Viscosity control
Consider the viscometer (Figure 2.17). A gear pump is driven by a constant speed electric motor through a reduction gear. A small, constant quantity of oil (fuel in this case) is passed through the fine capillary tube. As the flow is arranged to be streamlined (laminar), as distinct from turbulent, the differential pressure across the capillary tube is directly proportional to the viscosity of the oil. This differential pressure in two tappings, shown as + and -, is led from the viscometer to a differential pressure transmitter (DP cell). Differential pressure from the viscometer is applied across the diaphragm of the transmitter. Increased differential pressure, caused by increased viscosity, causes the diaphragm and balance beam to move to the left. The air inlet nozzle is closed in and air pressure builds up in the feedback bellows due to a relay (not shown in Figure 2.17), supplying increased air pressure to the feedback bellows. Equilibrium occurs when the feedback force equals the originating force; under these conditions air escape is minimal. The feedback bellows pressure is the control output signal. Transmitter diaphragm chambers are filled with glycerine or silicone as oil would clog the parts.