MKS Mass-Flo 1479A, Instruction Manual, Page: 41 / 72

Chapter Five: Theory of Operation Flow Path
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Chapter Five: Theory of Operation
The 1479 Flow Controller measures the mass flow rate of a gas and controls the flow rate
according to a given set point. The control range is from 2 to 100% of Full Scale (F.S.) with an
accuracy of ±1% of F.S.
Flow Path
Upon entering the flow controller, the gas stream passes first through the metering section of the
instrument for its mass flow to be measured. The gas moves on through the control valve for its
rate of flow to be regulated according to the given set point, and then exits the instrument at the
established rate of flow.
The metering section consists of one of the following:
•
A sensor tube for ranges < 10 sccm (N
2
equivalent)
•
A sensor tube and parallel bypass for ranges > 10 sccm (N
2
equivalent)
The geometry of the sensor tube, in conjunction with the specified full scale flow rate, ensures
fully developed laminar flow in the sensing region. The bypass elements are specifically
matched to the characteristics of the sensor tube to achieve a laminar flow splitting ratio which
remains constant throughout each range.
Measurement Technique
The flow measurement is based on differential heat transfer between the temperature sensing
heater elements which are attached symmetrically to the sensor tube. This senses the thermal
mass movement which is converted to mass flow via the specific heat, C
p
, of the gas. The
resulting signal is amplified to provide a 0 to 5 VDC output which is proportional to mass flow.