Thermal mass flow instruments that make use of a bypass (capillary bypass or bypass sensor) are what most people have in mind when they think of thermal mass flow instruments. What are the differences?
In instruments based on the thermal principle, power is applied to heat the sensor tube. Accordingly the temperature of the tube is measured at two points. With no flow measured, the temperature differential between the two points will be zero.
When the flow increases, the temperature at the first measuring point will decrease, as fluid carries away the heat. At the same time the temperature at the second measuring point will increase as the fluid carries heat to it. More flow will result in a greater temperature differential and this temperature differential is proportional to the mass flow.
Another technology used to measure mass flow is CTA (Constant Temperature Anemometry). In a CTA (through flow, straight tube) instrument there are two measurement “probes” inserted into a straight tube flow path. The first “probe” both heats and measures the temperature of the fluid, as the second “probe” measures the temperature of the fluid.
Again, as the gas flow increases the gas will carry heat from the first measuring point to the second one. In a CTA, however, the power is varied to keep the temperature between the two measuring points constant, and it is this power level that is proportional to the mass flow.
Each technology has its advantages and disadvantages which generally are application specific.
A clean, dry gas application where higher accuracy is as important as repeatability, may be a better application for a bypass instrument like the Bronkhorst EL-FLOW series.
An application with a dirty or slightly moist gas, or where lower accuracy but high repeatability and robustness is required, may be a better application for a CTA instrument like the Bronkhorst MASS-STREAM™ series.
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