Section 3 Operation; General; Theory Of Operation For Flow Measurement - Brooks Instrument GF40 Serie Manual De Instalación Y Funcionamiento

Installation and Operation Manual
X-TMF-GF40-GF80-Series-MFC-eng
Part Number: 541B161AAG
August, 2012

3-1 General

3-2 Theory of Operation for Flow Measurement

After the device has been properly installed in the process, it is ready for
operation. When initiating flow, slowly open any upstream shutoff valve to
avoid a flow surge. A bypass is helpful in bringing the flow on smoothly.
Avoid starting a pump to supply the device without the use of a valve
upstream of the device.
The thermal mass flow measurement system consists of two components:
the restrictor or bypass and the flow sensor. Figure 3-1 is a diagram of the
flow stream through the device, with an enlarged view of the flow sensor.
Gas flow entering the device is separated into two paths; one straight
through the restrictor and the other through the flow sensor.
The separation of the flow streams is caused by the restrictor. During flow
conditions, there will be a pressure differential across the restrictor that
forces gas to flow in the sensor.
The pressure difference caused by the restrictor varies linearly with total flow
rate. The sensor has the same linear pressure difference versus flow
relationship. The ratio of sensor flow to the flow through the restrictor
remains constant over the range of the device. The full scale flow rate of the
device is established by selecting a restrictor with the correct pressure
differential for the desired flow.
The flow sensor is a very narrow, thin-walled Hastelloy tube. Onto this tube
are built upstream and downstream temperature sensing and heating
elements. During no-flow conditions, the amount of heat reaching each
temperature sensor is equal, so temperatures T1 and T2 (Figure 3-1) are
equal. Gas flowing through the tube carries heat away from the upstream
temperature sensor and toward the downstream sensor. The temperature
difference, T2 – T1, is directly proportional to the gas mass flow.

Section 3 Operation

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Brooks GF40/GF80 Series
3-1