A thermal mass flow meter is a precise device used to measure the mass flow of a fluid running in a closed channel, such as a pipe or duct. It is also referred to as a thermal dispersion or immersible mass flow meter. The Tek-Thermal 1700B gauges gas mass flow using the thermal diffusion method and is specifically made for use with air and N2 (nitrogen). Its design is more compact, and its insertion tube probe is narrow. Use it on pipes ranging in size from 1 to 20 inches. Applications involving high pressure use it.