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Physikfan

Wed Apr 26 2017, 05:11PM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
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Posts: 304

For using gallium

1. the very low vapor pressure
2. it is not that toxic compared to mercury

Against using gallium

1. the high price
2. working temperature above 30Â°C

You should work with the Sprengel pump in Spain or Greece during summer time!

Physikfan

Wed Apr 26 2017, 05:43PM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

Hi jpsmith123 and klugesmith

"I also have some Edwards "Ultragrade 19" mechanical pump oil which is supposed to have a relatively low vapor pressure.
I'm not sure how low this system will go, but it rapidly buries my Varian 531 TC gauge; so I think it's well below 0.001 torr."

You are using a very good vacuum oil, Edwards "Ultragrade 19", about 25 US $/liter:

vapor pressure (mbar):
20 Â°C 1 x 10**-8
100 Â°C 1,0 x 10**-3

"What happens if you connect the vacuum gauge, and nothing else, directly to the pump inlet?"

I have done this experiment.
I got 3 x 10**-2 mbar with gas ballast and
I got 2 x 10**-2 mbar without gas ballast.

klugesmith

Sat Apr 29 2017, 07:41PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Physikfan wrote ...
..."What happens if you connect the vacuum gauge, and nothing else, directly to the pump inlet?"

I have done this experiment.
I got 3 x 10**-2 mbar with gas ballast and
I got 2 x 10**-2 mbar without gas ballast.

and, If I recall correctly, 4 or 5e-2 mbar with gauge on the chamber.
That suggests that your chamber vacuum might be improved by a factor of about two, at best, by using a fatter pipe and reducing leaks. JohnF says something isn't working as well as it should -- that could be the pump or the gauge. Don't you have other pumps?

Sulaiman:
Nice lab station you got there. Thanks for sharing.
It's great idea: to enhance a Sprengel pump by keeping the mercury cold. Never thought of that! Aside from substantially lower vapor pressure, the increased liquid density doesn't hurt.

Someone mentioned that working fluids less dense than mercury would require taller columns. Yes indeed, but nobody said a Sprengel pump needs to discharge directly to atmospheric pressure. A mechanical backing pump, even (say) a diaphragm pump, would substantially reduce the physical height requirement for any fluid. Always consider where the fluid will go in a hardware failure event.

Physikfan, your vapor pressure tables beat me to the punch! I was studying that, and making charts, and got in trouble for coming home late. Had just learned that most materials have a nearly linear relationship between the logarithm of vapor pressure and the inverse of absolute temperature. Here are some data points for Hg from several online references, including the tabular data you posted:

It was also news to me that gallium has a very low vapor pressure, esp. since it seems to have the second lowest MP of all metallic elements. One ref says its BP/MP ratio (in absolute temperature) is the only one higher than 8. That would have to be excepting helium .

Anyway, something about atomic bonding makes many high-MP elements relatively volatile, and low-MP elements very nonvolatile in the vapor pressure sense.

I hope my pretty chart is self-explanatory. Look how indium is way down there in vapor pressure. And how zinc vapor pressure is many orders of magnitude higher than that of lead, in spite of Zn being much harder to melt in a pot on a stove.

As for gallium in Sprengel pumps kept warm, there are also its lower-MP alloys (e.g. galinstan (tm) ) that are used in fever thermometers.

Physikfan

Sun Apr 30 2017, 07:42PM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

klugesmith wrote:

"I got 3 x 10**-2 mbar with gas ballast and
I got 2 x 10**-2 mbar without gas ballast.
and, If I recall correctly, 4 or 5e-2 mbar with gauge on the chamber.
That suggests that your chamber vacuum might be improved by a factor of about two, at best, by using a fatter pipe and reducing leaks. JohnF says something isn't working as well as it should -- that could be the pump or the gauge. Don't you have other pumps?"

At the moment I have no other pump.
The pressure gauges seem to be okay. Both meters show the same values as can be seen at my picture in my first posting.

Please, which software have you used for creating these interesting figures on vapor pressures?

Ad Proud Mary

"Mr Physik,
have you heard of The Bell Jar : Vacuum Techniques and Related Topics for the Amateur Investigator?
It is full of good stuff, some of it dating back more than 50 years. "

I think this i a very good stuff, it costs about 80 US$.

klugesmith

Mon May 01 2017, 07:44PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Physikfan wrote ...
Please, which software have you used for creating these interesting figures on vapor pressures?

The charts were made in Microsoft's Excel program. A tool I use every day at work; both the 2003 version and a "2007 or later" version. The latter can handle bigger arrays. But they changed the GUI extensively, in ways that make it horribly cumbersome to produce charts styled the way I want. The defaults seem to reflect the MS Office concept of marketing presentation trends around 2007.

There are free / open source software alternatives. e.g. gnuplot (from long ago). There are non-free programs, e.g. Matlab, that are much more versatile than Excel for rendering scientific data.

Excel has no vapor pressure feature, AFAIK.

All chart values were typed in by hand, or cut and pasted from internet pages, or generated from other spreadsheet cells using simple formulas. For example, the multi-element chart worksheet begins with:

One data point was obviously wrong in the chart. Manually adding a minus sign, missing in the source, brought the errant datum into line.

Paul F. Dietz, writing in 2000,

includes both mercury and lead among elements that have very low vapor pressures at their melting points.

By that measure, copper (bottom-most in my chart) is above mercury by a factor of 150.

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