The difference between thermal dispersion and calorimetric flowmeters

By Jesse Yoder, PhD

I believe I now see a difference between the thermal dispersion and calorimetric flowmeters. Thermal dispersion flowmeters measure flow by in two ways. One is by measuring the amount of power required to keep a constant temperature difference between a HEATED sensor and another temperature sensor in the flowstream. This is called the Constant Temperature method. In the Constant Power method, the amount of power or current to a heated sensor is kept constant. Mass flow is based on the difference in temperature between the temperature of the HEATED sensor and a second temperature sensor that measures the temperature of the flowstream. This is called the Constant Power or Constant Current method.

In the calorimetric method, there is also a heated sensor. But there is one temperature sensor upstream of the HEATED sensor and one temperature sensor downstream from the heated sensor. So this involves two temperature sensors, differently placed, and one heated element.

I concede that both are thermal type flowmeters, but they employ different principles. Here is my thesis. Thermal dispersion flowmeters were invented by Sierra, Kurz, and FCI in the mid-1970s, Mass flow controllers were invented by Hastings and others in the early to mid 1970s, but they are not thermal dispersion flowmeters but a different technology altogether. Nonetheless, they employ thermal principles. Calorimetric flowmeters were invented by Gunther Weber and others in the 1970s. They employ a thermal principle, but not a thermal dispersion principle. Instead, the principle is based on the displacement of temperature profiles.

I have written two articles on the history of thermal flowmeters for Flow Control. In each case, I have heard various people object to my analysis and say “We were first,” Well, say what you want, but the fact is that there are at least three distinct types of thermal flowmeters: thermal dispersion, mass flow controllers, and calorimetric. And If I write an article about thermal dispersion flowmeters, or about mass flow controllers, it is beside the point if people come out and say “We were first in developing some other technology that you weren’t writing about.”  Each of the three technologies should stand on its own and be treated historically as unique and different from the other technologies.