The output signal is fed to a controller (and recorder) to control the steam flow to the oil fuel heater which will cause viscosity adjustment. The actuator has a piston and valve positioner. The controller has desired value setting and incorporates a reset (integral) action. This detail has been simplified in Figure 2.17, lower diagram, so that the output pressure increase from the feedback bellows (due to viscosity increase) directly increases air pressure on a diaphragm valve to open up steam to the oil heater, to reduce fuel viscosity.