A transducer to measure pressure or temperature comprising a quantity of compressible material such as gas and a quantity of incompressible material such as liquid and proper means (16, 17) to detect volume and interface(s) (14) changes of said compressible material induced by pressure or temperature variation. Radiation may be used to detect said changes. Both source (16) and detector (17) of radiation may be separated from said compressible and incompressible materials using appropriate waveguides (10, 11) thus allowing for transducer miniaturisation. The only deformable element of the transducer is said compressible material, every other part of it being substantially rigid this feature offering robustness and simplicity of construction. Provision is made for the in situ calibration and flushing of the instrument when required.
