Primary gas and helium secondary gas flowmeters on the Metco 7MC and earlier plasma consols are calibrated with nitrogen at 50 psi @ 70oF. Using theoretical gas flow calculations, the actual flow rate of different gases ( argon, helium ) and gases at pressures other than the 50 psi can be calculated.
Theorectical gas flow calculations cannot be used on the 7MC hydrogen secondary flowmeters due to the positioning of the flow adjusting valve before the flowmeter tube. This causes the flowmeters operating pressure to be dictated from back pressure rather than the regulated input.
The actual flow of hydrogen secondary gas for a particular flowmeter setting was effected by back pressure in the system after the flowmeter.
The higher the back pressure, the higher the actual flow for a constant flowmeter setting for hydrogen secondary gas.
Primary and secondary gas flows, nozzle bore size and plasma on/off produce significant effect on back pressure.
7Mc current settings between 300-500 amps appeared to have an insignificant effect on back pressure and could be ignored.
GH/732 and GP/733 nozzles gave slightly different hydrogen flow rates for the same flowmeter setting. The narrow bore GP/733 nozzle giving higher back pressure. Only these two nozzle types were tested, but it is expected that wide bore nozzles like the G/730 or 720 would give similar results to the GH/732 and the narrow bored nozzles like the GE/731 or 704 would be similar to the GP/733.
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