Makeing an device that monitors air in tube, simple question: - Page 1
Forum section: electronics / general

author: Beamin
author_rank: Super Contributor
timestamp: July 11, 2018, 08:29:23 pm
content: I'm making a electronic device that's like an air sensor for a bunch of things kind of a universal application.First though if I have say 15psi/1bar in a sealed contiainer of air does that mean each cubic inch of air actually has 15 lbs of air in it since the air is just sitting and not being forced through a tube or manipulated in any way? And we just don't notice how heavy the air is because it is bouyant? I know from flying airplanes air is really thick. If you put a ci of air in a plastic box in a vacuum and pumped the air out would the scale on the bottom of the inside of the vacuum read 15lbs+ weight of box? That doesn't seem right. Or is that really just a measure of thermal energy and could be a very small mass of air?

author: CopperCone
author_rank: Super Contributor
timestamp: July 11, 2018, 08:35:51 pm
content: read gas laws, one square inch of the container will experience an equivalent of 15 foot pounds of force.The gas will expand to a nebula or such if it is free from gravity. If you compress it, there is repulsion between the molecules which manifests itself on a force.pv=nrtpressurevolumenumber of molesgas constanttemperaturewith air you have pressure (in metric) * volume (metric), R constant and temperature, if you solve for N you will get moles.So long its in a gas phase and not something weird like a triple point or whatever, you take the number of moles, then you use the ratio of the atmospheric gasses which is roughly 70/30 nitrogen/oxygen to calculate what the wegiht is.This is only true for static gasses and I don't know how much it will deviate from heavy gases, gases in motion, etc (fluid dynamics)

author: Beamin
author_rank: Super Contributor
timestamp: July 14, 2018, 11:20:04 pm
content: That's right PV=nRT my memory fades in and out.r is 0.06020 or something like that. A mol of air is the average of the n2 and o2 or the combined no of atoms? No wonder why I shied away from thermodynamics in school. Maybe I'll just run a hot wire and copy a mass air flow sensor, problem is it doesn't do much when the air flow is weak....or we will toss this in the half baked projects box.

author: CopperCone
author_rank: Super Contributor
timestamp: July 15, 2018, 01:50:52 am
content: a mole is a unit that you use 6.02^10^23 atoms.your equation gives you the answer in moles. You multiple it to see how many atoms you have in total. its like how many red and green balls there are in a box not counting trace gasses. each atom weighs differentIt is just a standard chosen to base equations around, like the kilogram, but with number of atoms instead of mass.there are trace gases and probobly small amounts of isotopes but thats usually irrelevant. You will also have some dust particles, fungal spores and maybe water aerosol and stuff, but again it sounds irrelevant towards your work.The pump will probably have lubricant and also it must shed some of its body as it works because there is friction. Unless you are running a chromotograph or other very sensitive instruments these won't typically be issues.There is industrial standards for 'clean dry air' , they focus on humidity but you should have mechanical filters too. These standards will illuminate you on what might actually be in your air.Very nasty air might destroy pneumatic tools and cause clogging, humidity may cause corrosion, and you can possibly get cavitation like effects happen that can wear down air motors if they get condensate in them because they spin very fast.Another 'typical' effect where you care about air quality would be for using compressed air in a plasma cutter. You can ready industrial literature all about wear and tare on plasma cutter heads and cut quality being effected by the quality of the air that you supply them with. If you pull air from a bad location you might get other contaminates like stuff being emitted by soil, radon, etc. For instance if you have a poorly maintained nasty ass compressor room that is added as an after thought and not finished well you might end up with funky air. I have seen this before, but it would only really effect sensitive things which would typically be well filtered anyway.I mean like the kind of place where you wonder why the fuck they don't pressure wash it and add a dehumidifier. Certain installations may spit semen like foamy discharges out of the air lines (not kidding). Air intake location matters too, like don't put your intake near by a cooling tower, sprinklered lawn, etc. Common sense shit.

author: IanB
author_rank: Super Contributor
timestamp: July 15, 2018, 02:12:59 am
content: Quote from: Beamin on July 11, 2018, 08:29:23 pmPsi is "pounds per square inch", not "pounds per cubic inch". So, no. Your other problem is that psi is "pounds (force) per square inch", not "pounds (mass) per cubic inch". Pounds (mass) per cubic inch is density, and the density of air at normal atmospheric pressure is about 0.0000443 pounds per cubic inch, or about 0.000708 ounces per cubic inch. That is to say, not a lot. Air is lighter than a feather.A pressure of 1 psi means that each square inch of containing surface experiences a force of 1 lbfacting on it.

author: Brumby
author_rank: Unknown
timestamp: July 15, 2018, 06:55:54 am
content: The standard air pressure of 14.7 lbs per square inch at sea level, does indeed come from the weight of the air in a particular volume.That volume is shaped like a square pyramid and turned upside down. The apex point would be at the centre of the earth. At the surface of the earth it would have a square section of 1" by 1". At 4,000 miles altitude, it would have a square section of 2" by 2".The volume we are interested in is that which extends from the surface of the Earth upwards into space - or at least until the atmosphere is essentially now a vacuum.The weight of that air pressing down on that one square inch at the surface is 14.7 pounds (Give or take variations due to weather.)

author: Brumby
author_rank: Unknown
timestamp: July 15, 2018, 07:08:33 am
content: There is a distinction here of which you should be aware. "Mass" and "weight" arenotthe same thing.Mass is a quantity that defines the amount of "stuff" an object has. It never changes - unless you add to or subtract from it. An object has the same mass whether it is on the surface of the Earth, in orbit or on the Moon.Weight is the force that acts on a mass due to gravity. The weight of an object will vary and will have distinctly different values when measured on the surface of the Earth, in orbit or on the Moon.

author: Beamin
author_rank: Super Contributor
timestamp: July 20, 2018, 07:03:31 pm
content: Quote from: Brumby on July 15, 2018, 06:55:54 amThat diagram is almost to scale too!I must have been tired when I was trying to figure this out so many rookie mistakes like PSI and cubic inches have nothing to do with each other. DOH! Like how some people think that a car engine works by pushing down pistons rather then air pressure expanding in all directions. At a hobby shop I kept buying glow plugs and the guy that worked there kept arguing with me that the force of higher nitro fuel was pushing up on the element and breaking the wire. No matter how many times I explained to him how air pressure works in all directions equally he kept insisting that it pushes up on the wire, breaking it. Maybe that's the reason why he worked there and wasn't in a more technical job. "Well I don't know nuthin' 'bout that but let me tell you how it works anyways." No.Unfinished projects pile it goes! Why can't you combine two half done projects into one working one? Because the second half is 90% of the work.

author: IanB
author_rank: Super Contributor
timestamp: July 21, 2018, 01:29:17 am
content: Quote from: Beamin on July 20, 2018, 07:03:31 pmWell, maybe he was right? Many times we may think we know what to expect, but when we do an experiment it confounds our expectations. I certainly wouldn't be quick to say he was wrong. Better to hold judgment until there is proof either way.

author: Beamin
author_rank: Super Contributor
timestamp: July 21, 2018, 03:12:54 am
content: Quote from: IanB on July 21, 2018, 01:29:17 amNo air pressure pushes on all sides at once. It pushes it down up and side ways all equally. The problem was using a heat gun to heat the block to aid in cold starts. The wire would absorb the heat faster and snap. A hair dryer or careful use of the heat gun worked every time. I also asked him if the glow plug had a dual role since it was platinum. He said no but that wasn't true. A catalyst is needed to ignite the methanol at low pressures and temps. I tested this with just a wire element and would not run as soon as the plug stopped glowing when the current was removed. Wonder if you could start it with acetylene and no hot glow plug since acetylene will detonate.

author: IanB
author_rank: Super Contributor
timestamp: July 21, 2018, 07:57:54 am
content: Quote from: Beamin on July 21, 2018, 03:12:54 amIf this is true, how does an airplane fly? If the air pushes on all sides of the wing equally, how is there an upward force to life the plane off the ground?

author: AndyC_772
author_rank: Super Contributor
timestamp: July 21, 2018, 08:10:45 am
content: There isn't, until the plane is moving. Planes don't just spontaneously rise into the air off the runway; you have to actively take off, using an engine.The upward force is due to the aerofoil shape of the wings. The shape of the wing forces air to move faster over the upper surface than the lower one, and this means the pressure on the top of the wing becomes lower than on the bottom. The pressure difference means there is a net upward force on the wing, which we call 'lift'.

author: IanB
author_rank: Super Contributor
timestamp: July 21, 2018, 08:57:34 am
content: Quote from: AndyC_772 on July 21, 2018, 08:10:45 amMy question was to Beamin...

author: G7PSK
author_rank: Super Contributor
timestamp: July 21, 2018, 09:35:05 am
content: Quote from: Beamin on July 21, 2018, 03:12:54 am

author: Dubbie
author_rank: Unknown
timestamp: July 21, 2018, 10:22:18 am
content: Quote from: AndyC_772 on July 21, 2018, 08:10:45 amThis is not quite right.A couple of questions to make you think.How do model planes with wings made of perfectly flat balsa wood manage to fly just fine?Secondly, how is it possible for some planes to fly inverted for as long as the pilot has the stomach?The main reason planes have lift is because their wings redirect air downwards when the wings are angled at an appropriate angle of attack. Newton’s 3rd law means that because tons of air is being redirected down, then there is a corresponding tons of lift (for a large plane) The Bernoulli effect counts for a very small portion of the lift.The shape of a wing is designed so that the air is as efficiently and as smoothly as possible redirected downwards in a laminar flow, without entering a turbulent state. Turbulent air swirls about and doesn’t redirect, so your plane falls out of the sky.

author: Beamin
author_rank: Super Contributor
timestamp: July 23, 2018, 04:12:40 am
content: Quote from: AndyC_772 on July 21, 2018, 08:10:45 amThat's not actually true hence why planes can fly upside down and have symmetrical airfoils. Most of the lift comes from angle of attack. There is a good video on YouTube, somewhere, that shows the air foil shape and Bernoulli's principle alone is not enough to make an airplane fly. When I was a kid I built a flying wing with two rigid tails no rudder, big ass engine in front, and a symmetrical wing from foam. Everything is done by aileron mixing. The wings are normally slanted up or the elevator is up slightly to produce level flight. A 747 was once flow upside down in a test flight.

author: Dubbie
author_rank: Unknown
timestamp: July 23, 2018, 04:35:29 am
content: Quote from: Beamin on July 23, 2018, 04:12:40 amThanks for paraphrasing my reply almost line for line

author: Beamin
author_rank: Super Contributor
timestamp: July 23, 2018, 04:43:34 am
content: Quote from: Dubbie on July 23, 2018, 04:35:29 amLooks like I clicked hit reply before seeing your post. Maybe I should heed that red warning more often. At least your post now has multi member support!

author: IanB
author_rank: Super Contributor
timestamp: July 23, 2018, 05:25:57 am
content: Quote from: Dubbie on July 21, 2018, 10:22:18 amIt is not the main reason planes have lift, it is theonlyreason planes have lift. The acceleration force of the air being redirected downwards is exactly equal to the upward force on the plane. This is true for all winged aircraft, large or small.QuoteThe Bernoulli effect is not applicable to wings, it only applies to fluids flowing inside enclosed ducts. So it does not count at all.Fun fact: the lift on an airplane is exactly equal to the reaction force of the air being redirected downwards. It isalsoexactly equal to the integrated difference in pressure acting on the lower and upper surfaces of the wings.

author: IanB
author_rank: Super Contributor
timestamp: July 23, 2018, 05:32:01 am
content: Quote from: Beamin on July 21, 2018, 03:12:54 amSince planes can fly, this disproves your assertion. The air pushes harder on the underside of the wing than on the top of the wing.Similar disproof is rendered by hurricanes and tornadoes that can cause considerable damage to structures in their path. If the air pushed equally on all sides of a building a hurricane would be harmless.In fact, air pressure acting equally in all directions is true only in the restricted case of static systems where there is no movement and no change with time. The combustion inside the cylinder of an engine is very much not static, it is highly dynamic. There is an explosion taking place, and as you know explosions can destroy things. You yourself used the word "detonation" in one of your posts above.