Comparing CFM vs SCFM

Ok, I know SCFM is corrected, so I assume it’s in theory a more real reading of air delivery. But how the heck can you compare apples to apples? I was looking at a kobalt compressor at Lowes and it delivers 11SCFM @90 PSI. that seems awfully low for such a monstrous compressor. (It’s the big 80 gal one) I know that’s enough for my current usage but I’m not so sure about future operations.

I guess my question is how to you compare CFM vs SCFM. I looked online and there’s not a conversion that I can find. Thoughts?

If it’s a Two Stage compressor, it’s not low. Two cylinder compressors will output more air volume, while Two Stage compressors will output higher pressure (the second stage compresses the air from the first stage).

With regard to this question, SCFM stands for ‘Standard Cubic Feet Per Minute’ which means that it is a well defined number of Air Molecules per minute all of which depends on a specific temperature/pressure relationship (see https://en.wikipedia.org/wiki/Standard_cubic_feet_per_minute) so @90 PSI not only determines the pressure, it also determines the temperature of the gas (at the Standard molecular weight). CFM is, in a sense, unit less since it doesn’t specify how much air is in the cubic foot. A very cold cubic foot of air has more ‘air’ than a very hot one at the same pressure.
The good news is that the accepted metric is SCFM at a given pressure when measuring a compressor.

Ok but that doesn’t tell you how to compare between them. If one compressor says 15 CFM and another says 11 SCFM, how the heck do know which is putting out more?

CFM and SCFM are the same thing. In this case its how many cubic foot of air it can put out in 1 minute.

IMO, this is like Sears advertising their lawn mowers with 3.5RP engines! It sure looked like HP in the advertising print, but it was clearly not. If they don’t use the standard unit of measure, then there’s something fishy about the spec in my book.

As @mechanic416 says, they SHOULD be the same thing - if the temperature and pressure are the same in both cases - that’s the ‘fine print’…

2 Likes

So you can just consider them the same? Then 11SCFM does seem a little low for a 5hp compressor.

Well, as a point of comparison, the twin cylinder (not two stage), 5HP, 60G Ingersoll Rand SS5L5 will supply 18.1 SCFM @ 90 PSI… For the same price (at Tractor Supply) as the Kobalt.

Correction, that is a 60Gal tank… not 80G.

There is a good discussion on this topic at https://www.pdblowers.com/tech-talk/scfm-standard-cfm-vs-acfm-actual-cfm/ .CFM is a measurement of flow at present conditions at the time of observation. It can also be a simple bore and stroke measurement multiplied by compression strokes per minute calculation. A manufacturer rating their compressor unit in “CFM” would have an interest in rating it under favorable conditions to get the best rating. SCFM is a measurement of flow under prescribed conditions. ACFM is a corrected actual measurement of flow applied to an observed set of conditions. So your SCFM rated compressor performance is still going to constantly vary because conditions are always in flux. At least this is my understanding of the topic.

I read all that before…but it’s useless as a measurement/comparison tool unless they are all using it, or if there’s a specific mathematic conversion. It’s crazy to me that there’s not a definitive standard. But I guess that’s the case with a lot of things. And mileage may vary. Real- world conditions have a big effect on performance (fittings, dryers, separators). I guess it’s buyer beware. And to my original reason for the topic, the Kobalt compressor seems to be under-performing in comparison to other 5hp compressors. I think I’m gonna go with a Quincy since it seems to be a solidly-built US-made compressor.

1 Like

Really? You really read it?

There is, it’s called SCFM.

in comparison to other TWO STAGE compressors, it isn’t. So much for reading what’s posted.

Concur. If there is not a regulatory body demanding all compressors get rated under a given standard then you are correct. The rating information has no choice but to be viewed skeptically. Sounds like this is exactly where you’re at. I suppose you could contact the manufacture and request information as to how they rate the equipment but in the world of “Made in China”… good luck!

Boy, you guys just don’t get it! There ARE standards that legitimate suppliers test to and publish. As long as you buy things that are published to those standards then you’ll get equipment that meets your expectations. If you buy crap that isn’t tested to those standards, you’ll get crap. Sure, call the mfr who didn’t publish a standard measurement. I’m sure they will tell you exactly what you want to hear…

1 Like

Not sure what your problem is since you just laid out the exact point being made. Your responses appear agitated about this. Why?

My ‘problem’ is this statement:

Which totally ignores and misstates the fact where there are standards that can be trusted and most of the compressor industry uses them.

1 Like

I’m not saying they can’t be trusted. But for a layperson, when one company uses SCFM and another uses CFM, and there’s no mathematical conversion, how can you be sure you’re comparing apples to apples? I think it was answered enough for me. Like I said, I intend to get as high quality, American-made as I can afford. No need for everyone to get mad here. Just trying to understand .

You aren’t. The only way to compare compressor performance is through the use of SCFM.

It’s not like meters & feet that have a valid conversion factor. CFM is like oranges and SCFM are apples. You can’t convert an apple to an orange. They’re just different. Mfg who only publish CFM ratings are doing it purposely because their SCFM performance isn’t good vs the competition at the same price point or power levels.

3 Likes

Thanks James that’s exactly what my point was. But ultimately I was trying to choose a compressor, and I think I’ve done that.

Lowes sells a Quincy air compressor with great specs 15.5 SCFM@90 psi, 2 stage 230V, 5 HP 60 or 80 Gallon. Its on their website. American made. But it is a special order. We bought one for work and like it very much.

That’s the one I have - Quincy QT54 - it’s on sale at Northern Tool. Solid compressor - very happy with mine. My store had it in stock and I picked it up, but I think they have free delivery as well. If you pick it up, make sure you’ve thought thru how to get it unloaded - it’s a handful!