Vogtlin SmartTrak 100 Series, Руководство пользователя

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9
THE CAPILLARY THERMAL OPERATING PRINCIPLE
The operating principle of the SmartTrak instruments is based on heat transfer and the first
law of thermodynamics. During operation process gas enters the instrument’s flow body
and divides into two flow paths, one through the sensor tube, and the other through the
laminar flow bypass. The laminar flow bypass (often called LFE which stands for
“laminar flow element”) generates a pressure drop, P
1
–P
2
, forcing a small fraction of the
total flow to pass through the sensor tube (m
1
).
Figure 1-1. Flow Paths through the Instrument
Two resistance temperature detector (RTD) coils around the sensor tube direct a constant
amount of heat (H) into the gas stream. During operation, the gas mass flow carries heat
from the upstream coil to the downstream coil. The resulting temperature difference (∆T)
is measured by the SmartTrak microprocessor. From this, SmartTrak calculates the output
signal. Since the molecules of the gas carry away the heat, the output signal is linearly
proportional to gas mass flow.