chemrox
International Hazard
Posts: 2961
Registered: 18-1-2007
Location: UTM
Member Is Offline
Mood: LaGrangian
|
|
vacuum system controlled leak or control
What is wanted is a realtively simple system for controlling vacuum probably by controlled leak. Highly controllable metering valves that function
well at low pressures are relatively expensive. To be a little more informative, we want to control vacuums in the 1-20 torr range.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
|
|
|
bio2
Hazard to Others
Posts: 447
Registered: 15-1-2005
Member Is Offline
Mood: No Mood
|
|
A tiny multi turn needle valve will work and they are much cheaper than true "metering" valves which have a calibrated stem. Just mark the body and
count the turns.
If finer control is needed then a very small gauge hypodermic needle attached to
the air inlet helps to attenuate the rate of change.
At any rate it takes some amount of time for the pressure reading to stabilize and
it works best if the vacuum is initially allowed to stabilize near full vacuum (if practical) then gradually open the valve.
Remember the gas flow must enter from the bottom of the needle. I have had good results using a needle valve intended for propane/natural gas with the
aforementioned series hypodermic needle.
|
|
|
chemrox
International Hazard
Posts: 2961
Registered: 18-1-2007
Location: UTM
Member Is Offline
Mood: LaGrangian
|
|
could you give me some specs or (even better) names for the needle valves you're talking about? Thanks!
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
|
|
|
bio2
Hazard to Others
Posts: 447
Registered: 15-1-2005
Member Is Offline
Mood: No Mood
|
|
http://www.coleparmer.com/Catalog/product_view.asp?sku=06393...
Ones made of brass are much cheaper and this vendor is known for high prices
but this will give you the idea. A 'true" multi turn (10 turns) needle is not really needed as 3-4 turns are used in the cheaper brass valves usually
found in angle pattern.
The flow rate of the valve and vacuum source needs to be considered and
a 26 guage or smaller hypodermic needle depending on the capacity of the vacuum pump/aspirator.
With a 1.5 CFM pump a 30 or 32 gauge insulin syringe works to control a vacuum in the 28"+ Hg range when used with the needle valve.
|
|
|
chemrox
International Hazard
Posts: 2961
Registered: 18-1-2007
Location: UTM
Member Is Offline
Mood: LaGrangian
|
|
bragging rights
I have to brag a little. I bought a Seargent Welch 1400 on ebay or labx for $25. It's rusty inside. I ran some cleaning oil through it and hooked
it up to a Bennert manometer yesterday. The column height difference is less than I can read. Under 1mm; maybe a lot under...The SW 1400 is a real
workhorse. Get one if you can.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
|
|
|
chemrox
International Hazard
Posts: 2961
Registered: 18-1-2007
Location: UTM
Member Is Offline
Mood: LaGrangian
|
|
| Quote: | Originally posted by bio2
The flow rate of the valve and vacuum source needs to be considered and
a 26 guage or smaller hypodermic needle depending on the capacity of the vacuum pump/aspirator.
With a 1.5 CFM pump a 30 or 32 gauge insulin syringe works to control a vacuum in the 28"+ Hg range when used with the needle valve.
|
After reading this I thought of taking a 1 ml 30 g syringe, putting a little glass wool inside and then putting in some tire repair foam. Might just
allow ~ 10mm. I'll let you know.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
|
|
|
MagicJigPipe
International Hazard
Posts: 1554
Registered: 19-9-2007
Location: USA
Member Is Offline
Mood: Suspicious
|
|
I have a teflon stopcock with a built in "needle valve" that came on a 25ml titration buret I bought not too long ago. As I have never messed with a
"real" needle valve before, how do they compare in principle of design? Is the stopcock one better than a brass one? I wouldn't imagine so.
I know these are common but I could post a picture if I need to. Also, I got it for $5! I know the stopcock alone, brand new, would be at least $20.
[Edited on 13-3-2008 by MagicJigPipe]
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
|
|
|
bio2
Hazard to Others
Posts: 447
Registered: 15-1-2005
Member Is Offline
Mood: No Mood
|
|
A stopcock is generally for on or off duty only. Also called a "plug" valve.
However they are used on separating funnels as flow control.
A needle valve is for accurate flow control and has a long tapered "needle" that inserts into a tapered seat using multiple turns.
The high quality ones are 10 turn and have a calibrated dial on the stem. They are repeatable with high accuracy. Also very expensive.
Cheap ones are maybe 3 or 4 turns but are suitable for many uses.
Look on a makers site for descriptions of the various types of valves.
|
|
|
bio2
Hazard to Others
Posts: 447
Registered: 15-1-2005
Member Is Offline
Mood: No Mood
|
|
A stopcock is generally for on or off duty only. Also called a "plug" valve.
However they are used on separating funnels as flow control.
A needle valve is for accurate flow control and has a long tapered "needle" that inserts into a tapered seat using multiple turns.
The high quality ones are 10 turn and have a calibrated dial on the stem. They are repeatable with high accuracy. Also very expensive.
Cheap ones are maybe 3 or 4 turns but are suitable for many uses.
Look on a makers site for descriptions of the various types of valves.
|
|
|
The_Natural
Harmless
Posts: 15
Registered: 11-5-2008
Member Is Offline
Mood: No Mood
|
|
I have a 3 cfm, 75 micron vacuum pump to be used in a dst. setup using flat bottom, round reaction flask up to 1L.
RBF's are restricted here.
I intend on using the system described by you bio, utilizing a 5/16 flare needle valve with syringe connected to an inline t-fitting. The main purpose
of controlling the vacuum is to avoid imploding glassware.
"At any rate it takes some amount of time for the pressure reading to stabilize and
it works best if the vacuum is initially allowed to stabilize near full vacuum (if practical) then gradually open the valve."
Would this be something that I should avoid?
|
|
|
MagicJigPipe
International Hazard
Posts: 1554
Registered: 19-9-2007
Location: USA
Member Is Offline
Mood: Suspicious
|
|
Thanks bio2 but I meant that the stopcock itself has a valve inside that pushes a plastic "needle" into the stopcock hole every time the needle's
handle is twisted. It's kind of like a cheap needle valve built into a stop cock to adjust the rate of flow.
It's all made of teflon, BTW.
[Edited on 5-22-2008 by MagicJigPipe]
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
|
|
|
bio2
Hazard to Others
Posts: 447
Registered: 15-1-2005
Member Is Offline
Mood: No Mood
|
|
A 1 liter flat bottom boiling flask will take full vacuum with no problem up to 250+ degrees.
|
|
|
The_Natural
Harmless
Posts: 15
Registered: 11-5-2008
Member Is Offline
Mood: No Mood
|
|
Do you think this could actually work? Using a compound of known boiling point to calibrate the vacuum on a system utilizing the propane needle
valve/syringe system.
I want to set my vacuum to somewhere between 1 to 10 torr. Was thinking about using xylene (bp- 138.5 @ 1 atm) which is at about 22 deg c room temp
here.
The boiling point applets tell me that xylene would boil at approx. 22 deg c if the pressure is reduced to 5 torr. Perfect! well... maybe.
I would let the system charge up to full vacuum and xylene would come to a boil, then slowly open the needle valve allowing air in, adjusting bit by
bit until the xylene almost stops/stops boiling, this would be close to 5 torr theoretically. I would then mark the valve body and the dial at this
point.
One would then remove the xylene and replace with the mixture that is to be distilled, tighten the needle valve back to full, then slowly turn the
dial back to the marked position (5 torr), apply heat etc...
Assumtions: 1. Accuracy of reduced pressure boiling point calculator applet, 2. Some kind of repeatability with this propane needle valve system,
doesn't have to be exact.
Reasons for not buying a manometre: I am a broke student, and a cheapskate.
|
|
|
bio2
Hazard to Others
Posts: 447
Registered: 15-1-2005
Member Is Offline
Mood: No Mood
|
|
Those vacuum nomographs are only an approximation. Your idea would work but
you need to use an actual reduced pressure boiling point from the literature for any
reasonable accuracy.
Better to choose something that will boil above room temp at the pressure you desire.
An automotive quality vacuum guage can be read to 1/4 inch which is 6.35mm and would be more repeatable than some cheapo needle valve that has been
marked.
A closed end manometer can be easily made with a few cents worth of glass tubing and a couple grams of Hg. Cost would be less than a bourdon tube
gauge. There is no cheaper or easier way to measure vacuum and it is very accurate within it's range.
|
|
|
The_Natural
Harmless
Posts: 15
Registered: 11-5-2008
Member Is Offline
Mood: No Mood
|
|
Interesting, I already have the mercury and some spare 1 ml pipettes.
Can pipettes be bent when heated with a propane/butane torch ?
There is heaps of info available on how to do this, can't see why I shouldn't give it a go.
|
|
|
bio2
Hazard to Others
Posts: 447
Registered: 15-1-2005
Member Is Offline
Mood: No Mood
|
|
Read up bubbha, this is very simple!
You are asking questions that you already have the answer for.
All you need to know has been posted on this forum.
Use the thick wall 3mm tubing and place a constriction in the appropriate spot.
Many pipettes are not made of normal glass. Place one in the flame and observe.
Takes less time than asking questions with obvious answers.
|
|
|
The_Natural
Harmless
Posts: 15
Registered: 11-5-2008
Member Is Offline
Mood: No Mood
|
|
Please excuse my lack of knowledge on the subject.
I read the other thread in which you described how you made your manometer.
It seems to work with a lot less mercury than those described in the literature, how many mm does the mercury occupy in your U tube.
The manometers described in all the literature contain enough mercury to occupy most of the right arm of the tube and the bottom U shape. Measurements
are given by the difference in mercury heights between each arm of the U tube.
Do you measure yours in a different way?
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
