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Jeff Liebermann Jeff Liebermann is offline
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Default Saving LCD screens that were under water

On Sat, 09 Jun 2018 05:08:37 -0700, mike wrote:

On 6/7/2018 10:46 PM, Jeff Liebermann wrote:
On Thu, 07 Jun 2018 22:13:46 -0700, mike wrote:

On 6/7/2018 4:32 PM, Jeff Liebermann wrote:

(...)

In that case, I don't think there's anything you can do to fix it.
Once the water has invaded the pixel area, it's all over.


Yep. The secret to engineering is knowing when to give up.

I was using an Edwards E2M-1.0 2 stage rotary vacuum pump:
https://shop.edwardsvacuum.com/products/r1/list.aspx
It's been run well past its 30,000(?) hr major overhaul point and is
probably leaking around all the rubber vanes and seals. As I recall,
it went down to about 200 milliTorr and refused to go lower, probably
because of leaks in the reinforced fish aquarium I was using for a
test chamber. Water has a vapor pressure of 25 Torr (0.5 PSI), so
that should have easily sucked all the gas and water vapor out even
with all the leaks. I had hoped to see some boiling near the hole,
but didn't see any.

I was experimenting with making heat pipes years ago. I don't remember
the numbers, but 0.5 PSI sounds WAY higher than I was trying.


Vapor pressure of water in Torr:
http://www.wiredchemist.com/chemistry/data/vapor-pressure
and in PSI:
http://www.pumpworld.com/vapor-pressure-chart.htm
25 Torr or 0.5 PSI look right.


If my google is right, HVAC systems are typically evacuated below
500 microns to get all the water out. That's more like 0.5 Torr.


The point at which water starts to boil at room temp is 20,300 microns
of Hg or 0.393 PSIA or about 20 Torr. Presumably, that's when the
water starts to turn to vapor which can the be removed by the pump.
However, at this pressure, it will take forever, so a much lower
vacuum is probably more practical. Whether it needs 40x more, I don't
know.

My memory is vague, but somewhere under 2000 microns was where
the pressure stopped decreasing due to water contamination.
With even the slightest
leak in the system, I couldn't get there.


When I filled my aquarium with water, applied a vacuum, and looked for
bubbles, in order to check for leaks, I found quite a few. These were
duly plugged with RTV. I had no trouble going down to what I thought
was 200 milliTorr (0.00386 PSI), but now I'm not so sure. I have the
vacuum pump and gauge and might be able to try again, but not for a
while. I would need to build a new vacuum test chamber and don't have
the time, bench space, fresh vacuum oil, fittings, rebuild kit, etc.

As a test, I put the vacuum gauge right on the suction port of the pump.
It went down to where water was supposed to vaporize and stuck there
until I gave up. Reading about it suggested that you have to suck all
the water (and other contaminants) out of the pump oil before you can do
anything useful. They recommended FRESH oil for every evacuation.
I bought some pump oil, but never got around to trying it.


That could have been my problem. At the time, all I had was the oil
that was in the vacuum pump when I bought it used. It probably was
rather dirty and likely contaminated with some water from
condensation. Looking at the pump through the oil level sight glass,
it looks dirty. New oil is about $30/gallon. I was eventually going
to buy the o-ring and basic rebuild kit, at which time I would buy
some new oil. However, I was getting a usable vacuum, so I didn't
bother.


--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558