Abstract: A method of testing a fluid is disclosed which comprises passing a main flow of a fluid through a conduit; inserting into said conduit a probe having an inlet opening which faces downstream of the conduit, or is disposed in a chamber communicating with the conduit, the inlet opening communicating with an outlet opening which is disposed in an end surface of the probe; the said main flow, in passing through the conduit, causing a sampling flow of the fluid to be drawn from the conduit, and to be passed through the probe from the inlet opening to the outlet opening, and so back to the conduit; and testing said sampling flow.

Abstract: An apparatus for measuring the density of a fluid including a coil assembly comprising a housing of magnetic material having first and second circular end walls and a cylindrical permanent magnet disposed between and interconnecting said first and second end walls, said second end wall having means defining a centrally disposed circular aperture therein; a pole piece which extends from the first end wall and into the said aperture; and a coil which is mounted in the housing with the said pole piece extending axially therethrough, magnetic flux passing outwardly of the housing only through said aperture and being focussed by the latter.

Abstract: Apparatus for measuring the density of a dirty fluid, said apparatus comprising a hollow body adapted to have said dirty fluid passing through its interior; a rigid support member within which the hollow body is mounted and to which it is connected to provide a space therebetween adapted to contain a clean fluid; drive means for exciting said hollow body to vibrate at a resonant frequency; detecting means for detecting a density signal representative of the frequency of such vibrations, the frequency of said density signal in operation being dependent upon the density of said dirty fluid; and pressure transmitting means for transmitting the pressure of dirty fluid passing through the interior of the hollow body to the clean fluid in said space, so that the exterior of the hollow body is always subjected to substantially the same pressure as the interior thereof.