Thermal Mass Capillary
The Capillary Thermal principal of operation is based on heat transfer and the first law of thermodynamics. During operation, process gas enters the instrument’s flow body and it divides into two flow paths. The vast majority of the gas flow passes through the Laminar Flow Element (LFE) bypass. A very small portion of the total flow is diverted through a small “Capillary” sensor tube with an ID between .007 to .028 inches.
The LFE bypass is designed so that the process gas maintains a smooth, uniform flow profile called laminar flow (in other words, the gas has a Reynolds number below 2000). As a result, the small fraction of the total flow measured in the sensor tube remains in direct proportion to the gas flowing through the LFE bypass. By measuring the flow in the sensor tube, the total mass flow rate can be calculated. The instrument is calibrated over its full mass flow range.