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HFM-E-200/HFC-E-202
Page 12 of 30
3. Theory of Operation
This section contains an overall functional description of HFC Flow Controllers. Detailed schematics and
parts lists can be found at the end of the manual in Section 6.0. In this section and other sections
throughout this manual, when a power supply is mentioned, it is assumed that the customer has a
Hastings Power Supply. These sections are not applicable if another type of power supply is used.
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-E-200/HFC-E-202 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, heats 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 (see
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 flow.
3.3. Electronics:
The Hastings HFM-E-200/HFC-E-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 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.
Figure 3.1