Sierra Instruments, Inc.

Industrial thermal mass flow meters Pt.1 -Pt.2 - measurement & control

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Courtesy of Sierra Instruments, Inc.

An industrial thermal mass flowmeter measures either the mass velocity at a point in a flowing gas or the total mass flow rate through a channel or pipe.

Figure 1 shows a typical thermal mass flow sensing element. It consists of two sensors: a mass flow sensor and a temperature sensor, which automatically corrects for changes in gas temperature. Both sensors are reference- grade platinum resistance temperature detec- tors (RTDs). The electrical resistance of RTDs increases as temperature increases; thus, they are the most commonly used sensors for accu- rate temperature measurements. The elec- tronics pass current through the mass flow sensor, thereby heating it to a constant tem- perature differential (T0-Ta) above the gas temperature, TaJ and measure the heat, q{, canned away by the cooler gas as it flows past the sensor. Hence, it is called a 'constant tem- perature thermal anemometer.'

Since the heat is carried away by the gas molecules, the heated sensor directly mea- sures gas mass velocity (mass flow rate per unit area), pU. The mass velocity is typically expressed as Us in engineering units of nor- mal meters per second, or nm/s, referenced to normal conditions of 0° or 20°C in tempera- ture and at 1 atm. If the gas temperature and pressure are constant, then the instru- ment's measurement can be expressed as actual meters per second, or m/s. When the mass velocity is multiplied by the cross-sec- tional area of a flow channel, the mass flow rate through the channel is obtained.

Customer comments

  1. By John Azbil on

    Azbil North America manufactured the following products like liquid level sensors, limit switches, process controllers, process heaters, pressure transmitters, fiber optic sensors and temperature controllers etc.