Buying a compressor

[Dan Gibson]

My output estimation is also based on time to fill versus change in pressure, not reading the flow meter. The flow meter was just another way to confirm what I already suspected. On this small of a compressor, there is a huge difference between 3.5 and 2.6 cfm. Rjacks 12 lpm and my 10.6 lpm is also pretty significant. The fact that I can reproduce a mix pretty closely should suggest that my output is pretty consistent. I have to believe that my output is no where near the rated output despite what methods of estimating output is used.

My fill times versus change in pressure have typically been at different pressure levels over a significant enough time period. I don't typically fill 80s one at a time, so I have to estimate, but I can't really be that far off. I'll give filling one 80 with air at shot to see if I can nail the numbers down, but I'll bet anyone my output doesn't approach the manufacturers ratings.

Dan, how the measurement is done depends on the premise, that is "what method represents the performance of a compressor?" My objective is to replicate manufacturers and government tests, to normalize results with the intention of obtaining output numbers for the block and to delete the effect of filling condensators and filters. Thus, I don't use flowmeters to determine compressor output because results will vary with the size of the filter set. Well, the test could be done at the output pipe coming off the block and produce a valid result. Test procedures specified by governments always involve the filling of a standard size test bottle, sometimes 1 liter @ 200 bar, sometimes more. The method for testing is to fire up the compressor and run up the pressure in the filter set to 200 bar, open the valve on the bottle and time the run to 200 bar. The result is obtained by dividing the capacity of the bottle (200 liters, etc) by the fill time.

I understand the concerns of continuous flow blending and how real time numbers are needed for this technique. However, comparisons of the numbers in this context can give different results from that of the manufacturers claims and other test results.

 

[Dan Gibson]

I usually have two sets of 104s going at a time. What would you suggest for dumping intervals (both dumps)? I'm sure I probably leave mine open just a little too long, but not enough to cut output by about 25%.

Roughly, this is how I have been calculating, er guessing, my output.

The larger the cylinder your filling, the less having your condensor or filters muck with the precision of the timing. Bank bottles being ideal for estimating rate, doubles a good second choice. A single 80 is harder to accurately measure IMO.

 

[pescador775]

My O2 flow is 'only' 12L/min which theoretically shouldn't be enough for 32% (but it works). I have assumed the guage or orifice is off a bit. My nurse wife says the med O2 flow gauges are notoriously unreliable in the hospital.

So instead, I have timed filling double 100s and get between 3.0 and 3.1 cfm out of my Alkins. A bit less in hot summertime (2.9 or so). More in winter, maybe 3.2 when its 40-45F out.

I bought it 12/2005 and have no idea of its speed. I have #10 wires feeding the panel, shouldn't be much voltage drop (although I have never checked).

The flow meter is correct: .11/.79 X 3 X 28.3 = 11.82 lpm.
Pesky

 

[Hoosier]

My records were besed on 119s and 130s fills for your reference...

 

[rjack321]

The flow meter is correct: .11/.79 X 3 X 28.3 = 11.82 lpm.
Pesky

Yes if you assume 3 cfm. But last I saw alkins rates this as a 3.5 or 100 lpm compressor. That would be 14 lpm O2 to make 32%

Not that it matters that much to me, I'm very happy with the unit as is. And I knew that its cfm rating was probably optimistic when I bought it (aren't they all?)

 

[rjack321]

I usually have two sets of 104s going at a time. What would you suggest for dumping intervals (both dumps)? I'm sure I probably leave mine open just a little too long, but not enough to cut output by about 25%.

I only open for 5-7 seconds. With my temps (cold compared to alot of places - I pump alot at temps <60F) this seems to be enough to blow out more than enough water. There are always a few drops remaining in the primary filter. YMMV

 

[pescador775]

Yes if you assume 3 cfm. But last I saw alkins rates this as a 3.5 or 100 lpm compressor. That would be 14 lpm O2 to make 32%

Not that it matters that much to me, I'm very happy with the unit as is. And I knew that its cfm rating was probably optimistic when I bought it (aren't they all?)

I am not assuming anything. You reported that your compressor produces 3 cfm and a flow meter reading of 12% to produce 32% NITROX. Your flowmeter appears to be quite accurate. I think the issue is a misunderstanding based on the tense used in your narrative.

 

[Hoosier]

This performance issue reminds me of a Japanese auto manufactures&#8217; horse power and gas mileage issues. It is well known fact that they used to jack up the number on their description. The consumer never be able to see those advertised number in reality.

The following is from Airetex.com

rjack321 is right on an AD model. It technically outputs less CFM (3.0 CFM to 3.5 CFM).

Based on their answer below, Alkin is nothing different with any other manufactures in my case. Mine is 2.8 CFM (3.0 CFM on the spec.) It is 4.3 ***SCFM&#8230;.:blinking:


&#8220;Generally **SCFM numbers are approximately 35% more than actual. 4.3 **SCFM = 2.78 actual FAD approximately So 3.48 FAD is much more actual CFM than 4.3 **SCFM which is well under 3 actual FAD.&#8221;

Q.Why does a Auto-Drain compressor put out less CFM than a manual bleed?​

a) With an Auto-Drain it dumps ALL air on a timer for a preset time usually 10-15 seconds every 15 minutes. After the solenoid valves close then your compressor has to pressurize the whole system again. When you bleed a system manually you only remove the moisture and do not lose much if any pressure (CFM). Some systems are more efficient than others but generally manual is much more efficient Our PLC allows you to set the drains to the most efficient settings for the humidity in your area.​

Q.What is the difference between Free Air Delivery (FAD) and **SCFM? ​

a) Free Air Delivery is the actual CFM your compressor puts out. To measure Free Air Delivery you start with an EMPTY tank and you pressurize your compressor to the final fill pressure you want and then you open the tank valve. (You want to measure the time to fill the tank not the compressor, separators and purifier) You fill the tank to the desired pressure and measure how long it took. Then you divide the Cubic feet of the tank by the minutes it took to fill it. For instance a 80 Cu. Ft. SCUBA tank that takes 23 minutes to fill from empty puts out 3.48 CFM actual. **SCFM starts with a partially filled tank and fills it the divides the time as if it filled an empty tank. A confusing way to make poor performance look better. Generally **SCFM numbers are approximately 35% more than actual. 4.3 **SCFM = 2.78 actual FAD approximately So 3.48 FAD is much more actual CFM than 4.3 **SCFM which is well under 3 actual FAD.​

 

[rjack321]

I am not assuming anything. You reported that your compressor produces 3 cfm and a flow meter reading of 12% to produce 32% NITROX. Your flowmeter appears to be quite accurate. I think the issue is a misunderstanding based on the tense used in your narrative.

Yes there were a couple of different thoughts going on simultaneously.

 

[rjack321]

Despite the reduction in capacity, auto drains sure do sound like a nice feature when you have 800cf to pump.