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Chapter 1 Introduction Series 830/840/860 Instruction Manual
1-4 IM-83/84/86-H
The Series 800 Flow Sensing Principle
The operating principle of Sierra’s Side-Trak and Auto-Trak flow
meters and controllers is based on heat transfer and the first law of
thermodynamics. During operation process gas enters the instru-
ment’s flow body and divides into two flow paths, one through the
sensor tube, the other through the laminar flow element bypass.
The laminar flow element bypass generates a pressure drop, P
1
–P
2
,
forcing a small fraction of the total flow to pass through the sensor
tube (m
1
) which is then monitored.
Figure 1-1. 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
(∆T) is detected by the RTD coils.
Figure 1-2. Flow Measuring Principle