An apparatus for use in determining the relative vapour pressure of a fluid in an environment in which the apparatus is located, the apparatus comprising a first layer (512) configured to enable a flow of charge carriers from a source electrode (505) to a drain electrode (506), a second layer (513) configured to control the conductance of the first layer (512) using an electric field formed between the first (512) and second layers (513) and a third layer (514) positioned between the first and second layers to prevent a flow of charge carriers therebetween to enable formation of the electric field, wherein the second layer (513) is configured to exhibit a charge distribution on interaction with the fluid, the charge distribution giving rise to the electric field between the first and second (513) layers, and wherein the second layer (513) is configured such that the charge distribution and electric field strength are dependent upon the relative vapour pressure of the fluid in the environment (516), thereby allowing the relative vapour pressure to be derived from a measurement of the conductance of the first layer (512).