It has legs which held it in place rotationally when the fan spun (if that makes sense) but was not fixed to the fan, so was able to move up and down freely and lift off of the fan (which it does when there is a stir bar).
We did consider using washers below the heating plate, however there is not enough clearance between the measurement PCB and the lamp on the side PCB, so we would also have to shorten the legs between the heating PCB and the measurement PCB, or cut a slit into the measurement PCB.
Further, even if we raised it enough to prevent the stirrer being lifted off during normal operation, if it was knocked in such a way to jolt the stirrer up, it does not naturally fall back down, so we would have to take the sides off of the device to reset it, and it may not be immediately obvious.
So that is why we simply used tape to affix the stirrer to the fan, so now it cannot move up. Yes it is not a permanent fix, and so we will need to find something more permanent/occasionally replace the tape, but it does work as a simple quick fix which at least allows us to immediately conduct experiments while we sort out something better.
