Summary: The O-Line control unit is a pneumatic instrument with proportional band and reset-rate adjustments. It is used to maintain furnace pressure by positioning a damper in the stack, suitable for differential pressures with inverted-bell models for small pressures. The text also discusses thermocouple-type vacuum gauges and ionization gauges for vacuum measurement, along with a discussion on instruments for measuring and controlling pressure and liquid level, as well as recording viscometers for measuring viscosity. Additionally, the text covers the operation of valve positioners, pilot devices, and electronic control systems for automation and accuracy in industrial processes, as well as methods of measuring power input for viscosity determinations. The Beck triple-function mechanism and the hydraulic viscometer are highlighted as examples of instruments used for flow and viscosity measurement in industrial applications.