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

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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.

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  1. 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. http://us.azbil.com/

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