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Answers to previous questions on Mechanics of Vacuum Technology:

Actuating Pneumatic Valves

Question

We're in the process of pumping down and our first stage of pumping is accomplished by using a mechanical pump with a 0.5 mm orifice - this works fine - this isn't where the problem lies.

Our second stage of pumping is using a diff pump with the appropriate roughing mechanical pump. There are two valves. One between the mass spec chamber and the diff pump and the other between the 2 stages of pumping.

We're using compressed air to open the solenoid valves. The valves also have an electronic component which will be hooked up later to a control panel. Do the electronic components need to be connected in order to open the gate valves - or is the compressed air enough? Maybe this is where our problem lies. The gauge controller isn't measuring a change in pressure - it seems as though our gate valve isn't opening properly.

Any advice would be greatly appreciated. Do the gate valves have manuals?

Answer

Hmmm! Your explanation has confused me and I think we may have a "glossary" problem. Let me give you some definitions and statements (I'm not trying to be a smart aleck, I'm just trying to get us both on the same page).

1. A solenoid valve is actuated by electricity to open or close. Which happens when depends on whether it is 'normally open' or 'normally closed' before actuation.

2. A pneumatic valve is actuated by compressed air put into at one end of an actuation cylinder. Moving the valve in the 'other' direction is either by a return spring or by connecting the compressed air to the other end of the actuation cylinder.

3. Since the forces of atmospheric pressure can be very large on a big diameter valve, most are pneumatically actuated. Solenoid valves are reserved for small diameter vacuum valves. Indeed, while I know of many solenoid actuated foreline valves, I don't know of any solenoid actuated gate valves.

4. However, connecting compressed air first to one end of a gate valve's actuation cylinder and then to the other, is a pain up with which no-one will put (to quote Winston Churchill).

5. So, every pneumatic actuated valve has a small solenoid valve attached. A single compressed air line is fed into the solenoid valves 'action part'. Without electrical power, the solenoid directs the air pressure to one end of the pneumatic valve's actuator cylinder - typically in the 'close valve' direction. When electrical power is applied to the solenoid valve, it changes 'state' and pressurized air is supplied to the other end of the pneumatic valve's actuator cylinder.

6. It is not possible, therefore, to actuate a pneumatic valve without either (a) connecting up the solenoid valve and switching the power on and off or (b) removing the compressed air tube from the solenoid valve and attaching it directly first to one end of the actuating cylinder and then to the other. And that -- if you have ever seen the "Princess Bride" -- leads to the expression "To the pain."

Now, you may have already knew this stuff, or this may be a revelation. Either way please contact me because what I'd really want to talk about is that 0.5mm hole in the rough-down line. Is that designed in to produce choke flow or has someone never heard the word 'conductance'?

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Tightening Flange Bolts -- Part 1

Question

I would like to know the recommended procedure for tightening CF (knife edge) flange. I use mostly 2.75" CF flanges.One person tells me to go in a continuous circular pattern and another tells me to tighten using a cross pattern. I have used both methods and they both seem to work.

Answer

Yes, for a 2-3/4" CF flange probably both methods will work. The caveat is don't "lean" on one bolt when the rest are loose. That is, try to add a little compression at a time around the circle.

The "official" way to tighten bolts on this size flange is: starting from the bolt designated as 1, the tightening order going clockwise around the flange is 3, 6, 2, 4, 5.

Happy tightening

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Tightening Flange Bolts -- Part 2

Question

What is the procedure for bolting conflat flanges before and after baking?

Answer

The recommended sequence depends on the flange diameter. But basically, you always tighten opposing bolts (at 180°).

You start by choosing a bolt as bolt #1 in direction 1 Bolt #2 is in direction 2 at 180° to direction 1 Bolt #3 is in direction 3, one bolt position clockwise from direction 1 Bolt #4 is in direction 4, one bolt position clockwise from direction 2 Etc

Finger tighten all bolts first and then do a small amount of turn (1/8?) on each bolt until compression if sufficient or you have reached the appropriate torque for that bolt size (80 - 100 in. lb. for 1/4" bolts, 120 - 150 in. lb for 5/16" bolts)

Follow-up Question

Do I retighten after 400°C bake? If it is not required and I do it anyway, does it cause any problems?

Follow-up Answer

First a word of advice about flanges and baking. I strongly recommend that you get Ag plated bolts or some high temperature MoS2 goop to lubricate the bolts. Baking plain stainless nuts and bolts to 400°C gives a high probability of seizure.

Re-tightening after bake should not be needed. I can "invent" a theory why. But please understand, as the old maxim says, this theory and $2.50 will probably get you a cup of Starbuck's finest.

Tightening the knife-edges into the gasket work-hardens it to a 'pretty hard' condition (note the use of precise technical terms). The copper between the knife-edges is quite thin. So thin, that the copper's differential thermal expansion, compared to the stainless, does not cause enough force to exceed Young's modulus for the copper. So, as the knife edges "recede" during cooling, the copper elastically returns to its original thickness.

But if you wish to "just check" the torque.... feel free. As long as the flanges are not metal to metal, it can't hurt.

Starbucks, here I come!

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Standards in the Vacuum Industry

Question

I am working on a vacuum chamber design. Are there "standard" sizes for ports e.g. diameter of tube (between flange and chamber wall), length of tube between flange and chamber wall? If so, what are they?

Answer

ConFlat (or CF) flanges have a 'natural' tube size associated with the flange OD. For example the 1-1/3" OD CF takes 3/4" OD tube while the 2-3/4" OD CF takes a 1-1/2" OD tube. The unfortunate part is, it is easy to use any smaller OD tube than this 'natural' size. Worse yet, for the 2-3/4" CF, the US 'natural' ID of 1.38" (34.9mm) contrasts with the European 'natural' ID which is between 37 and 40.1mm depending on manufacturer. While US manufacturers are slowly adapting to the European standard, given the global commerce in vacuum equipment, this difference causes not insignificant problems for customers.

QF and ISO flanges are supposed to be sized according to the ID of the tube in mm. For example, a QF40 flange is attached to a tube of 40mm ID. Except, in the US it is often the closest inch equivalent tube -- a QF25 may have a 1" ID tube. A QF40 may have a 1.5" ID tube.

ASA flanges have so little standardization, they are almost not worth mentioning. An 11" OD flange may have a 6" or 8" OD tube (or, indeed, anything else that does not break into the o-ring).

As for "half nipple" lengths (which is what flanged ports are called before they are welded to a chamber), each manufacturer has its own standards (tempered, to some degree, by what other manufacturers do). The result is, the length of a flanged nipple may vary within a few tens of thousandths depending on the manufacturer.

I'm sorry I am unable to be more positive about standards in this industry.

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Specialty Roller Bearings for Vacuum

Question

I would like to know if you sell bearings for deposit systems and vacuum systems. I looking for rotary ball or roller bearing. I'd like to know if it needs grease (if yes, it's necessary to do not degazed or splashing) or if it's sealed.

Answer

We have special ball bearings made for us that are coated with the dry lubricant tungsten disulfide. The construction material is 440 stainless and the bearing has a 0.25" ID, 0.625"OD, and is 0.196" thick.

We buy them in large quantities to reduce the unit cost and I understand we do sell them as individual items.

We use these bearings extensively in our deposition systems. However, they cannot be placed in any direct line-of-sight position where the sputtered/evaporated materials can deposit on the bearing surfaces. So, if you buy bearings from us (or another company) they must be shielded in some way.

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Regenerating Micromaze Traps

Question

We have micromaze traps on some of our mechanical pumps. Is it necessary to isolate the chamber from the pump and trap when baking it out? What is the correct procedure for baking out the traps? How long? How often? Any information would be appreciated.

Answer

Oh yes! You must isolate the chamber from the trap or the trap will simply act as a secondary oil vapor source. Vapor will quickly backstream into the chamber defeating the very reason for putting the trap in the foreline.

The "best" way to regenerate a trap is to have a small bleed of gas between the trap and the upstream valve while it is being heated. For example, dry nitrogen flowing at a rate that keeps the trap at roughly 0.1 to 1 torr. This helps sweep the oil vapor from trap to pump. Unfortunately, some pumps object to having a high inlet pressure and promptly dump a load of oil vapor out the pump's exhaust.

Our Micromaze have a built-in heater. Since the Micromaze material has a low thermal conductivity, it's better to ramp up power using a Variac than put full power to the heater immediately.

How long and how often depends on the oil loading and that depends on the oil's vapor pressure, the pump's operating temperature, the pump's design, and the amount of gas let into the chamber and pumped away through the trap during normal operation.

My typical answer is: once a day is too short; once a year is too long. So strive for something in the middle.

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45° Elbow Radius of Curvature

Question

Hi, I have a question about a 45° elbow. It is not clear to me from the webcatalogue drawing what the "A" dimension refers to. I need to know the radius of curvature of this piece.

Answer

Interesting question. Never been asked this one before.

I assume the radius of curvature you wish to know is that of the elbow's center-line.

As I'm sure you see, that radius depends on the tube diameter. So, the real question is, how do we calculate the radius of curvature given the tube diameter? Well, to help, we give you the "A" dimension which is the height of the center-point of the center-line above the base flange.

That is, if we connected another elbow to the bottom of the first so that we formed 1/4 circle (rather than 1/8 when considering a single elbow), then 2 x A is the length of a chord between two points on the circle that happen to be 1/8 of the total circumference apart.

Since the angle subtended at the the circle's center point for the total circumference is 360°, then the angle subtended by 1/8 of the circumference is 360/8 = 45°.

It's perhaps better, now, to draw a circle; put a chord somewhere; draw lines from the ends of the chord to the circle's center; and mark the included angle at the center as 45°. Now, drop a perpendicular from the chord's center point to the circle's center. You get two identical right angle triangles with a base length = A and an included angle at the circle's center of 22.5°. This means the angle between chord and the radius drawn from the end of the chord is (90 - 22.5) or 67.5°.

So, from here a little trig should do it. A right angle triangle with base of "A" and angle 67.5° gives the hypotenuse (the Radius of Curvature) as.

"A"/Hypotenuse = cos 67.5 Hypotenuse = "A"/(cos 67.5)

My advice is, check what I've just written before you accept it.

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Ground on MHV Connectors

Question

I have a question about the Instrument Feedthrough. I want a MHV feedthrough that Part Number is IFTMG012038. But I am curious about the connection. I have an electric wire with the ground line. If I use this feedthrough, where and how can I connect with the ground line?

Answer

There are three ways to connect two wires (one 'live' and one ground) through an MHV feedthrough:

1. If the ground wire is truly "system ground" then you select a single MHV feedthrough with a "grounded" shield. The live wire goes to the central wire of the MHV connector pin and the ground wire is attached to a MHV connector shield. This means the ground wire is directly connected to the chamber

2. If the ground wire is a "relative ground" (that is, relative to the live wire and not be connected to the system ground), then select a single MHV feedthrough with a "floating shield. The wires are connected as in (1) but since the shield is floating there is a insulator between it and the chamber. This type of connection is used when the signal wire and the ground wire are both floating at some potential above or below system ground.

3. If the live wire and the ground wire both have a high impedance (resistance) with respect to ground, then both may act as "antenna" and pick up RFI or other electromagnetic radiation. Under these conditions, the signal and the ground lead must both be shielded. Both are used as the central wire in two separate co-ax cables. At the chamber, two grounded MHV feedthroughs are used. They can be on one flange (IFTMG022038B) or on separate flanges (IFTMG012038)

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Rubber Gasket in Vertical CF Flanges

Question

I have a large SS vacuum chamber with 16.5" conflat flanges (from KJLC of course) on either end, horizontally mounted. Eventually we will probably want to use copper gaskets to achieve UHV conditions. However, before then, it would be very nice to use viton gaskets. The copper are very time consuming to install and expensive. I've tried to use 16.5" viton gaskets but without much success, they tend to fall off before I can lift the flange into place. Any suggestions?

Answer

Yes, this is a toughie.

Some folks have been successful using copper wire, scotch tape, or a combination of the two.

The scotch tape idea uses small lengths of tape with the inner edge **just** overlapping the outer edge of the rubber gasket. Try not to place the tape over any bolt holes. For a flange this size you may need 5 or 6 pieces around the gasket's rim.

The copper wire requires you to find a wire thinner than the rubber's thickness yet thick enough to have some rigidity. Form a semicircle of wire with a bit sticking out halfway round. From the side this will look like the cross section of a champagne glass. With the flange flat, position the gasket and hold it in place (at the flanges "top") with one piece of tape. The wire then goes around the gasket's bottom two quadrants and the "champagne glasses stem" is over the flange's rim and taped to its edge.

In all cases, when the flange's bolts have been "snugged up", remove the tape or wire before squashing the gasket appropriately to make the seal.

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Cooling a Chamber with LN2

Question

My chamber has a water cooling jacket integrated in the chamber walls.

Could I cool my chamber with liquid nitrogen (LN) instead of water? I am not thinking of a continuous LN2 flow but filling the cooling jacket and re-filling it when the level drops. During usage both inlet and outlet would be open to atmosphere to prevent an overpressure. Do you see any reasons why I should not do this (like overpressure, stress of weld seams, etc.)?

Answer

Your questions raise so many issues I can't touch them all. This is just a short list of those that spring to mind instantly.

1. If this is an o-ring sealed chamber. then LN2 cooling is definitely a non-starter. When designing a water-cooled chamber, one critical point is ensuring the o-rings are well cooled. That is, the water channel and the o-ring flange cannot be separated by a low thermal conduction path. Replacing water with LN2 will freeze the o- ring.

2. If you push the closed end of a test tube into LN2 in a dewar, after a few minutes a liquid collects in the tube. I suspect it's mostly liquid oxygen. I read once of a accident that arose because LN2 was running through an uninsulated pipe close to the floor. The details escape me now but, as I recall, drops where seen falling from the tube to the floor just before a fire broke out. (I think the source of this information convinced me it was not an urban legend. 100% O2 is dangerous stuff) A large surface cooled to LN2 temperatures without a vacuum insulating jacket is a bad idea in my opinion.

3. As you point out, the potential for an over-pressure exists. With only 30 seconds thought, I'd advise you to do some calculations along these lines. Let's say you're using the pressure of the LN2 dewar (10psi? 20psi?) to force the LN2 into the cooling inlet. Given the dimensions of the inlet water-cooling tube and LN2's (low!) viscosity, calculate the likely flow rate into the cooling jacket. it may translate into quite a few liters/min. Now, conversion from LN2 to N2 changes increases the volume by roughly 1000.

Will the outlet be able to handle that gas flow without a considerable pressure build? Again, you should be able to roughly estimate the maximum outflow for a series of over-pressures. I wouldn't want the jacket pressurized more than a few psig

If the outlet can't handle the full flow, the alternative is to limit the ingress rate of the LN2. But, of course, that limits the cooling watts. You can roughly calculate this from LN2's flow rate and it latent heat of vaporization, plus N2's specific heat as the gas temperature rises from 77K to, say, 273K.

You can perhaps balance the cooling watts against the thermal input with some more calculations. Guess an emissivity value for the stainless and the chamber's surface area. The thermal radiation transfers from the room is then pretty easy to calculate (use a shape factor of 1). But I suspect the major heat input is convection- conduction (and at atmospheric pressure that's so far away from vacuum technology that I don't have a clue how to estimate that). Anyway, after all this huffing and puffing, you might find the chamber will only reach, say, -40°C. You might be able to reach that using solid CO2 slush in alcohol or some glycol-based refrigerated chiller).

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Cleaning Up a SEM After a Venting "Event"

Question

When cleaning the aluminum and SS parts of my SEM column, should I use methyl (denatured) alcohol or isopropyl? Methyl is the better solvent, having a much smaller molecule, but I hear IPA discussed most frequently. Do you have a recommended cleaning procedure?

Answer

Yes, methyl alcohol is a 'better' solvent than isopropyl but that's not saying a whole lot. Neither is a particularly universal solvent.

But the issue really hinges around -- what are you trying to remove from the SEM column? Adsorbed water? Finger print oils? Hydrocarbons, esters, polyethers, silicones, or perfluoropolyethers from pump oils? All of the above? Methyl alcohol is OK for some but pretty useless for others on this list.

If you know what your contaminants might be, send me the list and I'll see what I can think of.

But please, two points are very important even at this stage:

1. If you intend to use methyl alcohol understand that its vapor is not good for you and the liquid is adsorbed through the skin.

2. Whatever solvent you end up using, remember if is works on your SEM column, it also works on its storage container and anything it came into contact with during manufacture. Use high quality solvents and don't store them in plastic squash bottles. To do so invites more crud to deposit on the surface as the alcohol evaporates.

Follow-up Question 1

Apparently the power failed during a run, leading to an inrush of air from the vent valve. This must have bubbled through the DP fluid, because I have small droplets of Santovac on the walls up in the column (the pump column which parallels the SEM column). These droplets are dense and large on the cold trap, and are more sparse and smaller as we proceed up the column through the gate valves and some minor coupling units. The final riser to the gun chamber is free of droplets, but must have been comprehensively filmed. More than backstreaming, unless what I am really seeing is the results of accumulated backstreaming of vapor which has slowly condensed into these lovely droplets. Perhaps vac plumbing should all be glass so this kind of thing could be observed in action. In any event, the column is wet, and will require a complete teardown, unless I can do something interesting like flood (fill) with alcohol to effect an in-place rinse? (acetone flood would deteriorate the o- rings, I believe) Anyone ever tried that?

Follow-up Answer 1

A foreline vent opening in a normal DP doesn't let gas go through the boiling liquid but it does the next best (worst?) thing. It blows oil vapor curtain up into the chamber. I suppose the only good thing I can think of is, the increased pressure immediately raised the Santovac's boiling point. So boiling ceased instantly.

I suspect the droplets you see are the result of coalescence on heavily oil contaminated surfaces. Every surface has to be thoroughly cleaned.

Can you fill the column with solvent? Maybe.... but I definitely wouldn't. First, the o-rings will get loaded with whatever solvent you use (including alcohols). But more important, I strongly doubt you will be able to remove all the oil from the nooks and crannies (notice my use of precise vacuum technology terms) where the column parts mate at the o-ring seals. As the solvent is emptied out, oil streaks will form as the solvent evaporate. This will probably happen for many washings and you won't be able to detect when it doesn't.

My suggestion, and I recognize you don't want to hear this, is:

1. Strip the SEM column completely
2. Water/Alconox detergent wash every part
3. Vapor degrease all parts, solvent wash only as a last resort
4. Dry each component in a vacuum oven to 120°C
5. Re-assemble in dust free area; using gloves with no talc, lint, or plasticizers; and replace ALL the o-rings with new ones (getting those o-rings may actually be your biggest challenge).

If you use a professional vapor degreasing service make sure they use something like Lenium ES (which is environmentally much more friendly than trichlorethylene). To choose a solvent for washing, I'd do a series of tests. I'd take a few test tubes and put a small drop of Santovac 5 in each. I'd cover the drop with a cc or two of solvent. For the first test, I'd shake the mix. If the Santovac drop disappeared, meaning it and the solvent are miscible, I'd do a second test using the same solvent but letting the mixture stand. I'd want to see how quickly the drop dissolved. After I'd tried a number of solvents I'd choose the one that dissolved quickest in the static test.

As for glass vacuum systems, I cut my eye teeth on them and I'm very glad to have left them far behind. Remember, contamination levels that are intolerable are one to several monolayers thick. For a clear liquid like Santovac, you just can't see a film several monolayers thick

Follow-up Question 2

Sigh. I suspect you are absolutely correct about the immersion / flood technique. Unless it was a hot, pressurized, turbulent pump-through, it would certainly leave residue. And the rings would suffer. Speaking of rings, as long as they retain their elasticity and are not solvent soaked, why not re-use them? They will have a light film of grease anyway, and any residue of pump oil remaining after a detergent / water wash would be sealed in, in a sense. I've even heard of pump oil being used specifically to lube rings - does that strike you as an acceptable practice?

Follow-up Answer 2

As long as the oil used to lubricate the o-rings has a low vapor pressure (in the same range as a grease) then... no, I have no problem with it. But who knows what is coating your o-rings? The VP of fresh Santovac is excellent. Vented air into boiling Santovac and who knows what chemical cracking that caused? Maybe none, but do you want to risk taking the SEM apart for a second time when the pressure is still too high for successful operation?

Follow-up Question 3

What's the benefit of vapor degrease vs the toothbrush in a pan of solvent followed by rinse in technical grade technique?

Follow-up Answer 3

We're talking about trying to remove stuff down to the molecular layer level here! A toothbrush and pan of solvent simply spreads the crap around. Any physical rinsing you do can only involve a tiny fraction of the solvent volume a vapor degreaser 'uses' in a few minutes. And remember, by definition, the vapor in a vapor degreaser is freshly distilled and therefore free of higher molecular weight crud even though the liquid volume may be loaded with it.

Follow-up Question 4

If, as you suggest, the oil drops built up slowly over quite a period, I suppose that would argue the cold trap was not functioning, or was so fully saturated itself it was no longer functional.

Follow-up Answer 4

No, I didn't say the 'contamination' built up over a long time. From your description of the vent problem, I think the major portion was quick. The air blew the DP's oil curtain into the column. To coalesce into drops may have been slower, but not days or weeks. Think about a bathroom window on a cold day when someone's taking a shower. The window mists quickly and droplets take time to form, but not days.

Cold traps function only if two conditions are met. (1) they are kept permanently cold AT ALL TIMES and (2) the pressure surrounding the cold surface is in free molecular flow at all times. I have no idea whether your trap met condition (1) but I know when your system vented condition (2) went out the window.

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Assembling Quick Connect Couplings

Question

Re: Your part number F0275XVC025, CF to Quick Connect Coupling

This coupling is made out of 4 sub-parts:

1- a flange with shaft
2- a knurled nut
3- an O-ring
4- an O-ring receptacle

Should the O-ring press up against the flat surface of the shaft's portion of the flange, or up against the inner flat surface of the knurled nut?

Answer

When assembling o-ring seals, one thing to avoid is capturing the o- ring between one stationary and one rotating flat surfaces. This is almost guarantees to add undesirable shear forces to the desired compression forces.

So, put the o-ring under the part you are calling the o-ring receptacle and let the flat of the knurled nut tighten down on the receptacle.

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