FastFlo™ 620S Instruction Manual
Chapter 1 Introduction
IM-62S
1-3
The FastFlo 620S Flow Sensing Principle
Sierra’s unique FastFlo 620S sensor probe is responsible for the unsur-
passed accuracy and reliability of Sierra mass flow meters. The sensor
consists of two sensing elements–a velocity sensor and a temperature
sensor which automatically corrects for changes in gas temperature.
When power is applied to the flow meter, the transducer electronics
heats the velocity sensor to a constant temperature differential above the
gas temperature and measures the cooling effect of the gas flow. The
electrical power required to maintain a constant temperature differential
is directly proportional to the gas mass flow rate. The meter electronics
measure this power and convert it into a linear 0-5 VDC (0-10 VDC op-
tional) and 4-20 mA output signal.
The FastFlo sensors are reference-grade platinum resistance tempera-
ture detectors (RTD) encapsulated in glass. The platinum RTD wire is
wound on a rugged ceramic mandrel for strength and stability. The sen-
sor is located at the tip of a 0.375 inch (3/8”) diameter, 304 stainless
steel probe which is inserted in the gas stream. The 620S electronics
are packaged in a weather-proof NEMA 4X (IP65) enclosure mounted ei-
ther directly on the sensing probe or remotely up to 100 feet (60 meters)
away.
Figure 1-1. FastFlo 620S Sensor Assembly
