Vacuum Chamber (potential safety issue)

Mr. Skinny said:

Wow! 192 dB sound pressure level? That's impressive.
I don't really understand what you mean though (1 atm is 192 dB SPL). Is there any way you can simplify this statement for the untrained?

QUOTE]

SPL is measured in decibels referred to a pressure variation of 20 uPa, i.e., 0 dB SPL = 2 x 10 ^-5 Pa sound pressure.

Normal sea level atmospheric pressure runs around 101 kPa. Assuming that the sound pressure created by the failure of the glass in the vacuum chamber would be equal to the pressure difference between the outside and the inside, we would have a peak sound pressure of 101 kPa.

To turn that into a dB SPL figure, you use db SPL = 20 log (P1/Pref), or 20 log (1.01 x 10^5/2 x 10^-5).

I get approximately 194 dB SPL, or, in really technical terms, a freakin' loud noise, much, much louder than what a shooter firing a rifle or pistol is subjected to.

The consequences for your safety regime might be as simple as requiring that anyone present in the chamber room wear appropriate hearing protection, or they might be more serious.

I can't say. I agree wholeheartedly with jj- get an expert opinion from someone who assesses acoustic safety professionally.
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ktesibios said:
SPL is measured in decibels referred to a pressure variation of 20 uPa, i.e., 0 dB SPL = 2 x 10 ^-5 Pa sound pressure.

Normal sea level atmospheric pressure runs around 101 kPa. Assuming that the sound pressure created by the failure of the glass in the vacuum chamber would be equal to the pressure difference between the outside and the inside, we would have a peak sound pressure of 101 kPa.

To turn that into a dB SPL figure, you use db SPL = 20 log (P1/Pref), or 20 log (1.01 x 10^5/2 x 10^-5).

I get approximately 194 dB SPL, or, in really technical terms, a freakin' loud noise, much, much louder than what a shooter firing a rifle or pistol is subjected to.

The consequences for your safety regime might be as simple as requiring that anyone present in the chamber room wear appropriate hearing protection, or they might be more serious.

I can't say. I agree wholeheartedly with jj- get an expert opinion from someone who assesses acoustic safety professionally.
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Thanks, ktesbios. That explaination helps a bit.

I'll have to look into the hearing protection issue further, but if memory serves, even the best hearing protection only offers a noise reduction rating (NRR) of about 28-30 dBA. Even if you doubled them up, e.g. ear plugs with headset muffs over the top, I'm not sure it would be sufficient protection.

I'm not sure if NRR's are strictly additive when used in combination. I'm also not sure what NRR I'd need to prevent damage from an "instantaneous" event such as a window failure.

We have a (company internal) standard of 85 dBA over an eight hour time weighted average. While I'm sure the noise from this failure event wouldn't exceed that limit due to it's short duration, it certainly seems as though some damage could be done due to a sudden peak SPL like the one described.

I appreciate that everyone is recommending a professional acoustic expert, but for now I'll look at some less expensive methods of abating the hazard. If I get backed into a corner over technical issues, I'll definitely recommend one.
 
As an engineer, my first thought is "Hire a professional to evaluate this."

I have questions pertaining to the certification of this pressure vessel. Who inspected it? When? Is it up to date? What is the window rated for? When was it certified? Is it up to date?

That is aside from the analysis of the forces invovled.

So, if you don't have the expertise to evaluate this yourself, hire someone who does have that expertise. There is life and liability invovled, and it's always better to be safe than sorry.

If you're the guy who'se responsible for the project, and something goes wrong, you better have taken all reasonable precautions.

There's a large amount of energy in the volume of air you describe. Don't mess with it. Be sure. Things can get very bad, very fast.

Best of luck!
 
Badger said:
As an engineer, my first thought is "Hire a professional to evaluate this."

I have questions pertaining to the certification of this pressure vessel. Who inspected it? When? Is it up to date? What is the window rated for? When was it certified? Is it up to date?

That is aside from the analysis of the forces invovled.

So, if you don't have the expertise to evaluate this yourself, hire someone who does have that expertise. There is life and liability invovled, and it's always better to be safe than sorry.

If you're the guy who'se responsible for the project, and something goes wrong, you better have taken all reasonable precautions.

There's a large amount of energy in the volume of air you describe. Don't mess with it. Be sure. Things can get very bad, very fast.

Best of luck!
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There is no problem with the pressure vessel, badger. Among other things, I'm a certified boiler and pressure vessel inspector :) . Vacuum chambers are not considered "code" vessels though for the most part since code doesn't apply unless you pressurize above 15 psi. The window itself is not certified per se, other than through experience with viewports on vacuum chambers (which are quite common where I work).

I'm not responsible for the project, but I am responsible for evaluating the risk (severity/probability) and recommending what actions need to be taken to make the risk acceptable to management. Since I didn't have a good feel for what could happen, I posted here to get the opinions of the smart people here.

I've got a few months to gather information, and my decisions won't be rushed, I guarantee you.

Appreciate your input though. Thanks.
 
Anecdotal evidence:

As a PhD student, I had a roomate who did experiments using a similar sized chamber at lower pressure and with several windows. We never gave it a second thought. Two people had desks beside the chamber, people were in and out of that lab all the time.

But hey, we'd have all put our life on the line for the cool physics they were doing :) (which apparently culminated in the best measurement of the fine structure constant to that time....)
 
Mr. Skinny said:

Wow! 192 dB sound pressure level?
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Bleah! It's 194dB SPL for a 1 atmosphere RMS sound wave, of course, like K said.

That is, of course physically impossible, you can't get the negative part, but the peak "loudness" of your event at the window at the break can approach that "level", although it's going to be so nonlinear that calling it a "level" is a joke.

But you need a formal assessment. Get it!!!!
 
JSFolk said:
I'm no engineer, nor am I a doctor, nor did I sleep at a Holiday Inn last night, but I would think that if someone's face were right in the glass (or, say, if they were sitting on it) that a sudden failure of the glass would be ugly. I mean, how much pressure can it possibly take to suck someone's internal organs out?
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More than one atmosphere. Have you ever kissed a girl?
 
Pragmatist said:


Even 1 atm can be pretty catastrophic.

From: http://www.osha.gov/pls/imis/accidentsearch.accident_detail?id=14321087

Accident: 14321087 - Employee Killed When Vacuum Chamber Implodes

Employee #1 went into a vacuum chamber testing area to look through the porthole opening in a pressurized vacuum chamber and observe the condition of a nose cone that was being tested. The 12 in. diameter glass in the porthole opening imploded, causing Employee #1's head and one arm to be pulled into the vacuum chamber. He died of crushing head injuries and asphyxia.

That was a 12 inch port of course. What worries me more about the case in question (which is a 4 inch port), is the shock wave AFTER the initial implosion. I suspect that porthole will act pretty much like a small cannon when the outgoing shock wave hits. It's a pretty large chamber, and there will be quite a bit of energy from the inrush of air which will have to go somewhere afterwards.

Although maybe the relatively small hole will slow down the inrush somewhat enough to damp some of the shock wave. Don't know. But personally I wouldn't like to be anywhere near it! :)
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And what exactly is a "pressurized vacuum chamber"? Off with her head!!
 
ktesibios said:
Mr. Skinny said:

Wow! 192 dB sound pressure level? That's impressive.
I don't really understand what you mean though (1 atm is 192 dB SPL). Is there any way you can simplify this statement for the untrained?

QUOTE]

SPL is measured in decibels referred to a pressure variation of 20 uPa, i.e., 0 dB SPL = 2 x 10 ^-5 Pa sound pressure.

Normal sea level atmospheric pressure runs around 101 kPa. Assuming that the sound pressure created by the failure of the glass in the vacuum chamber would be equal to the pressure difference between the outside and the inside, we would have a peak sound pressure of 101 kPa.

To turn that into a dB SPL figure, you use db SPL = 20 log (P1/Pref), or 20 log (1.01 x 10^5/2 x 10^-5).

I get approximately 194 dB SPL, or, in really technical terms, a freakin' loud noise, much, much louder than what a shooter firing a rifle or pistol is subjected to.

The consequences for your safety regime might be as simple as requiring that anyone present in the chamber room wear appropriate hearing protection, or they might be more serious.

I can't say. I agree wholeheartedly with jj- get an expert opinion from someone who assesses acoustic safety professionally.
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So where would children's party balloons bursting rate on that scale?
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So, is the issue the SPL difference between a very hard vacuum and atmosphere, or the amount of sound created by atmospheric air being drawn into the chamber through an irregular opening?

True, if you situated your head in such a way that one side of your eardrum was in the atmosphere, and the other was in the hard vacuum, you'd have this incredible pressure. Granted. But I don't think that is the situation being presented here.

I would expect a rupture of the glass to create a BANG!, followed by possibly a shriek, as air rushed into the chamber. As the pressure in the chamber increases the tenor and intensity of the sound would change, generally becoming less intense with time.

I don't know how to calculate the amount of sound the rushing air could make, perhaps there are ways of doing so. But it is NOT the same as the 192-194 dB of the total pressure difference.
 
That's kind of how I had envisioned it too garys2k, but I felt uneasy about assuming that.

I don't think the 192-194 dB figure is a prediction, but rather just a theoretical maximum, albeit one that deserves consideration (I mean, that's LOUD!)

And for TeaBag; I'd make a SWAG and say a childs party baloon bursting might be about 90-92 dB. It depends on the size of the baloon, how much pressure it contains, blah, blah....... (jj probably has a much better idea). :)
 
I would add my assent to jj's and others advice about consulting an expert. An acoustician will model the open space in the room and the chamber volume and all kinds of other factors ( reflectivity, structural materials in the room...) One thing tho the 192 db SPL is correct except that one of the most important factors -distance, is not taken into account. Normally acoustic measurements include relative distances from the source. The 192db figure represents the absolute maximum value of an ideal event a the point of origin.

Some other common measurements........130-150 dB SPL @ 100 feet behind airplane jet engine, train 70db SPL @ 100ft, pain 140db SPL. So the room will be plotted to give relative db SPL ( sound falloff, 6db per doubling of distance for a point source, the inverse square law) at specific points across the room or catwalk.
There's probably some pertinent OSHA standards as well you will have to consider.

Another thing to mention , can you retrofit the window with a hi density molecular plastic like lexan? Less of a chance of a catastrophic failure then even reinforced glass.
 
Mr. Skinny,

I just want to know what you need a vacuum chamber for at Burger King?
 
Till ,

I fully appreciate the advice you and others have given me to consult an acoustics expert. I will certainly do that it if it becomes necessary.

However, I currently still have the option of doing the data collection from a location that is remote from the catwalk. For example, I can require that personnel must be in a lightly constructed control room some 50+ feet away from the window, and could just as easily require them to be on the other side of a 6 foot thick reinforced concrete wall (in the next bay over.......also about 50+ feet away).

Taking one of these options seems less expensive than hiring an acoustics expert, though more expensive than doing data collection from the catwalk.

As for the window, it has to have certain transparency characteristics that will allow a laser beam of certain wavelength, etc. to pass. I leave that to the laser experts.

As to OSHA, there are no standards relating to vacuum chambers/windows to my knowledge.
 
Someone may have mentioned this already but a 4" obsevation window pretty much means ones nose has to be very near the thing in order to observe. Not a good situation I think. Maybe you should duct tape some steel armor over the window. :D

[edit to add] Oh lasers... umm never mind then.
 
Blue Monk said:
Mr. Skinny,

I just want to know what you need a vacuum chamber for at Burger King?
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Employee cranial cavity evacuation; liposuction to supply the deep fryers; general dry and wet vacuuming of the store, you know, that sorta thing.

You act like you never worked in a fast food joint, Blue Monk.
 
Mr. Skinny said:

Employee cranial cavity evacuation; liposuction to supply the deep fryers; general dry and wet vacuuming of the store, you know, that sorta thing.

You act like you never worked in a fast food joint, Blue Monk.
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Hey, that's a cheap shot.

I'm only 49. I'm sure I'll be moving up anyday now.
 
Mr. Skinny said:
As to OSHA, there are no standards relating to vacuum chambers/windows to my knowledge.
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There are, however, acoustic noise limits in OSHA regs.
 
garys_2k said:
True, if you situated your head in such a way that one side of your eardrum was in the atmosphere, and the other was in the hard vacuum, you'd have this incredible pressure.
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However, as you move away, this level will drop by distance. AT this kind of level, nothing is going to be remotely linear, and you need an expert who deals with shock waves, etc, to cope, rather than a simple acoustician.

The size of the room around it also matters, it will determine the mid-time drop in pressure before the outside room equalizes. It will also put some PSI load (without more specifics than I have, or want) on those walls temporarily, might pull in a window or something there if it's not a great big space around it.

In general, I think it's more dangerous than people assume, and I again repeat my suggestion to get a real expert opinion.

Consider, if the room has 13.7 times the volume of the vacuum chamber, you'll have a temporary 1 PSI drop until the room leaks back up. 1 PSI is, lessee, um, 144 lbs/sq foot. (cough) That also works out to about 170db SPL, although probably at a low frequency, not at midband ear sensitivity... This port is 4"? I haven't figured out the Helmholtz resonance, but it's going to be low, you may get quite a few (low frequency) oscillations as it fills up, too...

GET AN EXPERT OPINION DUDE!
 
jj said:


There are, however, acoustic noise limits in OSHA regs.
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Gotcha, 90 dBA Time Weighted Average, or PEL if you prefer.

Hey jj, what's your SWAG about the exposure to people in the area? Will the instantaneous noise level be so loud it will burst eardrums, even with hearing protection, or will it be a bang and a "noisy" rush of air.

You can PM me if you don't want to SWAG this in public.

Thanks.

Edited to add: Just saw the above post jj. Thanks. That's the kinda SWAG I'm looking for.
 
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