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Our old no-name, government-surplus diffusion pump has a problem.? After pumping the diff pump for a while (minutes or hours, doesn't seem to make a difference) and after loading the mirror to be coated into the bell jar, this is what happens: when I close the connection between the mechanical pump and the diffusion pump, and begin pumping down the bell jar directly, the pressure in the diffusion pump all of a sudden rises dramatically, going up to ~200 millibars if I don't immediately switch back to pumping down the diffusion pump
directly again. It becomes quite a dance, closing one valve then opening the other, then reversing again, and again, and again. Eventually it always settles down and I get decent vacuum levels, but it's annoying for a while.
? Guy Brandenburg, Washington, DC?My blog, mostly on Education in DC:
My home page on astronomy, mathematics, education:or else ??=====================================
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Hi,
?
this seems very weird, I dont understand!! 200 mb
is very high. Can you pump the bell jar down without bypassing the diff pump?
what happens then? Because you can achieve a good final vacuum (how much?),
there cant be too much wrong with your system, I wonder if there is someting
wrong with the gauge(s), I've seen some pirani gauges do stupid things when they
have been contaminated,
?
sorry this is not much use, but just my
ramblings,
?
?
James
toggle quoted message
Show quoted text
----- Original Message -----
Sent: Sunday, December 26, 2010 8:48
AM
Subject: [VacuumX] weird diffusion pump
behavior
?
Our old no-name,
government-surplus diffusion pump has a problem.?
After pumping the
diff pump for a while (minutes or hours, doesn't seem to make a difference)
and after loading the mirror to be coated into the bell jar, this is what
happens:
when I close the
connection between the mechanical pump and the diffusion pump, and begin
pumping down the bell jar directly, the pressure in the diffusion pump all of
a sudden rises dramatically, going up to ~200 millibars if I don't immediately
switch back to pumping down the diffusion pump directly again. It becomes
quite a dance, closing one valve then opening the other, then reversing again,
and again, and again.
Eventually it
always settles down and I get decent vacuum levels, but it's annoying for a
while.
? Guy Brandenburg, Washington,
DC?
My blog, mostly
on Education in DC:
My
home page on astronomy, mathematics, education:or else
??=====================================
|
BTW I haven't been able to photograph the haze that sometimes occurs because it hasn't occurred.?
?
Guy?
From: Guy Brandenburg
To: VacuumX@...
Sent: Sun, December 26, 2010 3:48:42 AM
Subject: [VacuumX] weird diffusion pump behavior
?
Our old no-name, government-surplus diffusion pump has a problem.? After pumping the diff pump for a while (minutes or hours, doesn't seem to make a difference) and after loading the mirror to be coated into the bell jar, this is what happens: when I close the connection between the mechanical pump and the diffusion pump, and begin pumping down the bell jar directly, the pressure in the diffusion pump all of a sudden rises dramatically, going up to ~200 millibars if I don't immediately switch back to pumping down the diffusion pump
directly again. It becomes quite a dance, closing one valve then opening the other, then reversing again, and again, and again. Eventually it always settles down and I get decent vacuum levels, but it's annoying for a while.
? Guy Brandenburg, Washington, DC?My blog, mostly on Education in DC:
My home page on astronomy, mathematics, education:or else ??=====================================
|
Just a guess, but that sort of behavior has been linked to a weak heater on the diff pump.
The pump oscillates between backstreaming and proper behavior.
It's a long shot, but it's been known to happen...
Daryl
|
A weak heater, and not a hyperactive, too-hot one? Hmm.
On Dec 26, 2010, at 9:12 AM, "Daryl P. Dacko" < mycrump@...> wrote:
?
Just a guess, but that sort of behavior has been linked to a weak
heater on the diff pump.
The pump oscillates between backstreaming and proper behavior.
It's a long shot, but it's been known to happen...
Daryl
|
I suppose I could, theoretically, pump both the bell jar and the diffusion pump at the same time, but I think it would be a bad idea since the dp would rise to 1atm for a while. We get down to about? ~9 to 7*10^-5 torr. Can't seem to go any lower. The coatings look good and seem to last a reasonable length of time (years).
On Dec 26, 2010, at 8:16 AM, "James Blackett" < jamesrblackett@...> wrote:
?
Hi,
?
this seems very weird, I dont understand!! 200 mb
is very high. Can you pump the bell jar down without bypassing the diff pump?
what happens then? Because you can achieve a good final vacuum (how much?),
there cant be too much wrong with your system, I wonder if there is someting
wrong with the gauge(s), I've seen some pirani gauges do stupid things when they
have been contaminated,
?
sorry this is not much use, but just my
ramblings,
?
?
James
----- Original Message -----
Sent: Sunday, December 26, 2010 8:48
AM
Subject: [VacuumX] weird diffusion pump
behavior
?
Our old no-name,
government-surplus diffusion pump has a problem.?
After pumping the
diff pump for a while (minutes or hours, doesn't seem to make a difference)
and after loading the mirror to be coated into the bell jar, this is what
happens:
when I close the
connection between the mechanical pump and the diffusion pump, and begin
pumping down the bell jar directly, the pressure in the diffusion pump all of
a sudden rises dramatically, going up to ~200 millibars if I don't immediately
switch back to pumping down the diffusion pump directly again. It becomes
quite a dance, closing one valve then opening the other, then reversing again,
and again, and again.
Eventually it
always settles down and I get decent vacuum levels, but it's annoying for a
while.
? Guy Brandenburg, Washington,
DC?
My blog, mostly
on Education in DC:
My
home page on astronomy, mathematics, education:or else
??=====================================
|
Guy,
What do you mean: "when I close connection between the mechanical
pump and the diffusion pump"? If you close backing valve and open
gate valve, system won't work.? Only cryo and other sorption pumps
work with closed backing valve. Dif and turbo pumps must be backed
always.
Do you have gauge on the dif pump input flange under the gate valve
and on its output? What are the pressures?
Sometimes valves may leak in some position, for instance, there is
no leak when it closed and a great leak when it opened. So
everything looks fine when a gate valve is closed, and a big leak
appears when you open it for high vacuum pumping.
Best regards,
Vladimir
On 12/26/2010 12:48 AM, Guy Brandenburg wrote:
toggle quoted message
Show quoted text
Our
old no-name, government-surplus diffusion pump has a problem.?
After
pumping the diff pump for a while (minutes or hours, doesn't
seem to make a difference) and after loading the mirror to be
coated into the bell jar, this is what happens:
when
I close the connection between the mechanical pump and the
diffusion pump, and begin pumping down the bell jar directly,
the pressure in the diffusion pump all of a sudden rises
dramatically, going up to ~200 millibars if I don't
immediately switch back to pumping down the diffusion pump
directly again. It becomes quite a dance, closing one valve
then opening the other, then reversing again, and again, and
again.
Eventually
it always settles down and I get decent vacuum levels, but
it's annoying for a while.
?
Guy Brandenburg, Washington, DC?
My
blog, mostly on Education in DC:
My home page on astronomy, mathematics,
education:
or else ??
=====================================
|
In our setup, the mechanical pump has tubes and valves that connect either to the bell jar or to the rear end of the diffusion pump. I'll have to see exactly where the gauge for the dp is located.
On Dec 26, 2010, at 3:30 PM, Vladimir Chutko < chutko@...> wrote:
?
Guy,
What do you mean: "when I close connection between the mechanical
pump and the diffusion pump"? If you close backing valve and open
gate valve, system won't work.? Only cryo and other sorption pumps
work with closed backing valve. Dif and turbo pumps must be backed
always.
Do you have gauge on the dif pump input flange under the gate valve
and on its output? What are the pressures?
Sometimes valves may leak in some position, for instance, there is
no leak when it closed and a great leak when it opened. So
everything looks fine when a gate valve is closed, and a big leak
appears when you open it for high vacuum pumping.
Best regards,
Vladimir
On 12/26/2010 12:48 AM, Guy Brandenburg wrote:
Our
old no-name, government-surplus diffusion pump has a problem.?
After
pumping the diff pump for a while (minutes or hours, doesn't
seem to make a difference) and after loading the mirror to be
coated into the bell jar, this is what happens:
when
I close the connection between the mechanical pump and the
diffusion pump, and begin pumping down the bell jar directly,
the pressure in the diffusion pump all of a sudden rises
dramatically, going up to ~200 millibars if I don't
immediately switch back to pumping down the diffusion pump
directly again. It becomes quite a dance, closing one valve
then opening the other, then reversing again, and again, and
again.
Eventually
it always settles down and I get decent vacuum levels, but
it's annoying for a while.
?
Guy Brandenburg, Washington, DC?
My
blog, mostly on Education in DC:
My home page on astronomy, mathematics,
education:
or else ??
=====================================
|
