Friday, July 29, 2011

Critical Backing Pressure

Definition: Maximum backing pressure for a steam jet or vapor vacuum pump to function normally.

This now, I think, has been the major problem with getting my Diff pump working...along with leaks.

I have constructed my pump with a deflector to pump body gap of 5/16" or 4mm.

Not the 2mm as recommended by my ever patient and helpful adviser, Mr. Hart over in the UK. (Who, to his credit, has been consistently correct with his advice over the past few years of my blundering in the world of vacuum. I have always found myself coming around to doing it the way he advised... but only after trying 'my way' first!)

Proceedures in Experimental Physics. J Strong:

The 5/16" clearance between the edge of the umbrella and the condensing wall is not critical, although an optimum exists for any specified set of pressures.

Perhaps 'optimum' should be read as 'maximum' above.

Mr. Hart recounted that he had to heat his homemade diff pump with a blowtorch to get it working the first time. As his electric heater was not doing the trick alone.
Page 2:

The critical backing pressure is the highest pressure that a pump can tolerate in the backing line. If the pressure is higher than the critical pressure, the pump may stall. The critical backing pressure depends on:The pump design; the power of the heaters; and the fluid used in the vapor pump.

Now after the disappointment of my homemade diff pump not working, I have new hope of getting some results! I have a Edwards E2M5 on the way.

So the choices to get your pump going are:
-Heat it more.
-Lower backing pressure.
-Or smaller deflector to pump wall distance.

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