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Does anyone understand the air compressor ratings?
I just bought a Craftsman compressor rated 6.4CFM @ 90PSI. However it won’t keep up with tools rated at 4CFM when they’re used continuously.
Is there some sort of 50% duty rating that’s just not published? And if that’s the case, does everyone rate compressors that way? It seems to me that the more reputable brands are rated at 100% duty – i.e. Ingersoll Rand. I get that feeling because it takes a much heftier compressor to match the Sears ratings.
Does anyone know if brands like Porter Cable or Emglo rate their compressors the same as Craftsman? Or does anyone know where I might find out?
Thanks to any who can help.
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Some of the Porter Cables, the 6 gal. pancake for example and I think all of the craftsman are made by devilbiss. don't know much more than that but you might be able to find out a little more from them.
*Isn't there a "duty-cycle" stamped/printed onto a plate on the compressor somewhere ? It's the same plate with the pressures and current requirements. In any case, it sure seems like there's a catch 22 in the ratings; and, it sort-of leaps out at you when you see identical "compressors" attached to different sized tanks and having significantly different CFM ratings.
*Phill,There nameplate shows tank size, max. pressure, etc. No info. on CFM.The compressor housing says 6.4SCFM @ 90PSI. No mention of duty cycle. All I know is the thing in no way puts out 6.4 @ 90. It would keep up with a tool running only half the time, but if I run a cut-off tool or air hammer non-stop, the tank pressure drops to about 60 PSI before it levels off.I've also noticed the inline compressor type - also know as oil-free - the type where the motor & compressor are all one black box, seem to be be more over-rated than the old oil type. I can't help but wonder, if the new oil-free type is just wonderful, why does the old design still exist. It seems like the old type requires maintenance, but it runs stronger and is more likely to stand up to the test of time.
*Steve,Right-on - I called DeVilbiss & they do make the compressors for Craftsman. Yes there is a duty rating - it's 50%.But here's another tidbit I learned while on the phone. The compressor is designed to only run 30 minutes out of an hour - more than & it may overheat & shut itself off (which did happen to me).But since mine wouldn't produce 4CFM while running continuously, how can it produce 6.4 when it runs 30 minutes out of an hour if you're also using it for 30 minutes out of the hour?I'm not calling them big liars, but to quote our new President, there's some fussy math. It must have to do with how the tank capacity allows the compressor to catch up.So the Craftsman mystery is somewhat solved. But the real question is who uses the 50% duty cycle & who doesn't? I know I've seen Ingersoll Rand machines ratings as continuous use duty cycle. Since Craftsman and Ingersoll Rand represent the two ends of the spectrum (homeowner vs. industrial), what about the inbetween guys?
*I past mag. reviews, the logic seemed to be that the maunfactures do lie. They all use different rating systems to arrive at the "comparative" numbers which we use to compare similar products and buy. The lower end brands stretched their performance numbers more than the top of the line. Most reviewers seemed to think recovery time was as important, if not more important than cfm output. Maybe because cfm can be fudged, and recovery time isn't usually reported by the manufacturers. I got the Thomas based on several reviews, with decible rating and recovery time the top two concerns. Paid about twice as much for something "comparable"...but haven't been disappointed. The stacked hotdog, 4 gal, 2 HP with two pistons was a little over $400, and I had to order and drive an hour away to get it.....closer stores had the single piston in stock but wouldn't special order......and have been happy with the decision ever since. As strong, if not stronger,and quieter, than the Emglo oiler of similar stats. Jeff
*Well, recovery time is based on the CFM of the pump (i.e. how much air gets put into the tank). The CFM of the unit is how much can be taken out of the tank before it falls below the operating pressure. The duty cycle of the motor/pump is what percentage of the time it can run in a given time period, there should also be a maximum run time. There should be a last spec: CFM delivered per time period (e.g. CFM/Hour or CFM/cycle) - this is where some manufacturers seem to be fudging because they don't measure the CFM output, they rate their ability to run X CFM of "draw", the rating of the tools used, they count on intermittant usage. Sort of like the 3.25hp routers running on 110v/15A - if that were really possible, we'd all have 3hp saws. This reminds me of the hp ratings on outboard motors too: remember when it used to take 3 men and boy to carry a 25hp motor and you could ski behind it ? Now a 50 is considered too wimpy to pull a 10-year-old out of the water.
*Uh..... "CFM/Hour". CFM: Cubic Feet per Minute. What's messin' ya' up is the Scfm vs. Cfm.... Standard cubic feet per minute vs. Cubic feet per minute.... The two ain't the same.... S.
*Okay, you can't really have cubit feet per minute per hour, but Phill does make a good point.The CFM ratings are inaccurate because they aren't sustained numbers. I would like to see a rating to show how much it can put out in an hour because the longer time period would force the manufacturers to include recovery speed, amount the motor could operate in an hour without overheating, etc.And yes, you can pretty much disregard the HP number - complete crap. My background tells me the biggest motor you can put on a 120V / 20A circuit is between 1 & 1.5HP. Notice the wording "developed HP" on those machines.SCFM vs. CFM - what is the difference? Standard temperature?
*Do a search for "What those compressor ratings really mean" (sorry, this Unix box doesn't allow me to post the link). There was a pretty good exegesis regarding this. Sam
*Although not mathematically "correct" as written (it was late at night), this is the essence of what you want to find out: if run for a sustained period, say an hour, how many CFM can your compressor continuously deliver ? When I used to work on Jaeger, Sullair, and Gardiner-Denver compressors in the 60's, that was one of the specs (I don't remember the units, it was always shown as a graph.
* "What those compressor ratings really mean"
*SCFM Standard cubic feet per minute., SCFM or scfm. Flow of free air measured at some reference point and converted to a standard set of reference conditions (e.g., 14.4 psia, 80° F, and 60% relative humidity.) Scfm means cfm at standard conditions. However, standards vary and some care is necessary. In the United states, the usual standard is 14.696 psiA and 60°F. Some chemical engineers will use one ATA and 0°C, but usually will be specific about the reference point. Europeans normally use one ATA and 0°C. It is not the same to all people, therefore it is best that the reference pressure and temperature be definitely specified. (099) from http://www.impactrm.com/html/s.html#S
*Phill, Good link! Sam
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Does anyone understand the air compressor ratings?
I just bought a Craftsman compressor rated 6.4CFM @ 90PSI. However it won't keep up with tools rated at 4CFM when they're used continuously.
Is there some sort of 50% duty rating that's just not published? And if that's the case, does everyone rate compressors that way? It seems to me that the more reputable brands are rated at 100% duty - i.e. Ingersoll Rand. I get that feeling because it takes a much heftier compressor to match the Sears ratings.
Does anyone know if brands like Porter Cable or Emglo rate their compressors the same as Craftsman? Or does anyone know where I might find out?
Thanks to any who can help.