Teledyne 202 Manuale Utente

 

Page 12 of 24

 

 

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 flow 

meter 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 flow meter/controller. 

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. 

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/4 slpm).  The % mixture of gas A is 95.2 (80 slpm/84 slpm 

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/100 slpm x 5 Volts = 3.90 Volts x 50% = 1.95 Volts; 
1.95 Volts /5.00 v x 10 slpm = 3.9 slpm; 78 slpm: 3.9 slpm = 20:1) 

There are two controls for each flow controller connected to a Hastings power supply.  A switch labeled 

“OPEN; AUTO; CLOSED” (valve override THPS 400 only) and a potentiometer labeled 

“COMMAND”.  For normal operation, the valve override 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 override 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.