Brooks Instrument GF81 - Manual (Page 37)

3-1
Brooks ® GF40/GF80/GF81 Devices
Installation and Operation Manual
X-TMF-GF40-GF80-Series-MFC-eng
Part Number: 541B161AAG
June, 2013 Section 3 Operation
3-1 General
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.
3-2 Theory of Operation for Flow Measurement
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. This tube has
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.