Sierra Instruments Instruction Manual
Smart-Trak
®
2 Series 100
9
THE SMART-TRAK 2 FLOW SENSING PRINCIPLE
The operating principle of the Smart-Trak 2 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 Smart-Trak 2 microprocessor.
From this, Smart-Trak 2 calculates the output signal. Since the molecules of the gas
carry away the heat, the output signal is linearly proportional to gas mass flow.
Figure 1-2. Flow Measuring Principle