Optical Density (OD) Readings

Overview

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.

For more in-depth information about the OD hardware see here.

Most Common Problems

Variability in OD Sensing Range Between Vials

Vial 1 (left) shows a smaller range than vial 2 (right). However, vial 2 is still unable to accurately differentiate values under OD 0.5.
  1. If you already have a version of the 3D printed tube holder with defined OD LED and photodiode distances:

    1. Use the upgrade guide to double check your photodiodes and LEDs are correctly placed!

  2. Sometimes a given OD photodiode or LED can be variable as they are cheap parts. Swap both out for new ones. Part numbers here.

  3. Try different ADC resistor packs

    1. eVOLVERs come with a variety of these

    2. Ask on the forum for help

  4. Check that your vial construction is not variable

Noisy OD Data

  1. Stir bars jumping around during OD readings

    1. Decrease stir speed

  2. You are near the edge of your calibrated OD range

  3. OD blank is off enough to make your calibration fail

Incorrect OD

ie eVOLVER OD is offset from the cuvette or plate reader measured value.

  1. Bad OD blank

    1. Especially because you did not wait for temperatures of the vials to equilibriate

  2. Poor quality OD calibration

    1. You can check your OD calibration using the manual calibration script

    2. Different temperatures require different OD calibrations

No Change in OD

  1. Photodiode or IR LED not working

    1. Replacement guide

    2. A failed photodiode will show a raw OD value close to 64,000

    3. A failed IR LED will most likely show a raw OD value much closer to your other vials

    4. Check that they have the long (positive) lead in the correct positive terminal

  2. ADC board or motherboard broken

    1. Can happen after vial overflow

    2. See motherboard hardware and troubleshooting

Other Considerations

Pre-Calibration Considerations

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

  • 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

Non-standard changes

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