A fluid 10 is contained within a chamber 12. The chamber wall 14 has an inner surface 16 exposed to the fluid 10 and an outer surface 18 separated from the inner surface 18 by the material 20 of the wall 14. Ultrasound 22 is introduced into the material 20. Measurement of the time of flight of the ultrasound 22 through the body 20 allows a measurement to be made of the pressure of the fluid 10.