Optical density (OD) is one of the most integral parts of continuous culture. While chemostats do not innately rely on OD, it is still a useful metric for your evolution. Meanwhile, turbidostats and growth curves heavily rely on OD to function.
However, OD is the most easily disrupted parts of eVOLVER hardware. This guide serves as an initial list of things to check when troubleshooting OD.
Disclaimer: You may find that you are still not getting "good" OD readings after trying solutions in this guide. We hope that you will search the because others may have had your same issue.
After trying a fix, before attempting a whole new calibration, prepare a couple of standards in the OD range that you care about and check if you see a difference between them in the raw OD values (in the GUI Setup page or by starting an experiment and going to the data folder).
For more in-depth information about the OD hardware see .
Most Common Problems
Variability in OD Sensing Range Between Vials
If you already have a version of the 3D printed tube holder with defined OD LED and photodiode distances:
Use the upgrade to double check your photodiodes and LEDs are correctly placed!
Noisy OD Data
Stir bars jumping around during OD readings
Decrease stir speed
Incorrect OD
ie eVOLVER OD is offset from the cuvette or plate reader measured value.
Bad
Especially because you did not wait for temperatures of the vials to equilibriate
Poor quality OD calibration
No Change in OD
Photodiode or IR LED not working
Replacement
A failed photodiode will show a raw OD value close to 64,000
Other Considerations
Pre-Calibration Considerations
od_led value in conf.yml file (PWM value)
Experiment Considerations
Photodiode Temperature - make sure temperature is equilibrated before calibration and before start of experiment
eVOLVER recently turned on - causes rapid change in values
-
Non-standard changes
Vial holder reflectiveness - ie white vs black vs metallic
Sometimes a given OD photodiode or LED can be variable as they are cheap parts. Swap both out for new ones. Part numbers .
Try
Try different ADC resistor packs
eVOLVERs come with a variety of these
Ask on the forum for help
Check that your vial construction is not variable
You are near the edge of your calibrated OD range
is off enough to make your calibration fail
You can check your OD calibration using the script
Different temperatures require different OD calibrations
A failed IR LED will most likely show a raw OD value much closer to your other vials
Check that they have the long (positive) lead in the correct positive terminal
ADC board or motherboard broken
Can happen after vial overflow
See motherboard and
Use the splash guard that comes with your eVOLVER
Or using black acrylic
You may need to recalibrate if you calibrated with ambient light
Needles or other structures coming below 10mL line (close to OD PD etc)
Vial volume - a volume of 10mL will totally change OD compared to 30
Stirring with low vial volume - this will give noisy readings as the funnel created by stirring fluctuates and is more and less reflective
