During a protocol run, when the pipettes move to labware, you might notice:
- One pipette goes to the correct place.
- The other pipette is up to 1-2 mm off, and that error is consistent throughout the protocol.
This is due to a limitation in how calibration currently works on the OT-2.
Why this happens
Labware is only calibrated with one pipette. During a protocol run, the OT-2 moves that pipette to the correct place, just as you calibrated it.
To move the other pipette to the same position, the OT-2 relies on knowing the exact offset between the two pipettes' tips. It measures this offset during pipette calibration. (See How calibration works on the OT-2.)
In practice, though, that measurement is imperfect. The OT-2's idea of the offset between the two pipettes' tips won't exactly match reality.
So, whichever pipette wasn't used to calibrate labware can be slightly off-position when it moves to that labware.
How to work around it, for now
The exact error will change every time you do pipette calibration. If you reupload your protocol and redo pipette calibration, the error might be smaller.
If the new pipette calibration seems to do better, you can keep using it for subsequent protocol runs, instead of overwriting it with a new calibration each time. The next time you upload a protocol, try skipping pipette calibration.
Because every tip is slightly different, there will always be some error.
However, we're working on mitigating this problem in an upcoming software update. We plan to change the way that calibration works, to remove some sources of inaccuracy and imprecision.
Until then, if you need more precise pipette positioning for your protocol, reach out to Opentrons Support. Depending on the details, we might be able to help.
Let us know:
- How far off the bad pipette's positioning is from the good pipette's.
- How close you need the positioning to be.
- How often you need to swap pipettes on your OT-2.
- Whether you're comfortable with Python.
- Whether you're comfortable with the Linux command line.