I am working on a barometer that will operate t-ed in to an experimental vacuum chamber. The fluid column is composed of vacuum oil so it doesn't boil. I have calculated that given the oil's density of 0.92g/cm3, each cm of oil height indicates 90 pascals of pressure. The oil has a vapor pressure of 10^-6 mbar. Given a height of 12cm, it should operate between ~0 and ~1kPa. The problem is that there is gas evolution happening. When I vacuum it down, the fluid level drops corresponding to pressure, but continues to drop slowly even as I hold the pressure constant. This continues until gas bubbles out from the bottom of the column. I have already de-gassed the parts to the point where no more bubbles form. I'm not sure where this gas is coming from or how it's pressure could be greater than that of the barometer's environment? I.E. forcefully displacing fluid in the course of pushing the fluid level in the column below that of the basin it sits in.