168-082010_ 210-212 Meter/Controller Page 12 of 17
3. Theory of Operation
3.1. Overall Functional Description:
The HFC Flow Controller consists of a sensor,
electronic circuitry, a shunt and a valve. The sensor
measures the flow rate from 0 to 10 sccm of the gas
to be metered. The shunt divides the flow such that
the flow through the sensor is a precise percentage of
the flow through the shunt. The flow through the
sensor and the shunt is always laminar. The circuit
board amplifies the sensor output and uses this
output to control the valve position. The valve is an
automatic metering solenoid type; its height off the
seat is controlled by the voltage in its coil. All of these
components working together result in a fast, stable
flow controller.
3.2. Sensor:
The Hastings HFM-210/HFC-212 series operates on a unique thermal electric principle whereby a metallic
capillary tube is heated uniformly by a resistance winding attached to the midpoint of the capillary (see Figure
3.1). Thermocouples TC-1 and TC-2 are welded at equal distances from the midpoint and develop equal
outputs at zero flow.
When flow occurs through the tubing, heat is transferred from the tube to the gas on the inlet side, and from
the gas back to the tube on the outlet side creating an
asymmetrical temperature distribution (sees Figure 3.2).
The thermocouples sense this decrease and increase in
the capillary tube temperature and produce a millivolt
output signal proportional to that change.
For a constant power input, the differential thermocouple
output is a function of the mass flow rate and the heat
capacity of the gas. Since the heat capacity of many gases
is relatively constant over wide ranges of temperature and
pressure, the Flowmeter 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 Flowmeter is capable of measuring
a wide variety of gases. The HFM sensor measures
approximately 10 sccm, full scale shunt.
3.3. Electronics:
The Hastings HFM-210/HFC-212 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 is 0-5.00 VDC. This 5 volt output is sent back to the power supply and to the Flowmeter
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.
Fi
3.1
Fig 3.2
