Abstract: A apparatus for measuring at least one parameter of a fluid flowing between two conduit sections has a housing connected between the sections and having a throughgoing main passage communicating therebetween and an externally open branch passage extending from the main passage between the sections. The housing is formed at the junction of the main and branch passages with a valve seat. A valve body on the seat is formed with a throughgoing and at least generally T-shaped valve passage having a pair of arm sections alignable with the main passage and a leg section aligned with the branch passage. The body is displaceable relative to the main passage at the seat between a flow position in which the valve passage is aligned with the main passage and the fluid can flow through the main passage between the sections and a blocking position in which the valve passage is not aligned with the main passage and fluid cannot flow through the main passage between the sections.