Category: Thermal

By Jesse Yoder, PhD

The history of thermal flowmeters is fascinating. Thermal flowmeters were born on the West Coast of the United States —the result of independent development by first two, then three separate companies. One company was Fluid Components International (www.fluidcomponents.com), which began by developing thermal flow switches that were used in the oil patch. The switches detected the movement of oil in oil well pipes, but they didn’t evolve into actual flowmeters until 1981.

The second strain of early development in the flowmeter marketplace was a result of the collaboration of John Olin, Ph.D., and Jerry Kurz, Ph.D. Both Olin and Kurz worked for Thermo Systems Inc. (TSI) in Minnesota from 1968 until the early 1970s. They used hot-wire anemometers in their research on air velocity profile and turbulence. The anemometers consisted of a heated, thin-film element. While these anemometers worked well for research purposes, they were too light for industrial environments.  

While Olin and Kurz were doing research using anemometers, they were more interested in developing measurement products for industrial environments. This would require a more rugged device than an anemometer.  They approached TSI about developing industrial products, but TSI wasn’t interested. As a result, Olin and Kurz decided to start their own company, incorporating Sierra Instruments (www.sierrainstruments.com) in Minnesota in 1973. In 1975, they moved the company to California , packing the business up into two trucks, driving it across the Continental Divide to set up shop in Monterey .

In 1977, Sierra Instruments was making both air sampling products and thermal flowmeters. That year, Jerry Kurz decided to become independent and formed Kurz Instruments (www.kurzinstruments.com). Sierra kept the air sampling products, while Kurz Instruments kept the thermal flowmeters.  However, Sierra got back into the flowmeter market in 1983.

In the early 1980s, Sierra, Kurz, and Fluid Components were the only companies manufacturing thermal flowmeters. However, over time, more thermal flowmeter manufacturers arrived in the area of Monterey . These include Eldridge Products, Fox Thermal Instruments, and Sage Metering Inc.  Eventually, some of the larger flowmeter companies entered the market, including Endress+Hauser and ABB.  Magnetrol, a manufacturer of level and flow switches, also entered the thermal flowmeter market.  

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.