Why won't your compressor keep up with a small air tool? Because the CFM ratings are taken at the suction INLET of the compressor, not the pressurized air from the tank. I'll show you how a Rolair compressor rated at 4.1CFM @ 90 PSI only really delivers 0.9CFM @ 90PSI of useable air.
"They ain't lyin' to ya" — disagree
As a graduate of the Slashdot School of Car Analogies, I can explain why they definitely ARE lying.
Suppose I produce, and sell, a car with a horsepower rating based on the INPUT horsepower of the fuel consumed. This is a simple figure to calculate. And, with a little creative manufacturing, the horsepower of said car could be made VERY high. So, does it sound reasonable to advertise a car with a go-kart engine, that just sprays fuel into the air, as 1000 horsepower? Or maybe can we admit that these ratings are somewhat fraudulent?
So, 4.7 CFM at 90 PSI on a die grinder requires a 33.3 CFM compressor. A 20 CFM blaster requires a 180 CFM compressor…a 35 CFM rattle gun needs a 250 CFM comrpessor…
Why is that Snap On does not show the grinder operating Pressure in the specs??
Assuming a 90psi tool operating pressure you would need 6 of those green compressors manifolded l to run the grinder at full power continuously. This is why having the right tank capacity is critical.
Lying to the consumer only has consequences for the consumer.
Reminds me of the HP rating for some shop vac 😂
Change your hose size and remove as many QD's, filters, and any other line restrictions. You will see different results. However, you can only validate the compressor performance for rated flow and pressure at the STP (Standard Temperature and Pressure) used by the manufacturer. STP is a measurement standard to make sure everything is compared apples to apples. You have to adjust the compressor rating and tool rating for YOUR test conditions (altitude [pressure], temperature, and humidity) and you have to account for line friction. The short version is that as altitude, humidity, and temperature go up; the compressor flow capacity goes down and the tool flow demand goes up. The performance WILL change continously depending on those variables listed above. For my location in Eastern Idaho, there is a 15% loss due to altitude alone. July max temperatures reduces that capacity another 8%. This is true of ALL compressed air systems. The compressors I manage at work are manufacturer rated for 905scfm@125psi. What I actually get is anywhere between 705 and 770cfm depending on the ambient air temps. These compressors are performing EXACTLY as designed. The reality is the same compressed air system will perform differently in July than it will in January. It will perform differently in Denver than it will in Orlando.
Why does humidity matter you might ask, because when you compress air to 100psi, a large portion of the "gas" you compressed is water and it condenses out to rust your tank (assuming you don't drain it regularly). It is, for practical purposes, a normal expected power leak. The only way to eliminate that power leak is to live where there is 0% humidity. Compressors are usually rated at 0% humidity.
The problem with STP is there isn't only one universally agreed upon STP. There are many different standards, depending on the industry. ISO STP was 14.7psi before 1982 and 14.5psi after 1982. ISO uses 32degrees F. NIST uses 14.7psi and 68 degrees F, but calls it Normal temperature and pressure (NTP) and uses it for chemistry experiments. NIST also uses 60 degrees and 14.5psi for thermodynamics experiments. Most use 0% humidity, but some don't. SCFM (Standard Cubic Foot/Minute) is defined as some total number of molecules in a given volume at a specific temperature and pressure. SCFM or FAD (free air flow delivery) are the same thing, just with different units. The problem is air doesn't have a fixed density. It is dependent on pressure, temperature, and composition; so it can't have a fixed definition of molecules per given volume. In electrical terms SCFM is the comparable to Amps and Pressure is comparable to Voltage. It’s not a direct comparison, but close enough. Fortunately, Amps do have a single universally defined quantity. One amp is equal to one coulomb or 6.24x10e18 electrons worth of charge moving past a point in a second. Just like you can't get 100amps at 120 volts through a 28ga wire, you can't get 100scfm through a 1/4" hose at 100psi. Coming from a compressed air engineer, line friction is one of the biggest misunderstandings in any compressed air system. It doesn't matter whether you have a party balloon or the Goodyear Blimp worth of air behind it, there is only some much you can push through a soda straw. So the compressor manufacturer ratings are not lies or intentionally misleading, but they are often misunderstood and incomplete. The proper way to write an air compressor rating is 15 SCFM@100psi for ISO STP. If your test conditions don't match ISO STP and 100psi, it will be impossible to measure the same flow rate as the factory rating. IE: You will not be comparing apples to apples.
Also, the tool ratings are more likely the misleading rating. All air tools have a duty cycle rating, not a continuous flow rating. What this means, for example, if a tool has flow rating at 5cfm and a duty cycle of 30%, the 5cfm rating is the average of 15cfm for 20 seconds and 0 cfm for 40 seconds. Duty cycles often aren't published and are often overlooked when they are. Impact werenches are notoriously misleading because of this duty cycle idea.
Your standard box store air hoses–50ft long 1/4" and 3/8" often can't flow enough air for 15cfm and maintain 90psi at the end of the hose. Just like wire sizes and electricity, air hose performance is dependent on length and the desired load. At 15cfm, 1/4" hose has roughly 1psi loss/ft of length, and 3/8" is has about 0.1psi/ft. This does not include any losses from QD's, filters, check valves, water separators, etc.
By the way, while your math generally correct, your interpretation of CFM at the intake vs CFM at the output is wrong. A Standard CFM at 14.7psi has the same quantity of molecules as a Standard CFM at 100psi. It has just been compressed into a smaller volume. There is a difference between SCFM, ACFM, and ICFM. They each vary based on actual air density.
What AvE says is true, and I do wish tool CFM requirement specs meant the same as compressor specs. However, it does take a huge amount of power to deliver a lot of air at 150 pisg and high CFM. At my workplace, we have a 10 HP compressor that delivers 46 CFM at 125 psig, and that takes a lot of 3-phase AC Current, and I have no idea what it cost.
Thank god for cordless tools improvements replacing air tools, slowly but surely
Why hasn’t someone capitalized on this? The obvious results would be proof in the pecker and they would sell like a mother fixer. Whyyyyyyy!?!
Just like how tasers are marketed to put out OnE miLLiOn VoLtS lol
I have a late 1970's twin cylinder compressor that I bought used. It's rated for 1 hp and 6.9scfm at 40psi displacement is 9 cfm and stroke 2 inches each cylinder bore is 2.75 inch running at 900rpm on pump. This compressor easily handles harbourfreight purple sprayer rated at 6scfm. I know this setup cannot handle beyond 70psi required for bigger nozeel sprayer that works with non catalyzed lacquer because that is very thick. My experience with this compressor is it keeps up with heavy painting non interrupted. I know the examples you explained is true but when a manufacturer says 6scfm at 40psi on sprayer then the most important thing is your compressor should not drop below 40psi when you press the trigger, if it does, then you can't paint uninterrupted. It is possible that you are using a thick viscosity paint that require higher PSI to be maintained and a bigger nozzel with much higher cfm requirement of 20 or more. The compressor I am taking about is a well known compressor for spray painting across the world and made in USA. If you want more than 9scfm be prepared to buy gas powered compressor because even if you go for 220v keep in mind the bore size of cylinders should also be sufficiently big 3 inch or more. And it should be more than 1 cylinder also since spray painting is sensitive to psi maintained so you alsoneed to ensure compressor is continous duty not a 50% duty sold in box stores. Duty is something you cannot compromise then scfm and lastly psi. Tanksize can also give some relif but it will be temporary.
WoW
Ran into the same issue – compressor not able to run jack-sheets. Solved it by adding a cooler to the compressor heads output pipe, plumbing in 400l of additional storage, a big 5-run condensor loop with 1/2" internal pipe & stepped all the rest of the pipework up to 1/2" as well. Now, plugging in/unplugging a tool is scary & everything runs like a Ferrari, as opposed to sputtering like a Lada with the "as bought" set-up. They can lie, we can modify. 🙂
I have watched this video at least five times and I am with you, I still can't believe the audacity of these companies. So to use a little piss-ant die grinder continuously you need a $5000 compressor – Great!
Would this also be the case for oxygen concentrators?
AvE, thank you. This helps a lot. Please keep calling out companies on their BS marketing!