168-082010_ 210-212 Meter/Controller Page 9 of 17
2.6.1. Operating Conditions
For proper operation, the combination of ambient temperature and gas temperature must be such that the
Flowmeter temperature remains between 0 and 50°C. (Most accurate measurement of flow will be obtained if
the Flowmeter is zeroed at operating temperature as temperature shifts result in some zero offset.) The HFM-
210/HFC-212 series is intended for use in non-condensing environments only. Condensate or any other
liquids which enter the Flowmeter may destroy its electronic components.
2.6.2. Zero Check
Turn the power supply on if not already energized. Allow for a 1 hour warm-up. Stop all flow through the
instrument and wait 2 minutes. Caution: Do not assume that all metering valves completely shut off the flow.
Even a slight leakage will cause an indication on the meter and an apparent zero shift. For the standard 0-5
VDC output, adjust the zero potentiometer located on the lower outlet side of the Flowmeter until the meter
indicates zero. For the optional 4-20 mA output, adjust the zero potentiometer so that the meter indicates
slightly more than 4 mA, i.e. 4.03 to 4.05 mA. This slight positive adjustment ensures that the 4-20 mA
current loop transmitter is not in the cut-off region. The error induced by this adjustment is approximately
0.3% of full scale. This zero should be checked 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 0-100 slpm range with a 5.00 volt output at full scale. Flow controller B
has 0-10 slpm range with a 5.00 volt output at full scale. If flow controller A is set at 80 slpm, its output
voltage would be 4.00 volts (80 slpm/100 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 4 slpm (2.00 volts/5.00
volts x 10 slpm = 4 slpm).
The ratio of the two gases is 20:1 (80 slpm/4slpm). The % mixture of gas A is 95.2 (80slpm/84slpm and the %
mixture of gas B is 4.8% (4 slpm/84 slpm).
Should the flow of flow controller A drop to 78 slpm, flow controller B would drop to 3.9 slpm, hence
maintaining the same ratio of the mixture. (78 slpm/100slpm x 5v = 3.90v x 50% = 1.95v; 1.95v/5.00v x 10
slpm = 3.9 slpm; 78 slpm:3.9 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.
