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140-072011 200/202 Series Page 17 of 24
relatively constant over wide ranges of temperature and pressure, the flow meter may be calibrated
directly in mass units for those gases. Changes in gas composition usually only require application of a
simple multiplier to the air calibration to account for the difference in heat capacity and thus the flow
meter is capable of measuring a wide variety of gases. The HFM sensor measures approximately 10 sccm,
full scale flow.
3.3. Electronics
The Hastings HFM-200/HFC-202 series uses a thermal flow sensor to measure through a capillary
tube, which is a fixed percentage of the total flow through the instrument. This sensor develops an
output signal proportional to flow which is approximately 1 mV full scale magnitude. This signal is
amplified by the meter circuitry until it is 0-5.00 VDC. This 5 Volt output is sent back to the power
supply and to the flow meter circuitry, if applicable. At the power supply the 5 Volt output is sent to the
terminals on the back and to the decoding circuitry in the display which converts it to a 3-digit output.
The controller circuitry utilizes the command and the flow voltages as input signals. The 0-5VDC
command signal is subtracted from the 0-5VDC flow signal creating an error signal. This signal is
amplified and causes the solenoid valve to move. The amount and direction of the movement is
dependent upon the value and the sign of the error signal, and tends to minimize the error signal.
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-200/HFC-202 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 medium range
shunt consists of a corrugated
stainless steel ribbon wound into a
coil and fused. It is adjustable from
0-0.25 slm to 0-25 slm ranges.
Figure 3.3
Figure 3.4