary in this case to wait for a complete zero flux for a simple sensor test. A negative flux on the other side strongly indicates that zero oxygen has not bee reached jet (all the ditionite has been consumed by oxygen, therefore the oxygen back diffusion is visible as a negative flux). In this case another injection of dithionite is necessary.
A DatLab demo file showing a typical sensor test (from a working system) can be downloaded with this link from the OROBOROS Homepage.
Evaluation of a Sensor Test
Most parameters are best viewed by observing the Raw Signal. It will be helpful to compare the obtained test with the test of a functioning system, e.g from the DatLab demo file.
Informations gained by the sensor test include:
Signal stability at air saturation
Response time of the sensor
Flux at closed chamber near to air saturation
Raw signal at zero oxygen (zero current)
Signal stability at zero oxygen
