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 |
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| circuitry, a shunt and a valve. The sensor measures the |
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| flow rate from 0 to 10 sccm of the gas to be metered. The |
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| shunt divides the flow such that the flow through the |
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| sensor is a precise percentage of the flow through the |
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| shunt. The flow through the sensor and the shunt is |
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| always laminar. The circuit board amplifies the sensor |
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| output and uses this output to control the valve position. |
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| The valve is an automatic metering solenoid type; its |
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| height off the seat is controlled by the voltage in its coil. |
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| All of these components working together result in a fast, |
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| stable flow controller. | Figure 3.1 |
3.2. | Sensor: |
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| The Hastings | |
| metallic capillary tube is heated uniformly by a resistance winding attached to the midpoint of the | |
| capillary (see Figure 3.1). Thermocouples | |
| midpoint and develop equal outputs at zero flow. |
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| 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. |
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| 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: |
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| The Hastings | |
| 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 | |
| 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 | |
| The controller circuitry utilizes the Command and the Flow voltages as input signals. The |
command signal is subtracted from the
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