168-082010_ 210-212 Meter/Controller Page 13 of 17
3.4. Shunt:
Measurement of flow rates higher than the
10 sccm full scale is achieved by dividing
the flow with a fixed ratio shunting
arrangement, as is illustrated in Figure 3.3.
This is accomplished by placing the
measuring capillary tube parallel with one
or more dimensionally similar channels,
called a laminar flow element (LFE).
Therefore, the sensor only needs to heat the
gas passing through the capillary tube
resulting in low power requirements, while
retaining all the mass measuring
characteristics.
The HFM-210/HFC-212 series has two
possible shunts. The low range shunt
consists of tubes inserted into a cylindrical
base. This shunt is adjustable for ranges from 0-10 sccm to 0-250 sccm (see Figure 3.4). The high range
shunt consists of a corrugated stainless steel ribbon wound into a coil and fused. It is adjustable from 0-0.25
slpm to 0-30 slpm ranges (see figure 3.5).
3.5. Valve:
The control valve is an “automatic metering solenoid” valve. While most solenoids operate in either the fully
open or fully closed state, the automatic metering solenoid valve is designed to control flow (see Figure 3.6). A
spring, connected to the plunger assembly,
holds a magnetic plunger tightly against an
orifice to shut off flow. The magnetic plunger
is surrounded by an electrical coil, which when
energized with electrical current lifts the plunger
off the orifice and allows flow to pass between
the orifice and the plunger seat. Controlling the
current through he coil controls the distance
between the orifice and the plunger seat, thus
effectively controlling the flow through the
valve. This current is controlled by a feedback
loop that matches the transducer output with
the command voltage.
Fig 3.3
Fig 3.4
Fig 3.5
Fig 3.6
