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HFM-E-200/HFC-E-202
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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 0-10 slpm range with a 5.00 volt output at full scale. Flow controller B
has 0-1 slpm range with a 5.00 volt output at full scale. If flow controller A is set at 8 slpm, its output
voltage would be 4.00 volts (8 slpm/10 slpm x 5.00 volts = 4.00 volts). If the output signal from flow
controller A is connected to the command potentiometer of flow controller B, it then becomes a variable
reference voltage for flow controller B proportional to the flow rate of flow controller A.
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 0-5
VDC command signal proportional to the intended flow (0 volts = zero flow; 5 volts = 100% of rated flow)
must be applied to pin A of the “Edge” connector. A 0-5 VDC signal proportional to the flow rate
through the instrument will be present on pin 3 of the “Edge” connector. The control mode is selected
via pin J of the “Edge” connector. Apply +15 volts for full open, -15 volts for closed and allow pin J to
float for flow proportional to the command voltage. Refer to your power supply manual for the specifics
of implementing these parameters.