Abstract: Electrical current driven through one or the other of two coils (20 and 22) draws a ferromagnetic bob (28) along a chamber (26) containing a liquid whose viscosity is to be measured. The current that flows through the coil includes an AC component, and the resultant magnetic field causes in the other coil an AC voltage whose magnitude depends on the bob's position. A position detector (38, 40) monitors the electromotive force thus induced and concludes that the ferromagnetic bob has reached a predetermined end-of-travel position when the magnitude of the electromotive force has fallen to a predetermined fraction the maximum value that it had attained during the stroke, and a coil driver (36, 38) switches current drive from one coil to the other so as to begin driving the bob in the opposite direction.

Abstract: A densitometer (10) provides a chamber (24) within which a ferromagnetic bob (27) is shuttled back and forth by alternate driving of two coils (32 and 34). Measurement circuitry (FIG. 2) determines the density of the fluid inside the chamber (24) by comparing the stroke times in opposite directions.

Abstract: A viscometer (10) includes a chamber-defining cylinder (22) within which a bob (26), including a ferromagnetic collar (30), is shuttled back and forth by alternate driving of two coils (32 and 34). Measurement circuitry (FIG. 2) determines the viscosity of fluid inside the chamber (24) by measuring the time required for the bob (26) to make a round trip consisting of one stroke in each direction. By employing round-trip time rather than single-stroke time, the viscometer (10) reduces the sensitivity of the viscosity measurement to orientation with respect to gravity.

Abstract: A viscometer (10) employs a tube (12) with a check valve (30) at one end and a ferromagnetic bobbin (16) disposed inside and freely movable with respect to it. A pair of coils (38 and 40) are alternately energized, causing the bobbin (16) to move against fluid in the tube in one direction, in which the check valve (30) prevents fluid flow, and in the other direction, in which the check valve (30) permits fluid flow within the tube. When the bobbin (16) moves in the direction in which the check valve (30) permits flow, it draws a new sample of fluid into the tube, and the time required for the bobbin (16) to move in the other direction, in which fluid flow is prevented by the check valve (30), is measured as an indication of the viscosity of the fluid. The position of the bobbin is sensed by measuring the mutual inductance between the coils (38 and 40).