Reactor Volume Control

[Kate]

Hi Guys,

We’ve been using our ChiBio for a few weeks now with quite a lot of success. Would be great to have some clarification around volume control in the glass reactor tubes.

We’re using your recommended 3D printed lids which have a ‘tail’ on the outflow hole that sticks into the reactor tube to allow it to draw out liquid. Obviously when the liquid level falls below the end of the tail, it only pumps out air. We’re using the OD regulation programme so the pump rates are set internally and the volume seems to stay constant. As a test for something else we added some extra rubber tubing to the end of the tail, effectively extending how far it sticks into the glass tube. Somewhat to our surprise the volume of the liquid in this reactor was much lower – it remained constant but below the level of end of the rubber extension (so <<20ml).

My question is – did you design the 3D lids with a tail length that corresponds to 20ml volume? Can you think of a way to keep a more accurate track of the volume (beyond e.g. setting up the reactors on a scale)?

[harrison]

Hello Kate,
The system solely regulates volume based on the length of that tube. It will pump out more than it needs to (i.e. above the inflow rate) such that most of the time it is pumping air. Thus in practice it should pump down to the level of the tail and then stop (as then it is pulling air). If you extend the tail it should go and regulate to a lower volume. I am not sure how it could end up with volume less than the bottom of the tail (extension) in general. One special case this could happen is if yuou are pumping out when the liquid is being stirred in which case the vortex means it has a non-flat surface so you might be able to pull it lower (i.e. the liquid rises up the sides so if your tail is there it could reduce it to below the liquid level with no stirring).

I think we designed them to be approximately 20ml volume with that tail, but I think we also adjusted it slightly over time so it is likely far from exact. In practice I think they do a pretty good job of keeping volume fixed (or close to it); if you need to change the fixed volume you change the tail.
Hope this answers your question?

[Kate]

Ok yes thanks that helps. So stirring can affect the volume pumped out for a given pump rate? i.e. potentially marginally more volume removed than if stirring was turned off? Our stir rate is set to 0.6 so shouldn’t be stirring for a full cycle, but I guess it depends if the stirring and pumps overlap (e.g. they both initiate at the beginning of each cycle and are therefore always running at the same time?).

For one of our applications it’s fairly important to have a good handle on volume regulation as we’re adding a fixed amount of product to the system without pumps on (but always stirred), leaving it for a while and the using the pumps to wash through fresh media. We therefore need to calculate the dilution rate / how many cycles it will take to wash the product out using the pumps at a given level. Thanks!

[harrison]

I think it SHOULDN’T make a difference in general since we schedule in the software for liquid removal to happen when the reactor is not stirring. But if you have made some edits to software or custom programmes maybe this is no longer the case so it would make a difference.

I don’t know what else in specific might help your application. I suppose if I were doing that I might try adding a fluorescent dye with whatever you are adding, so you can see the fluorescence spike when you add the chemical and then watch it go down as it is diluted out during growth. THis would give you a very precise measurement – much more so than trying to rely on the pump data to confirm growth rate.
If you ARE trying to get the growth rate estimated accurately (to back-calculate to dilution rate) I suggest using the Zigzag mode on the UI (Dither OD button) which lets the OD go in a “z” shape trace so you can more accuarely fit a growth rate model to it. It is discussed in the PLOS Biology paper about Chi.BIo if you wish to learn more.

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