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Re: u-gauge markings

Posted By: John Anderson
Date: Tuesday, 17 July 2007, at 2:16 a.m.

In Response To: Re: u-gauge markings (SVP Neon Equipment)


> Do you have a standard U-tube 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 DC-704. 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 U-guage(and not have to re-adjust 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 equal-tube manonometer.

(This is 0.249 inches - close enough for 1/4in graph paper in the context of neon tube filling)


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