Abstract: The probe is of the withdrawable type and consists of an elongated cylindrical body made up of a tube 10 joined to an upper T-connector and to a lower tip portion 18. The tip portion 18 is of stepped shape and has a through-passage 34 beneath the step. A pressure tapping 36 opens at a port 38 in the wall of the through-passage 34 to sense reduced pressure at a vena contracta in the passage 34. Another tapping 40 opens at a port 42 in the step face 26 adjacent a concave face 28 which causes a pressure higher than static pressure. The total pressure difference can be three velocity heads and is communicated through tubes 20, 22 to a differential valve so that changes in flow rate in the pipe 16 affect the valve. The probe is relatively of small diameter and is especially applicable to control of the pressure in relation to the rate of flow of natural gas in distribution systems.