Reasons to be careful

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At sea level the air pressure is 15PSI. Every 2" of vacuum is equivalent to a 1 PSI drop in pressure. So at what point do you consider a vacuum a vacuum? And how does a gasoline engine operate when its intake manifold is in a vacuum if there was no oxygen? IOW's how many inches of vacuum required before all oxygen is boiled off, and that includes the oxygen from moisture.

An overheated bearing would have to be above 425 degF as I recall to burn wood. A spark is a different animal. A spark hitting a muffler packed with steel wool sets it on fire which sets the sawdust on fire and minor implosion in a vacuum system, and hey! It was my imagination so don't be a party pooper, K?
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On 2/24/2016 8:06 PM, OFWW wrote:

Well hopefully there is no combustion inside of the manifold and having said that, typically the fuel is combusted when it is under higher than atmospheric pressure. And engines work better when the fuel bypasses an intake manifold altogether and is atomized, by an injector, at the optimum location and time inside the cylinder. BUT YES you need a vacuum inside the manifold to draw fuel into the heads and cylinders from a throttle body or carburetor. IIRC sometimes up to 15~30 lbs of vacuum in the manifold at idle, considerably less during full throttle with the butterflies fully open.

And I am not trying to argue with you at all here but there are a lot other considerations to worry about that can make your DC combust aside from static spark.
I cant tell you how many times I have cut through a finishing nail with my TS. Surely there was a really hot spark that traveled into the collector hose.
AND with marginal powered table saws or those with dull blades it is not at all uncommon, at times, for the wood to actually get hot enough to smolder and for the dust from that to go into the DC.
I totally believe the static spark thing is a threat with any DC if the DC is used to clean up something other than wood saw dust. I think the caution labels are a blanket statement for what ever the DC might be used for. Maybe some one uses them to clean up grain elevators. ;~) I know I use mine to suck up anything that is on my shop floor including my son's hair when my wife cuts his hair. Yeah is is 28 but sometimes this is the only way we get to see him. LOL
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wrote:

15-30lbs.?? Atmospheric pressure is only ~14psi.

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On 2/24/2016 9:18 PM, krw wrote:

Vacuum, not pressure.

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wrote:

Exactly the problem. You can't pull a vacuum higher than the outside atmospheric pressure. That is, once you suck all the air out of the room (14psi), you can't suck any more. Well, The Donald can, but...

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On 2/25/2016 9:00 PM, krw wrote:

I think he meant inches of Hg, a common measure on a vacuum gauge. 29.92 is perfect vacuum at 1 atmosphere.
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On 2/25/2016 8:21 PM, Ed Pawlowski wrote:

Yes! Precisely, I was mistaken. Inches not PSI, although the actual PSI is half the reading on the gauge.
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On 2/25/2016 8:00 PM, krw wrote:

OK, My mistake, not PSI, inches of vacuum. I was trying to recall what the vacuum gauge on my first car indicated. I thought it was between 15~30 "inches".. apparently inches on the gauges are double actual PSI.
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wrote:

know if you were talking about in.Hg, or what. Yes, PSI is half in.Hg (though I hadn't thought of it that way) by the simple fact that 1ATM = ~14PSI = ~30"Hg.
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1ATM = 14 PSI = 0" Hg
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On 02/27/2016 12:48 AM, OFWW wrote:

You've mixed reference -- 1 atm is ~760 mm Hg (29.9"), 0" Hg _gauge_ pressure relative to atmospheric. Negative is then vacuum, positive is pressure like 28 psi for tires is 28 psig(auge) or 28 psi _above_ atmospheric.
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On 02/27/2016 8:17 AM, dpb wrote: ...

Or, when in "inches (mm) vacuum" then the negative sign is discarded but numeric value is same of course.
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Possibly, I didn't verify what ATM stood for, just accepted it as one Atmosphere at sea level. I almost made the mistake of typing psig but caught myself since that would be 14 above atmospheric pressure.
Now 0" is the equivalent to the air pressure at sea level. And yes, it is gauged vacuum. since everything above 30" is actually positive pressure.
The air pressure at 20,000 feet is 6.8 Psi equivalent to 13.6" hg. And yes, there is oxygen at that level. Maybe not enough to keep us functional, but it is still there.
So if you want to dry out wood quickly run it up the mountain it would be less expensive than pulling a vacuum on it. ;)
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On 02/27/2016 12:32 PM, OFWW wrote:

But you can't drop the reference from the units-- 0" Hg _vacuum_ is equivalent, yes, but _NOT_ just 0" Hg. The units are important, not just the number.
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True, but I don't 0" should be understood as 14 psi since I know of no zero measurement for board feet, etc. Since any identifying mark would be larger than zero. Or maybe I am just used to say 0" as meaning no vacuum in my old trade.
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On 02/27/2016 1:24 PM, OFWW wrote:

If you don't say it's 0" vacuum, then it isn't a complete specification. Granted, if one is working within an environment where vacuum is the norm it'll become a shorthand and will be understood _in_that_environment_. But, it's still a shorthand and isn't precise without the qualifier.
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If you say so, "0" is neither pressure nor vacuum.
So, we can extend that out to making sure one specifies PSIA or PSIG in all instances, and as there is no absolutes in vacuums it will need to be specified in PSI? or PSI? if you know what I mean.
An interesting time wasting discussion. Along these lines I have thought about the possibility of creating a near impossible large diameter pipe. With the right diameter inside, and if it was created with and interlock say 100 ft from its base. And a large door at its base, one could put something inside, open the interlock and open another door lower than the object and launch the object into space with no energy consumed.
The only drawback would be if someone opened the door and left it open all of the earths atmosphere would be sucked into space.
So that will be the last of my craziness for now. :)
Of interesting note for this topic, I found an addon for sketchup which works with both Pro and Make.
It is like some add on libraries for AutoCAD and my understanding is that it is free. There is an offline version, for downloading.
It has softwood libraries with the "finished " sizes, S4S, it has HVAC and ductwork fittings and stuff.
It has various piping fittings for plastic, steel, copper, etc.
http://sketchup.engineeringtoolbox.com/
Multi language
The wood sizing list is to me a bit on the weak side, but at least on designs if you were to say a 4x4 it would give you the real s4s measurements.
Could be valuable for vacuum piping design.
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Right. Context is everything. One has to mentally add (or subtract) the absolute reference if the context requires it.
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On 02/27/2016 1:24 PM, OFWW wrote: ...

Yes, apparently you are. That, as noted above, would be a shorthand within that context that would be understood but is incomplete and potentially misunderstood if translated to another context wherein the missing reference units would not be automagically inferred.
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On 02/27/2016 1:24 PM, OFWW wrote:

0 "Hg Vacuum _is_ 14.7 psia by definition. It has to be else't the weather measurements for barometric pressure would have to 0" plus/minus instead of 30" plus/minus with fluctuations...and that isn't so.
Or another way to say it which I know you know innately is that no vacuum is one atmosphere pressure.
You conflate two things of different relative measurements by comparing to bd-ft (and yes, I know you were trying to somehow get it back to w-working and that's a noble attempt) but there can be 0 bd-ft; you just don't have any wood. But the difference is that you can't have negative board-feet but you can have negative pressure or temperature or any of several other physical measurements. It is simply what the definition of the units reference point is.
Consider temperature, there are at multiple common scales, one of which is absolute and the two other of most familiarity each have positive and negative values but their zeros are at different absolute temperatures. For pressure, simply for convenience, units were developed historically from a mean atmospheric pressure and hence, are positive and negative around that value. That was purely convenience as it's nice when doing the ordinary things like tire pressure, etc., to not have to worry about that background 14.7 psi. Similarly for vacuum work, it was/is convenient to define a new zero at that point and measure how far away from that point one is and in fact "inches of vacuum" throws away the negative sign and speaks of less as more...kinda' like scoring in golf! :)
So, currently barometric pressure is heading up and is about 30" Hg, _not_ 15!!! :)
<http://www.weatherlink.com/user/dpboz
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