2.0OPERATING PRINCIPLE
2.1Operating Principle
Hastings Linear Mass Flowmeters operate on a unique electrical principle whereby a capillary tube is heated uniformly by a transformer. The temperature distribution is symmetrical about the midpoint at zero flow (see Figure 2.1) and external thermocouples
When flow occurs through the tubing, heat is transferred to the gas and back again creating an asymmetrical temperature distribution. For a constant power input, the differential thermocouple output is a function of the mass flow rate and heat capacity of the gas. Since the heat capacity is relatively constant over wide ranges of temperature and pressure, the flowmeter may be calibrated directly in mass units for any given gas. Changes in gas composition only require simple multiplier applied to the air calibration (see table, page 19) to account for the difference in heat capacity, thus the flowmeter is capable of measuring a wide variety of gases.
High ranges of flow are achieved by dividing the flow with a fixed ratio shunting arrangement, as is illustrated in Figure 2.2. By placing the measuring tube in parallel with one or more dimensionally similar channels (laminar flow elements), viscous restrictions are
created. Therefore, the sensor need heat only a small portion of the total gas which results in low power requirements while retaining mass measuring characteristics.
Fig 2.1
Fig 2.2
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