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Chapter 1 Introduction Series 820 Instruction Manual
1-4 IM-82-C
The 820 Series Flow Sensing Principle
The operating principle of Top-Trak transducers is based on heat
transfer and the first law of thermodynamics. During operation
process gas enters the instrumentÕs flow body and divides into two
flow paths, one through the sensor tube, the other through the lami-
nar flow element bypass. The laminar flow bypass generates a pres-
sure drop, P
1
ÐP
2
, forcing a small fraction of the total flow to pass
through the sensor tube (m
1
).
Figure 1-2. Flow Paths through the Transducer
Two resistance temperature detector (RTD) coils around the sensor
tube direct a constant amount of heat (H) into the gas stream. In ac-
tual operation, the gas mass flow carries heat from the upstream coil
to the downstream coil. The resulting temperature difference (DT) is
detected by the RTD coils and gives the output signal. Since the
molecules of the gas carry away the heat, the output signal is linearly
proportional to gas mass flow.
Figure 1-3. Flow Measuring Principle