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Part 2: Capillary Thermal Flow Meters vs. Differential Pressure Devices


Jan 26, 2012

In my last post on capillary thermal flow meters, I talked about volumetric flow and how it must be temperature and pressure corrected to obtain mass flow. Wouldn’t the world be a simpler place if we could simply measure mass flow directly? In fact, we do! Capillary thermal is likely the most ideal technology for accurately and economically measuring and controlling gas mass flow rate of low flows (under 1000 slpm) and is widely used for this very reason.

Our video on precision gas flow control provides a helpful illustration of the capillary thermal principle of operation, which 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 a Laminar Flow Element (LFE) bypass. A very small portion of the total flow is diverted through a small heated “capillary” sensor tube with an ID between .007 to .028 inches. As the gas flows through this very small, evenly heated tube, the molecules of the gas carry some heat from the upstream section to the downstream section. This creates a temperature differential between the two sections which yields the output signal. This output signal is directly proportional to the number of molecules taking the heat away and is thus a direct measurement of mass flow. No pressure (P) or temperature (T) compensation required.

Since the early 1980’s Sierra pioneered these capillary thermal instruments and currently offer the most advanced sensor, valve and laminar flow element technologies available. Our user-friendly 100 Series Smart-Trak 2® mass flow meters and controllers (a meter with an integral control valve) gives you control, flexibility and independence with innovative solutions like Dial-A-Gas capability and the Pilot Module interface.

While ΔP meters will always have a place, I encourage you to take a look at some of the newer mass flow meters using capillary thermal technology. I think you’ll be excited to see how far this technology has advanced.

Scott Rouse, Product Line Director
Written By:
Scott Rouse, Product Line Director
Sierra Instruments

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