Gauge
Posted: Mon Dec 13, 2004 5:36 pm
Atm.pressure doubles when 30 feet under water. . . hum then this gauge is under water ??? 
not quite
The Dictionary, physics books, Fluids books and international standards committee bodies define Vacuum..
Vacuum =Absence matter., a space empty of matter, a state of emptiness or void., void of gases, By definition......Pressure in a vacuum is deined as absolute Zero..... same term used for absolute temperature temperature coldness..... (yes the two are related), American Heritage, Websters, # Fluids books, Hyper dictionary,
Your "in a nutshell comment" is technically incorrect and based on a common misconception, similar to the misundrestanding between centrifugal force and centripetal acceleration, a very common misunderstanding.
Pressures within Aircraft intake manifolds are referenced to theoretical absolute O pressure
You may be referencing common differentail pressure instruments,commonly used boost pressure measurement in SC's or flow meters..pumps etc....
I am a an engineer pilot, have flown, and worked on recip and turbine aircraft, and have been on instrumentation standards working groups. As well as having over 20 years getting paid in the design and modification of systems of instruments and controls.
Enginering pressure measurements used are identified as Moe has detailed...(mostly wink wink
)Turbine engines also measure stage pressures as differentials and referenced to absolute zero, as Moe Identified except......
You generally wont find "manifold Vacuum" in any engineering text, physics book, or standard. I have seen it in sales literature, catalog cut sheets, and discussions of diferential pressure, but it is not a unit of measurement or metric. Ambient and absolute pressures are used very frequently in service manuals and pressure curves performance data sheets etc...lol...
his "Manifold Vacuum Gauge" is also somtimes called suction pressure...
9I know, aan oxymoron in itself and incorrect) is is very common to display on the inlet side of centrifugal pumps.(the differential pressure across the pump).. The reason for this common instrumentation preference is to determine where on the preformance curve a particular pump is operating... for example in a large electrical engine that has protections for overcurrent and overspeed.... And the utilization of one differential instrument introduces less uncertainty into the actual differential measurement than using two separate gauges referenced to a relative pressure.
and your statement
"Why on earth would you ever have positive pressure inside an intake manifold (while the engine is running properly" is not technically correct.
Pressure (inches of mercury, inches of water, lbs, per square inch, pascals etc).. are/is a unit measurement. It has no vector value until the frame of analysis, or in thermal dynamics terms...envelope, etc is identified. When pressures are given with no definition of frame, the assumption is ALWAYS without exception referenced (By adoption of standards committes) to absolute vacuum, Absolute zero presssure... eg....as in the case of barometric pressure....29.92 in HG (inches of mercury) you can see in a mercury barometer....for example, your statement would only be technically more accurate if you said- lower pressure referenced to ambient, or X inches of differential pressure.
another example of a similiar standard reference frame is highway speed limits. , Typical temperature units are not.... since the theoretical basis of O deg Kelvin was not reproducible until we had good measurement devices for pressure and temperature at low pressures.. and gabriel Fahrenheit was one of the first ones to document his measurement scale....lol , and we are still stuck with it 400 years later.
Vacuum is not "negative pressure in relation to ambient atmospheric " It is is defined even in the dictionaires.....as absence or lack.
But not to worry, many people make that misconception and commonly refer to a pressure lower than ambient as a vaccum,
This does not make it correct tho.
and yes I do know something about venturiis, partial pressures and the like..lol and know the equations regarding static and dynamic pressures, weight of the gas, Isotropic props, and velocities. differential pressures and physical geometry of the both sides of the (throttles, chokes, orifices... etc. )venturii control the velocity of the stream of intake air and fuel metered into the intake manifold.
and yes I do know how airplanes fly....lol I have helped cut Foam for wing planiforms...and helped build aircraft ( Joining the EAA Experimental aircraft association was kick And tho admittely I have only owned a couple of aircraft.... For aircraft fly because they have sufficient air flow crossing the lifting surfaces in the proper direction.)
Ina carb BTW Fluid is pushed out of the jets by ambient pressure in the fuel bowl, becasue the static pressure of the gas (inlet air) is lower than the static pressure in the bowl. a gas with velocity has defferent physical properties (Static and dynamic pressure) than a gas at rest at the same temp. The Fuel is not sucked. I know I know..theres that engineering thing again......
And While I agree that there are many ways to be taught to fly (theoretical physics isnot a strong element of ground school)...... a lot more is taught in 4 to 5 years of an engineering education than what I learned in my ground schools.... and correcting wording errors in class was a lot of fun for one of my instructors..... A cal poly Physics prof who lliked to fly...lol
As I read your discussion elements I understand what you are trying to say, but because of your choice not to use the standard definitions and terms that the world has accepted for these engineering elements, puts you at a bit of a disadvantage to trying to convince others. I am not saying engineers are smarter or wiser or the only ones that know, However they hav adopted a standardized vocabulary and defintions for what something means..... and this allows the disccusion and the exchange of information with careful consideration to the metrics (repeatable measurements).
In all honesty, I felt Moe was trying to push his point here... rather harsh at times, true... but in general he is trying to get some standardization of terms so that the discussion can continue.. He is not implying anyone is any smarter than anyone else.. but the training is showing here. And as you note, without common terms and definitions, discourse is difficult..... (ie the board gybes.... etc...) (blue is faster)
Moe, Chip, mark, yourself, me etc in generally try clarify with our discussions, and if thator this seems like a personal attack I sincerely apologize since I dont think anyone here has the purpose to attack or terrorize... (and yes I have brain farted and been caught by moe also....... specifically because I was using a term different than what he was using on (Excitation Field current) so I have eaten my share of humble pies also....
and I guess I have to make one last comment on the phrase "rigid semantics "
That is what engineering and analyis is about.....the adoption of those rigid definitions is one of the major reasons science advances....(we build upon the work of those that have gone before us)....
So believe it or not, there are hundreds of thousands of engineers in the country who would love to find mistakes in the laws (not really laws but repeatable observations and metrics) of physics and thermodynamics... Could make any of us rich...... but we are saddened by the onerous undetectable fallability of these observations on our daily lives..
And I will argue on semantics with moe on a couple of points also here
the statement "When the throttle is backed off, manifold vacuum RISES" Again Vacuum, is defined at zero, , it not technically accurate to use the term manifold vacuum, I could go into other similiar errors in the assumptions......
for example comparing Constant speed props and someone using the term constant speed airplanes......and engines...
Again, you have the ability to change pitch on the CS prop on the aircraft and you cant fairly compare output power curves when the MAc does not have an adustable prop... 25 x 25 (25 inches absolute manifold pressure, and 2500 prop rpm) would be difficult to reproduce on the mac....lol.....
and cat's comments about why planes shake..........(Study of supersonic and hypersonic shcok waves on control and lifting surfaces sounds like a wonderful discussion on this thread lol.) and dimensionless numbers assoicated with Froude (that would take a few pages, ....) and I probably should not go into that here.
Also, since you are all concentrating on the motors, some of the statements of what is happening at full and partial throttles are not always the full story.
Prop physics can greatly unload the engine... You can reach high rpms on some of the smaller props because of the higher slip numbers, (One of the reasons the bigfoot outboards are poplular).. This is evident during acceleration especially when my engine is indicating 5200 rpm, for many seconds, but boat is not anywhere near the speed of 16 mph yet...... The ventilation of the prop ((relative decrease in water pressure on the leading edge of the prop) causes the formation of of even more bubbles (liquid water turning to gas) which allows the prop to slip more and spin faster (almost like lubricating the prop) ..with less of a load....
So, perhaps a little tighter boundary case definition on horespower, speed and coupling might be in order....lol and help the discussion......
So..... adieu for now.....
Damn it was cold on the water but beautiful saturday.....this weekend.......lol
And I would pick on Moe more.... but he is pretty careful in his statements....and it takes time to read through his entire comments hunting for the tech shortcomings....lol...... heheheheh
But us pickers of nits...... and performers of metrics be a careful bunch mostly, even though we do make mistakes when going from the math world to the spoken and written non tech descriptions....We do try to be accurate...
Enjoy
hmmmm7:49??