How deeply the photodiode and IR LED are pushed in to the vial greatly affects OD readings


After building and rebuilding eVOLVER vials several times, I noticed a great deal of variability in OD measurements. I thought that pushing the IR LED and photodiode (PD) as far as possible into the vial would give the vials an equal starting place. Instead I found that many vials were now unable to read ODs above ~0.1 - 0.2. However, I had failed to push the LED in all the way on Vial 0, which gave good results:

Anecdotally, myself and others also have the problem of accidentally moving the PD or LED while using the eVOLVER. This voids the OD calibration and leads to puzzling results.

Experimenting with Locations of PD and LED

I altered the locations for the PD and LED and recorded the resulting OD for a single vial.

eVOLVER OD Settings:
I’ve been able to measure ODs between 0 and 4 using these hardware settings, but the same results are true for various settings.

  • PD angle = 90 degrees
  • PD resistor pack = 500KOhm
  • od_led Setting = 4095

Experimental Conditions

  • Plate Reader OD = Culture OD was measured on a plate reader to be 1.65 and allowed to come to 37C before measurements.
  • OD Post-Calibration = measured OD after I previously had calibrated and before I started changing PD and LED positions
  • In = LED or PD was pushed in to the vial as far as possible
  • Middle = I attempted to place the LED or PD as close to the center of the vial as possible
  • Out = LED or PD was pulled out as far as possible while still remaining in the vial


  • Out/Out = much lower measured OD
  • In/In = measured OD exceeded the range of the calibration (OD was off the charts)
  • Middle/Middle = I wasn’t able to get the PD and LED back into their original positions


I designed a new vial with defined locations for the PD and LED. They’re pressure fit in there and can only be inserted so far. Will be uploading to the GitHub once I characterize it. Hopefully this will solve the problem.


Thanks for documenting this Nate. This is something we’ve observed in the past but characterizing it is super helpful!


1 Like