The Neon Bender's BBS!

Re: ugauge markings
Posted By: John Anderson
Date: Tuesday, 17 July 2007, at 2:16 a.m.
In Response To: Re: ugauge markings (SVP Neon Equipment)
> Do you have a standard Utube manometer? Or
> one where one side of the tube is larger
> diameter than the other? Each type requires
> a different scale.You guys must me talking about a situation where you don't move the scale... correct? The distance 'between' the two columns will remain the same regardless of the diameter.
With a large diameter or 'pot' on one side, that leg will barely move compared to a much thinner tube. Doing such you can have a scale you really don't have to move each time to 'zero' it. But the 'difference' between the two columns will always be 12.65mm for DC704. Doesn't matter if one side is 5mm and the other is 2 inches.
Using a big pot on one side and a narrow tube on the other, you can effectively make your marks 12.65mm apart, as one side will hardly move. If your two tubes are equal, and you don't want to move your scale each time  then 6.32mm per division will work. Having one larger leg effectively doubles the sensitivity of the gauge  and this is a definite plus! It also provides other advantages, like having a large reservoir to absorb an accidental backflow.
I think this is what you guys are talking about  just wanted to clarify.

In case anyone cares  the calculation for this is as follows:
704 diffusion pump fluid (NOT butyl oil!) has a specific gravity of 1.07 (water has a SG
of 1.0). Torr really means "mm of Hg", so we have to convert that 1.07 (which is referenced to water) to the much heavier Hg.Hg has a specific gravity of 13.546 (thus it is 13.546 times heavier than water). Dividing 13.546 by 1.07 = 12.65
So, one "torr" (or mm of Hg) will support a difference in your two columns of 12.65mm. Since it's common to just measure one side of the Uguage(and not have to readjust the scale every time) this is divided by 2 to give 6.32. There should be 6.32mm distance between each of the marks on an equaltube manonometer.
(This is 0.249 inches  close enough for 1/4in graph paper in the context of neon tube filling)

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