Vibrating wire viscometers

A vibrating wire viscometer includes a housing 402 which includes a flowline 406 through the housing to expose a wire 404 to a downhole fluid (44, fig 3). At least one cavity (408,410) in the housing is provided to hold a magnet 412, 414 and to conduct one or more additional wires 442,444 from the flowline to a signal generator 422. First and second electrically conductive posts 416,418 are mechanically coupled to the housing to hold the wire in tension within the flowline, and a seal mechanically coupled to the housing to prevent access to the magnet by the downhole fluid. The remaining space within cavity 408,410 may be filled with an encapsulation material 450, 452 such as an epoxy rubber or the like. A moulding may be used to protect the encapsulation material from the downhole fluid. A number of vibrating wire viscometers are described. The vibrating wire viscometer may be located in a downhole tool (10, fig 1) and is used to measure fluid viscosity.