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Manual 111-082010_Nall Mass Flowmeters Page 6 of 23
2.0 OPERATING PRINCIPLE
2.1 Operating 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 TC-1 and TC-2 develop
equal and opposing outputs.
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