Sierra Instruments” Multi-Trak Model 670S is an instrument for measuring mass flowrates in very large ducts or stacks that have non-uniform velocity profiles, high turndown requirements, dirty gas streams, wide temperature ranges and fast velocity and temperature changes. The instrument dynamically compensates for changes in the flow profile by using up to four independent mass flow sensing points to measure the instantaneous average gas mass flow velocity. Commonly used in petrochemical refining, coal-fired electric power generation, steel manufacturing and other industrial processes that face the challenge of accurate and repeatable gas mass flow measurement in very large pipes or ducts. A  microprocessor-based Human Machine Interface (HMI) controls all functions of the instrument. The HMI is used to collect, visualize and store flow data, as well as set up the individual sensor points, thus allowing the system to be easily configured in the field. Unlike single-probe insertion sensors where each sensor point is on the same insertion probe, each sensor point is completely independent of the others, allowing for field swap-out or cleaning of individual transmitters.