The best and most accurate way to monitor viscosity is through online continuous viscosity measurement, where a small quantity of product is circulated through a viscometer mounted in an independently temperature-controlled enclosure near the reactor.

Such a system is used for alkyd, polyester, epoxy resins and polycondensation reactions. It presents several advantages, including the following.

• The signal generated by the viscometer is directly correlated to the product viscosity. It represents a real-time measurement and is a reliable way to monitor the reaction process.
• This viscosity monitoring can be made at temperatures different from the reactor temperature, thus allowing flexibility with instrument calibration and selection of the viscosity range.
• The end of the reaction can be exactly determined, thus eliminating off-spec product and waste.
• The process can be entirely automated, thus reducing batch time and operating costs.
• Product quality as endpoint viscosity can be achieved for each formulation.

Disadvantages to online continuous viscosity measurement are as follows.

• To operate the system, a flanged connection must be available or installed on the lower section of the reactor. This may not always be an option with existing equipment.
• The system is limited to resin manufacturing reactions where viscosity evolves during the batch and represents a valuable parameter to monitor.