MKS 1579A, Инструкция по эксплуатации

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1579A/579A Chapter 7: Theory of Operation
59
Chapter 7: Theory of Operation
Technique of Measurement and Control, Electronics
The design of the 1579A/B flow controller incorporates an advanced flow sensor, a new control
valve and an optimized bypass. The latest generation two-element sensing circuit provides
accurate, repeatable performance. Low temperature effect from ambient temperature change and
a low attitude sensitivity effect are also ensured. The newly optimized sensor/bypass arrangement
minimizes the flow splitting error for gases with different densities, which dramatically improves
measurement accuracy when gases other than the calibration gas are used.
The surface mount digital, processor controlled electronic circuitry allows optimum adjustment of
the sensing and signal conditioning circuitry and provides tuned flow control for fast response to
any set point in common with excellent stability.
The flow controller type 1579A/B measures and controls the gas flow rate accordingly to a given
setpoint signal, which may be an analog signal or a digital command when using the Profibus
version. The control range is from 2 % to 100% of full scale. The accuracy of the flow
measurement is
±
1 % of full scale for both the analog units and Profibus versions.
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 a sensor tube and a parallel bypass.
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 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 then amplified, digitalized and linearized. The corrected digital signal is then transferred to
the control section (controllers only) and also converted into a 0 – 5 V analog signal.
Analog versions of the units described herein provide just the analog flow signal, Profibus versions
provide both the digital information and the analog signal.
The measurement principle of keeping temperatures constant results in much shorter response
time than conventional principles.