Is pressure **always** constant?  Probably not if combustion is involved.

However, once allowed to cool, pressures will be constant. Volumes change. This is done so *volume* (as residual oxygen) can be measured and compared with the original volume of oxygen.

Hydrocarbon analysis is based on ratio of carbon (atomic weight 12) and hydrogen (atomic weight 1).

More moles of oxygen are consumed for a given mass of hydrocarbon if the hydrogen content is higher, as in one mole of CH4 as compared with C8H18.

Alkali is used to strip CO2 from the combustion mixture, leaving residual oxygen.

Atmospheric pressure is used to keep pressure in the reactor constant (just hope the barometer does not change much while running the experiment).