Awesome stuff. If the humidity in the room is substantial I can see some water vapour diffusing into the detectors against the gas flow, even though it should be constantly purged out again by the P10? But there would be a strong concentration gradient for the water vapour. It might be possible to calculate this from thermodynamics? Water molecules are very effective for charge compensation in variable pressure/environmental SEM. Any water in the detector will probably have some effect.
In any case, probably another reason to put something on the exhaust. I tried filling our bubbler with diffusion pump oil (to avoid the somewhat nasty dibutylphthalate) but it was way too viscous at room T. So now I'm trialling Alcatel 200 rotary pump oil, which seems to have the right sort of viscosity (with bubbler close to half filled 26 nicely shaped bubbles per minute, with P10 pressure regulator set at 16 kPa, flow regulator to around 1.15 ml/minute). From the specs it is hopefully fairly clean and long-term stable ("double distilled hydrocarbon fluid, low backstreaming ... strong oxidation resistance, ... for corrosive applications..."), so I'll see how that goes. It is a double chamber bubbler so even in the case of a detector window failure it shouldn't suck the fluid all the way back into the detectors (in theory, at least...). If anyone has a better idea what to use let me know. I'd still be interested in the back pressure regulator setup even at low altitude such as in our case.
Cheers,
Karsten