Instruction manual

Instruction Manual Smart-Trak
®
50 Series
IM-50 Rev. A.2 7
The Smart-Trak
®
50 Series Flow Sensing Principle
The operating principle of the Smart-Trak
®
50 Series instruments is based upon 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 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
).
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
®
50 Series microprocessor. From this, Smart-Trak
®
50 Series 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
Figure 1-1. Flow Paths through the Instrument