Air compressor inspection advice (cfm measurement)

[Bmw4life]

I've been researching used air compressors for a while, probably for months, reading up about different ways to test them.
Only today have I stumbled upon a couple of formulas on how to calculate the actual cfm. Which it seems would be a really important test, yet I don't see it recommended for people buying used compressors.

So I was getting ready to buy a compressor, a medical compressor of all.
It's actually rated 4.5 hp, 15 cfm at 80 psi.
So I went to check the compressor, AirStar AS70.

From an empty tank, the manual says it should take 2min and 40 seconds to fill up.



I timed it, and it took around 4 min and 36 seconds until it turned of.

I then let the air out until the compressor kicked back in, and waited until it stopped again, 50 seconds this time.
So I'm wondering what these results mean. I have no idea what gets worn out on the compressors, but just not sure if it's on its last legs or not. How much life does it have left, or how many hours?

Also, based on the calculations from the manual, the output is around 9-10 cfm, not 15. Because the manual says that the compressor should full in 2 min 40 seconds. And based on that, it doesn't get even close to 15 cfm.

Here's the formula I used

Actual cfm = TANK GALLONS x .536* x PSIG divided by SECONDS

(30x.536x90)/270=5.3
I rounded the numbers. This is based on how long it actually took to fill it up from empty.

 

[tre873]

I'll start by saying I'm not very good at math so I use all the help I can get. There are several online calculators that you input the tank size, cut-off pressure and time to fill, then it gives you the CFM. I've used a couple of different ones and they were fairly close to each other.

 

[MacMcMacmac]

Sounds like something is not pulling its weight. Unfortunately, there is no interstage to plug a gauge into to check for bad valves, so the best you could do is see if it could be run from 0 to cut-out and check the pump up time against the stated performance of a single pump unit and see which one is running lame. Looks like they might be switchable according to the attached electrical box.

 

[GeoBruin]

I hope you're not planning on paying $8,000 for that? Unless you are a dentist?

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[Bmw4life]

It was for sale for $400 hehe, so I considered it very seriously. But in the end purchased a second 20 gallon speedaire compressor, which does it fact produce 10 cfm.
I was still contemplating buying this medical unit for resale or parts, but just don't have the time to do that...too many projects haha!

 

[stockerwithalocker]

The calculation is tank volume in gallons, divided by 7.48 to get cubic feet. Multiply that by (pressure in psig plus 14.7)/14.7. Divide by minutes it took to fill tank. This doesn’t account for the temperature rise but it gets you close.

From your example:
(30 gallons / 7.48) x ((90+14.7)/14.7) all divided by (276/60). Poor math on my cell is ~ 6.2 cfm.