C.F.M

Try Boyle's or Charle's law for ideal gases...

PV/T = P'V'/T'

A sufficiently large storage tank would result in no difference. The temperature of the stored air is what degrades the airflow.

RchH

As Electroman said for a closed system that looses no heat the Boyle's law applies but keep in mind the temperatures are in °K.

Also forget about using CFM you are mixing SI and imperial systems so stick with liters or m³.

I do this for a living. One of the most important things you need to do is define where you are measuring flow. Boyle's Law and the Ideal Gas Law apply here as mentioned previously by my fellow contributors, at least two of which have contributed to other discussions I have participated in and in whom I have considerable respect.

If you need to know what happens to the actual flow rate of the gas (air?) in terms of volume as it is cooled from 80C to 40C, then v2=(p1*v1*T2)/(p2*T1). The pressure and temperature terms must be described in absolute temperature (Kelvin)and pressure units (reference absolute vacuum, not atmospheric pressure). It's quite easy to calculate the effect once you understand this.

There is a confusing array of compressor performance methods which report performance in ICFM which stands for Inlet Cubic Feet per Minute. There is also Standard Cubic Feet per Minute (SCFM) which varies depending on which "Standard" is referenced. What is usually different is the reference temperature.

Ft³ or L³ or bushels. The ratio of one bushel to one half bushel is the same as the ratio of one liter to 500 cc. Also the ratio of 273 K to 546 K is the same as 457 (0 degrees F) to 914 (457 degrees F).

S I units are not a part of the law. They are an accomodation. The thing to know is what the law means. A ratio is a ratio no matter what units you use, as long as your 1 bar or 29.97 inches of mercury or 2 cubits is measured from a nano-torr or less absolute pressure.

If it's twice as hot, it is either twice the volume at the same pressure or twice the pressure at the same volume.

RichH

NoSciFi:

Rich,

I understand where you are coming from, and you made a valid point concerning ratios.

However, I do disagree with your comment:

"S I units are not a part of the law. They are an accomodation."

They are NOT an accomodation and we are very much a participant in the international SI process of definition and standardization.

http://www.physics.nist.gov/cuu/Units/

They are as much a part of the law as something can be:

"In the 1870s the French made a crucial decision to turn control of the system over to an international body. In 1875, most of the leading industrialized countries (including the United States, but not Britain) signed the Treaty of the Meter. The treaty established the International Bureau of Weights and Measures, which has presided ever since over what we now call the International System of Units. It also provided for distribution of copies of the metric standards throughout the world and for continuing consultation and periodic revision and improvement of the system through regular meetings of a General Conference of Weights and Measures. The 22nd General Conference met in October 2003."

The link from which the above quote was taken:

http://www.unc.edu/~rowlett/units/metric.html

****************************************************

"In the Omnibus Trade and Competitiveness Act of 1988, the United States government designated the metric system of measurement as "the preferred system of weights and measures for U.S. trade and commerce". The legislation states that the Federal Government has a responsibility to assist industry, especially small business, as it voluntarily converts to the metric system of measurement. This process of legislation and conversion is known as metrication, and in the U.S. is most evident in labeling requirements on food products, where SI units are almost always presented alongside customary units.

However, metrication in the United States has been less forcefully imposed than in other countries, and has encountered more resistance from industrial and consumer market forces, so customary units are still widely used on consumer products and in industrial manufacturing; only in military, medical, and scientific contexts are SI units generally the norm. (Mars Climate Orbiter crashed on Mars because one team used newtons and the other used pounds-force.) Since everyday weights and measures are mostly non-SI, children in U.S. public schools are generally taught customary units before SI, although many schools are now attempting to teach SI units at an earlier age."

The link from which the above quote was taken:

http://en.wikipedia.org/wiki/U.S._customary_units#History

As members of the scientific/engineering community we should be at the forefront in SI advocacy, since it was established by scientists, is based on scientific definitions and was intended especially for use by scientists and engineers worldwide to eliminate ambiguity.

Regards,

Greg

Hi Greg G,

I will respond to you last post using the Australian vernacular with the simple statement,

Bloody ripper mate!

Mssrs Charles and Boyle were trying to teach you something. Not some "preffered" system very nearly as arbitrary ( else 1 cc would equal 1 ml) as pounds, ounces, gills, and grains. Among other things, absolute zero is still a vaguely derived ( a little more well defined today than in Boyle's day) value, since there is no reference source to compare it to.

What I'm trying to impress you with is that if you learned what the law represented, you would always be able to put it together yourself, and you could use it "on the wing" , maybe even with a slide rule if you couldn't ball park it in your head.

One of the better effects of winging it before putting it to paper is that you can spot a missed calc or scaling or formula if the answers aren't in the same ballpark.

As soon as they stop making 6" water pumps, and 1500 ton cranes and carats of diamonds and 6030 welding rod and 7/8 inch grade 5 bolts and 20 yard dump trailers and A36 steel and acre-feet of water and numeric drill bits and...and...and... .

Tell me, what size shoes do you wear? And is that the English, French, American or Asian size? What size hat do you wear? How many stone do you weigh?

The convenience of a standard unit system is a wonderfull concept. Until we all use the same system, which still looks like another 365,250 days away, learn what the law means, rather than the rote monkey see monkey do of the way things are taught today.

I'm sorry. When I went to school, light went 186,000 miles per second, and rarely did we run into 2.99 * 10^10 cm/sec, the diameter of the earth was 8,000 miles, the earths orbital radius wasn't 2.99*10^11 m, geostationary orbits were 22,000 miles up there (They didn't have them then. Sputnik was running around at about 125 miles going "beep, Beep, Beep.), and American radio was Cheering Fidel and Che on in their battle against Batista.

Wire is still measured in circular mils, my car still gets miles per gallon, and my steaks are priced per pound. My favorite beer still comes in less than a pint so that it can make a head. My furnace and air conditioner are still rated by BTU's, tons, pounds of steam, horsepower, my sling psychrometer still refers to grains per cubic foot, and I'll probably be buried 6 foot under.

It's not that SI doesn't nor shouldn't exist, it's that when you get over to Mozambique and don't have any concept of what ever units they are using are, you still need to be able to use the marvelous relationship you are so fond of calling a law. Can we fine gases when they no longer function like ideal gasses?

Sorry, I didn't mean to be rude, but astronomic extragalactic gasses operate under a very poor hybridization of the laws, quarks don't even believe in your laws, even the baryonic masses don't.

RichH

"Wire is still measured in circular mils, my car still gets miles per gallon, and my steaks are priced per pound. My favorite beer still comes in less than a pint so that it can make a head. My furnace and air conditioner are still rated by BTU's, tons, pounds of steam, horsepower, my sling psychrometer still refers to grains per cubic foot, and I'll probably be buried 6 foot under."

But only in Myanmar, Liberia and the USA. Every where else

Wire is measured in mm, cars uses liters per hundred kilometers, steaks are priced in Kg. My favorite beer comes in 375 ml or 750 ml cans and bottles, my air conditioner is rated in Kw, my sling psychrometer reads in °C , the psychrometric chart is in gm^-3, gl^-1 and joules and they will bury me 2 m under.

Seriously though the imperial system of weights and measures is a cobbling together of a whole stack of different ideas that numerous people thought up over the centuries with no real coordination. The SI system is a systematic method of measuring standards that is based on reproducible scientific standards. It just makes sense that since we are all engineering the same sorts of things that we all use the same units.

The other thing to keep in mind is that there are 300 million Americans using imperial weights an measures and 5,500 million people using SI units so just who is in the minority here?

Hi Masu

I agree fully with Rich's point on understanding the gas laws (or other formulas, relationships, laws etc).

On the SI system though he misses the point because he wants to. I was ready to respond, but then saw that you did it so well.

I assume he isn't stuck like I am astride the English and SI systems, with some info in one, some in the other, and the royal pain of endless conversions. In CAD, I have to work in inches, but more and more purchased parts I have to incorporate in the models are specified only in SI units. In designs, I'm using items whose performance specification are in SI units with items specified in English units. As with the measurements, increasingly only the SI units of force, flow, pressure etc are given. More and more items have metric dimensions for threads, shafts, tubing sizes, etc. Parts inventory, tooling, and required tools are all needlessly complicated. And often these parts are from American companies that also deal in exports or have foriegn factories that manufacture for the world market, for some of their product lines.

I am very fondly attached to the English system because it is so familiar to me, but for engineering I'd be thrilled never to see it again, and I'd accept it in everyday life happily, just to get it over with. It is the sloooooooowwwww changeover and need to convert that is so annoying.

Regards,

Greg

"It is the sloooooooowwwww changeover and need to convert that is so annoying."

I couldn't agree more. When Australia converted from ₤ to $ back in the mid 60s the change over was carried out in the space of a few days. It went really well and everything was well and truly running on the new system well before the final cut of date.

When the decided to move from the imperial weights an measures the whole process was drawn out over decades and to a certain extent it's still mucking everybody about. When I started as a cadet engineer the American company that I worked for was still wallowing in the imperial system. I steadfastly refused to use them and it didn't take long for everybody else to realize that they had been doing it the hard way for years.

I suppose we will need to cope with the mixed measurements for a few years yet. Maybe I should stop encouraging the use of imperial units by supplying the imperial conversions in my posts. By the way the statement back in post #3 was made as deliberate attempt at stirring the pot and I must admit that I am pleased with the response.

Stirring yje pot is not the way to win frends. It makes people think in ways that they haven't wanted to. At the desk, Si are a marvelous improvement over less than sensible Imperial system. It makes EVERY quantification and operation so very much similar. I would never espouse the continuation of such an out-moded system.

However, I live in a real world America, and when I walk into Home Depot and watch someone selecting a new sump pump for his basement, if it says 500 gallons per hour at 10 foot head, and right next to it is the SAME pump packaged in metric at 1900 lph and 3 m head, guess which one stays on the shelf, even if the metric is priced somewhat cheaper.

America has about 300,000 engneers among its 300,000,000 peoploe. The serious attempt to start metrification started in the early '60s, and I would be more than happy to change over tomorrow, but I don't see it happening within my lifetime, or even my kids. In the meantime, we live with a barrel meaning between 30 gallons and 55 gallons, depending on the industry.

My tirade was brought on by someone who's reaponse to another asking for help, derided the possibly lesser educated but maybe equally intelligent seeker by saying the mix of SI and Imperial units wouldn't work. Pointing out that pressure starts at the CONCEPT of 0 in any system, and the temperature started at what is termed absolute 0 regardless of the units used. And whether you are working in m³hr^-3 or bushels per hour, the relationship will still work for the man asking for help in the terms he is familiar with.

Speaking of SI, wouldn't it be more convenient convet to quads and drop the angular degrees minutes seconds that all navigation, astronomics, surveys, and even GPS are based in, and then do the same with time, converting it into say 100 unit days with 1/1000ths? That would mean that the whole world would have to relearn a sensible, readily quantifiable system? 100,000 units replacing the 86,400 seconds per day?

Of course that would require recalibrating all of the standards we all use, and even as a result the absolute definition of the meter. Imagine how much easier ghz would convert to the replacement for angstroms.

Imagine the worldwide sales to be made of clocks, wristwatches, reprograming of instruments, microwaves, coffee pots, speed limit signs, whether metric or not.

Sounds great, but that would throw out your joules, and the plumber would be the only one who would still buy his fittings in quarter-bends etc., and I'm sure he would quickly convert his billing from $100/hr to $25/newtimeunit instead of the $24 that he should have. I'd say he's not too dumb to take advantage of the system.

Would it be hard? Yes, but if we can expect the US to sensify one part of their lives, why don't we do it all of the way, and then do it right?

Thank GOD for the opposing thumb, or we'd be using an octal rather than a decimal system. Then again, there would also be advantages to changing everything to a binary system, wouldn't there? I guess I'd have to watch SUPERBOWL 101001, on channel 101, along with my 10 kids and have to work 101 of the 111 days a week.

There are a lot of things that would make sense in one way or another, including hexadec, but that's not going to happen, is it?

Maybe just for the scientific/engineering Professionals?

If you are going to proport a decimal system, carry it to it's logical and sensible end, not some convenient halfway point, resulting in a hodgepodge nearly as senseless as the imperial unit system.

RichH

RichH:

With all due respect, I think you are stirring the pot at least as much as I am. I am not here to make friends, (God knows there are enough places on the web to do that) but to learn as much as I can, and hopefully contribute something in the process. Likewise, despite my blunt style at times, I am not here to offend anyone either.

Reading as you take one side, then the other, some in jest, some not, I suspect we are probably on the same page pretty much.

You are perceiving me as "pushing" some "decimalized" system based on abstract notions of logic, and I perceive you as unduly wedded to the familiar, and resisting change.

Here is where I stand:

I grew up with the English system, use it everyday for most things: its familiar and comfortable. As an engineer, I would be perfectly happy to work only in a "cleaned up" (lbm, lbf etc, and some standardized units) English system. Unfortunately I don't have that choice. So be it.

I've been around long enough to see many, many changes in all aspects of our lives. I don't fight it, except on a very personal and limited basis in my personal life. Otherwise, when I perceive change is inevitable, I embrace it, to keep up or ahead of the learning curve. I will not allow myself to become obsolete, and have no time or inclination to make quixotic tilts at windmills.

The SI system is a "fait accompli" ... the battle as it was is effectively over, and the fact that there are still pockets of resistance and troops that didn't get the word doesn't change the outcome. It is the wasteful skirmishes, slow retreat and the fact that I have no control over it, that wears me down.

My philosophy is "bite the bullet", get it over with, and move on, but a lack of political will (as in leadership) has caused our country to muddle along astride both systems of measurements, dragging out over generations a change that could have been accomplished in one. It hurts our global competitiveness, and needlessly complicates things. That is what I find illogical and frustrating!

Like you, I disdain attempts to impose the re-invention of things to conform to some "head in the clouds" concept of logic and simplicity (Esperanto, the duo-decimal system, etc), but this is not at all like that today, and whether the metric (as in predecessor to the SI) system was at times in the distant past awash in intellectual and/or political snobbery doesn't change a thing today. The only question before us is whether we will passively muddle on, or willfully make the changeover in as painless a way as possible, but make it, and move on. There are too many more important issues facing us, but then, we don't seem to be addressing them well either.

In another thread, I joked about setting my clocks to some fanciful "metric time" like you are now, but for my part, if such a kind of thing ever happened, and the entire rest of the world adopted it, I would hope we would too, and not have to have clocks for both time systems, converting between them constantly.

It's no more complicated than that for me: Since the change to the SI system is inevitable, I would just like to get it over with. It has nothing to do with my own personal preferences over which system I myself would prefer in the abstract. Were the situation turned on its head, and we were the last SI holdout of consequence in the world to the English system being adopted, I would want to change to it in an organized purposeful way.

Regards,

Greg

I was brought up initially with Imperial units only by the mid 60's the UK had decided to go metric in engineering at least, so my engineerings education was continuously mixed between Imperial and Si units, not only that but between the mks and the cgs systems of Si units....

For those that are unaware in those days there were the two metric systems in operation the mks (metre, Kg, Second) and the cgs (centimetre, gramme, second) metric systems...

So my engineering education had to encompass all three systems of units Bth, joules and ergs etc... etc...

It was damned difficult but at least now when someone refers to a measurement as 'furlongs per kilogram per pint' (I made that up!) I can just about manage to still do some useful calculations with it...!!

Today I'm constantly amazed at the lack of knowledge by younger members about the two main systems...

Recently in a pub I asked for a pint of Speckled Hen, the young bartender said we haven't any on draught but we have the bottles of it and they are over a pint... I asked what size are they and he looked and said they're a good 500 mL ... To which I had to quicklt remind him that there are 568 mL to a pint and the bottle was therefore much more expensive and smaller than a pint!!!

There you are, a practical example of mixed units causing numerous confusions amoungst the 'one system' taught people.

I am grateful for my education as it covers so many units and everyday I'm finding another unit that I've not come across before... but do I throw my hands up in horror? NO of course not, I just look for the conversion factor and continue with my work....

John.

Incidentally, the UK switched over to the metric with a minimal fuss over only 12 years....

John,

You said it well.

My frustration is that instead of dealing with the units system issue head-on, we here in the U.S. have buried our heads in the sand and it shows most in our schools. I blame a lack of political leadership.

As you say, the metric system(s) and the SI system are separate systems, only sharing some names of units and prefixes. The liter technically is not even an integral part of the SI system but is an accepted "accessory" unit (The SI base unit of volume being the cubic meter.), but now I'm splitting hairs.

Regards,

Greg

"Speaking of SI, wouldn't it be more convenient convet to quads and drop the angular degrees minutes seconds that all navigation, astronomics, surveys, and even GPS are based in, and then do the same with time, converting it into say 100 unit days with 1/1000ths? That would mean that the whole world would have to relearn a sensible, readily quantifiable system? 100,000 units replacing the 86,400 seconds per day?"

When GPS units first cam out I started playing with one and trying to interface it with a computer. It turns out that internally GPS units do the calculations in radians then convert it to degrees and minutes only for the display. If you wanted to interface with the early units the output was in radians, later models used the NEMA standard that had the readings in degrees.

As for a metric time astronomical date and time functions are measured in days after a particular point in the past. For example 27^th January 2007 at 18:21:19 UTC is 2454127.264803. Computers do it in a similar way internally except they generally start counting from 1^st January 1980 at 00:00 and local time rather than the time at the international date line.

So you see it's already been done, you just don't see it as it gets converted back before you see it.

"Thank GOD for the opposing thumb, or we'd be using an octal rather than a decimal system."

You have obviously never worked on main frame and super mini computers. From day one all the technical manuals have used octal. Primarily because its easy to convert to binary and the conversion can be done in your head. For some strange reason when the microprocessor was invented they decided to add six extra digits rather than subtract 2 and hex was born. Personally I prefer octal because it's easier to do in your head and I have no idea why they decided to use hex. Another classic example of a lack of communication I guess.

You may also be interested to know that in aviation they have not and there is no plans to convert the measurement of altitude from feet and airspeed in knots. Some of the eastern European aircraft have altimeters calibrated in metres and air speed in km/h but these are the exception. This is one thing that I must agree with you would be a nightmare to try and convert. It would require replacing every instrument in every aircraft and ground based ATC system as well as all the maps simultaneously world wide. You would then need to transit all the airborne aircraft to the new standard altitudes without causing a collision. Yesh, what a debacle, definitely a day to avoid flying.

The big haste with the US using imperial sizes is that when we get equipment from the US the parts and sizes are at different increments than every body else's. As a result it you end up needing to carry two sets of things like drills, taps, dies, nuts, bolts, etc.. One set for metric and another for items that come from the US and this can be costly and is a real pain in the posterior.

Ultimately I think you will be surprised at how soon the US starts to adopt metric sizes. The more international the market place becomes the more pressure there is going to be and you are outnumbered something like 20 to 1 so it's only going to get harder to not convert. NASA have decided to bite the bullet and have announced that everything to do with the Moon from now on must use SI units.

It seems to me that the main issue is definition of the moles (or alternatively the weight) of gas being delivered per unit time. Given the absolute temperature and absolute pressure of the measurement location, the volume delivered is directly defined per Boyles and Charles Laws. PV=nRT for gases not significantly far from ideal conditions.

Make absolutely certain you don't mean SCFM rather than just CFM.

Compressors are generally rated in delivered SCFM (Standard Cubic Feet per Minute) at various discharge pressures, which means the equivalent volume at atmospheric pressure (14.7 PSIA), and typically (there are different "standards" for temperature) at 68°F (20°C), and 0% relative humidity.

In the case of SCFM, it would not change by changes in temperature, (or pressure) but the effective volume of "air" would change based on the removal of water from the compressed air (but you don't want moisture in the air anyway).

SCFM ratings are made assuming perfectly dry air and therefore technically have to be lowered depending on the amount of moisture present in the air being compressed. This is why they sometimes refer to ICFM (I=Inlet).