For gas mass flow measurement and control, Sierra offers the broadest selection of thermal mass flow measurement products on the market. This is because our thermal mass flow products incorporate two types of advanced thermal technologies, each complementary to the other and each with unique advantages. These two thermal sensing technologies directly measure gas mass flow based on principals of heat transfer and are commonly called Capillary Thermal Sensor Technology and Immersible Thermal Sensor Technology. Taken together they cover the entire spectrum of gas mass flow measurement and control applications found in industry. Sierra is the only company in the industry to provide both thermal technologies to our customers.
The following tells “A Tale of Two Thermals” and is meant to educate our customers about Sierra’s thermal mass flow technology. Our thermal products are divided into two identifiable families—The Scientific Products Family which uses proprietary capillary thermal sensor technology and the Industrial Products Family which uses our advanced immersible thermal (sometimes called thermal dispersion) sensor technology. We discuss the virtues of each technology below.
Although both technologies have “thermal” in their name, their principles of operation are quite different. They both directly measure gas mass flow rate, not volumetric flow rate, because both have a heated surface that transfers heat energy to the molecules that bear the mass of the flowing gas stream. In the case of capillary tube thermal mass flowmeters, the heated surface is a capillary tube that transfers heat energy to the bulk, or to all, of the gas flowing through the tube.
In contrast, in the case of immersible thermal mass flowmeters, the heated surface is a cylinder immersed in the flow stream that transfers heat to the viscous boundary layer surrounding the cylinder. Therefore, the theoretical models expressing the first law of thermodynamics for each type of thermal mass flowmeter are not the same. To best serve the purpose of this paper, we have chosen to present easy-to-understand descriptions of the principles of operation instead of the complex solutions to the theoretical models.
We will start our technical discussion with Capillary Thermal Technology and follow with the discussion of Immersible Thermal.