periodically during normal operation. Zero adjustment is required if there is a change in ambient temperature, or vertical orientation of the Flowmeter/controller.
2.6.3. High Pressure Operation
When operating at high pressure, the increased density of gas will cause natural convection to flow through the sensor tube if the instrument is not mounted in a level position. This natural convection flow will be proportional to the system pressure. This will be seen as a shift in the zero flow output that is directly proportional to the system pressure.
2.6.4. Blending of Gases
In the blending of two gases, it is possible to maintain a fixed ratio of one gas to another. In this case, the output of one flow controller is used as the reference voltage for the set point potentiometer of a second flow controller. The set point potentiometer then provides a control signal that is proportional to the output signal of the first flow controller, and hence controls the flow rate of the second gas as a percentage of the flow rate of the first gas.
EXAMPLE: Flow controller A has
If the set point potentiometer of flow controller B is set at 50% of full scale, and the reference voltage from flow controller A is 4.00, then the command signal going to flow controller B would be 2.00 volts (4.00 volts x 50.0% = 2.00 volts). The flow of gas through flow controller B is then controlled at 0.4 slpm (2.00 volts/5.00 volts x 1 slpm = 0.4 slpm).
The ratio of the two gases is 20:1 (8 slpm/0.4 slpm). The % mixture of gas A is 95.2 (8slpm/84slpm and the % mixture of gas B is 4.8% (0.4 slpm/8.4 slpm).
Should the flow of flow controller A drop to 7.8 slpm, flow controller B would drop to 0.39 slpm, hence maintaining the same ratio of the mixture. (7.8 slpm/10 slpm x 5v = 3.90v x 50% = 1.95v; 1.95v/5.00v x 1 slpm = 3.9 slpm; 7.8 slpm: 0.39 slpm = 20:1)
2.7.Operation with External Devices
2.7.1. Operation with a Hastings power supply.
There are two controls for each flow controller connected to a Hastings power supply. A switch labeled “OPEN; AUTO; CLOSED” (valve over ride THPS 400 only) and a potentiometer labeled “COMMAND”. For normal operation, the valve over ride switch will be in the “AUTO” position. The “CLOSE” position removes all power from the valve, shutting off flow regardless of the command pot setting. The “OPEN” position applies full available valve voltage to the valve, causing it to open, regardless of the command pot setting. The “OPEN” position is useful for purging systems. It is recommended that the valve over ride switch not be left in this position for extended periods of time, with no flow through the controller, as a small positive zero shift may be observed.
The “COMMAND” pot adjusts the Analog command signal sent to the flow controller. The setting for each controller connected to the power supply can be observed. (Depending on how the power supply was set up, the display could indicate in flow units or percent of full scale).
2.7.2. Operation with a power supply other than a Hastings.
The flow controller must be connected to the power source as specified in section 2.6. In general, a
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