Only The Cart Stays The Same

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Humdinger

Humdinger

Member
Jun 3, 2015
1,157
716
Charlotte, NC, United States
My interest in PCPs began 18 years ago. I've bought and sold countless air rifles and lots of air tanks over those years. During that time I've owned seven compressors and sold the first six without ever having a part fail that couldn't be serviced at home. The first was an FX motorized pump in a box that could only fill rifles directly to 3000 psi. It was replaced by a Shoebox Max then a Shoebox F8. 

The rest have been 4500 psi dive compressors shown here. I noticed while looking through photos that they were all mounted on the same yard cart. Each is shown in its order of purchase. The first was a 20 year old Bauer Utilus which I used for 3 years then sold to a paintball field. They use it to fill a bank of storage tanks. It was replaced by an Alpha-Carette compressor which ran trouble free for 2 years. It was sold to upgrade to a Daystate LC-110 used during the past 2 years. I missed the heavy duty Bauer Utilus 220V unit so I upgraded to a new Alkin W31 three months ago. It is well made and seems to have the same build quality as the older Bauer. I feel confident that my compressor trading days are over. The W31 is a pleasure to operate and should last a lifetime. It effortlessly fills a 6.8 liter tank from zero to 4500 psi in 20 minutes.
Bauer12 copy.1599135983.jpg


Carette 004 copy.1599479748.jpg


Daystate 3 copy.1599134666.jpg


Alkin W31 003 copy.1599135751.jpg











 
I’ve been wanting to ask this question for quite sometime and since you’ve had both the Bauer and Alkin your the guy I’m gonna ask. I don’t know how long you’ve had the Alkin and there’s one for sale about 5 blocks from my house so my curiosity is about to get the best of me. One day when was doing a little research I went to the Scuba Boards compressor section and typed in Alkin in the search box and started reading all of the threads. I found a discussion about Alkin’s use of a overhung style crank. The way I take it is a overhung crank limits it to lower rpm, which some claim is better due to heat being generated but the crank isn’t supported at both ends. That means if one bearing fails the other has no other choice than to fail because the shaft isn’t supported at both ends. I’ve got experience in rebuilding water pumps, B&G and Aurora, and the shaft is supported at both ends. That has had me questioning the design ever since, especially when Bauer’s design is NOT overhung. I figured the Alkin would get me to the grave but in the back of my head I would know the potential issue. To confirm I did look at a parts schematic. There’s a big difference between a scuba shop business and what I’m gonna be using it for. I’m not bashing one design over another just a legitimate question.
 
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You are much more analytical and mechanically astute than I am. I am sure a mechanical engineer could find aspects of any brand's design that might be improved. The Bauer and the Alkin are very solidly designed and last for decades in commercial applications that are much more demanding than you would ever subject a compressor to.

I am very happy with the Alkin. It filled a 6.8 liter tank for me from empty this week in 20 minutes. The cooling coils on the output side were barely warm to the touch. A $3100 Alkin W31 is much more solidly built than the $4400 Bauer Jr. II. I purchased a 20 year old Bauer Utilus that is no longer made but its replacement sells for $8000 bare bones and is no more compressor than the W31 which includes more standard features. 

Both are top of the line brand name compressors. Alkin wins on value for the money, hands down. The U.S. Navy thinks enough of Alkin to use them on warships and to supply air for the Navy Seals. That's good enough for me and better than any brand I've seen other than Bauer. You might also consider Arctic compressors if you want a U.S.built compressor that has quality and durability. I have no personal experience with Arctic but know they are priced well above an individual hobbyist's budget.
 
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Thanks for your reply, and I know that they’re built like a tank. I was just curious if they were making a mountain out of a mole hill as they say. I’m no mechanical engineer either. I’m aware of the substantial price difference and my way of thinking I didn’t really need the Bauer to fill a tank every once in a while. I’m sure the Alkin would do everything I needed it do for the rest of my airgunning days, it’s just I had that one question about it. Congrats on your new one and have a nice holiday weekend!
 
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Humdinger, I am curious if you e ever tried topping off two tanks with the compressor pressurized?

Example- tank number one is at 3000 psi. You top it off to 4500. You take that manual valve on the fill whip and close it. You close off your tanks valve and bleed off the air between bothe the fill whips shut off valve and your tanks valve.

you connect to tank number two that’s also at 3000 psi. You start the compressor. When pressure is at 3000 psi that’s equal to your tank you open your tanks valve, then open that manual valve on the fill whip. 

Meanwhile thru all this, the entire compressors has remained pressurized. 

While we’re at it, what is that shut off valve on the fill whip for anyway?
 
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The W31 crankshaft is an overhung design. The reason it's not supported on both ends is to allow one piece piston rods and bearings to be slid on from one side. Here is a description from the owners manual. I'm not worried that this is a problem to be concerned with. This thing is built like an Abrams tank. It will outlive me for sure.

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I liked the idea of the low RPM, long stroke Alkin W31, but that requires some design compromises. The Coltri MCH6, using a shorter stroke and higher rpm, has the crank supported on both ends, AND it has the one piece connecting rods. The Coltri is a four stage compressor vs the three stage Alkin.

I looked at reputation/specs/price/availability of both the Alkin W31 and Coltri MCH6. Both seemed to be good compressors but I went with the Coltri.


 
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The W31 crankshaft is an overhung design. The reason it's not supported on both ends is to allow one piece piston rods and bearings to be slid on from one side. Here is a description from the owners manual. I'm not worried that this is a problem to be concerned with. This thing is built like an Abrams tank. It will outlive me for sure.

....
Click to expand...

I liked the idea of the low RPM, long stroke Alkin W31, but that requires some design compromises. The Coltri MCH6, using a shorter stroke and higher rpm, has the crank supported on both ends, AND it has the one piece connecting rods. The Coltri is a four stage compressor vs the three stage Alkin.

I looked at reputation/specs/price/availability of both the Alkin W31 and Coltri MCH6. Both seemed to be good compressors but I went with the Coltri.

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The MCH-6 would be my choice if I hadn't opted for the Alkin. They are both excellent compressors. There is no wrong choice between the two. I like lower rpms for less heat, quieter noise level, standard hour meter, and repackable steel filter cartridge of the Alkin.. My previous compressor was a Coltr built LC-110 and it was an excellent performer. I recommend it to anyone for the best option under $2,000. 


 
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Humdinger, I am curious if you e ever tried topping off two tanks with the compressor pressurized?

Example- tank number one is at 3000 psi. You top it off to 4500. You take that manual valve on the fill whip and close it. You close off your tanks valve and bleed off the air between bothe the fill whips shut off valve and your tanks valve.

you connect to tank number two that’s also at 3000 psi. You start the compressor. When pressure is at 3000 psi that’s equal to your tank you open your tanks valve, then open that manual valve on the fill whip. 

Meanwhile thru all this, the entire compressors has remained pressurized. 

While we’re at it, what is that shut off valve on the fill whip for anyway?
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I never start a compressor with back pressure but the W31 would most likely start regardless. What I always do to avoid Bigragu's scenario is bleed back pressure from the two moisture bleed valves on the water separator. There is a check valve in the air line between the 2nd and 3rd stage that retains high pressure in the filter housing. Even with the starting torque of the 3HP motor starting under 3,000 psi back pressure would eventually cause a parts failure. It's never good to stress a compressor and risk a breakdown. The little 12 volt Nomads and Hatsan sparks will break if restarted with high back pressure on the first try.

The new Alkin includes a heavy duty fill hose, gauge, and bleed lever assembly terminating in a 300DIN connector. I like the heavy duty construction of the compressor but I don't like the extraordinary weight of the fill hose assembly. I disconnected the factory hose assembly from my W31. The factory hose attaches to the compressor using a 1/4NPT male fitting. I replaced it with a 1/4NPT male to 1/8NPT male reducer fitting and male foster connector. This allows me to just snap on the tank hose by it's quick connect. The pressure level is monitored on the tank gauge and the hose pressure is released by the tank's bleed screw. Here's a closeup photo of the modification.

Alkin connector copy.1599344672.jpg

 
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The MCH-6 would be my choice if I hadn't opted for the Alkin. They are both excellent compressors. There is no wrong choice between the two. I like lower rpms for less heat, quieter noise level, standard hour meter, and repackable steel filter cartridge of the Alkin.. My previous compressor was a Coltr built LC-110 and it was an excellent performer. I recommend it to anyone for the best option under $2,000.
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I too would recommend the MCH3 or LC110 if someone wants to use with a standard 110v outlet. But the MCH6 and W31 can fill a tank in 1/3 the time.

Lower RPM does not necessarily equate to less heat. The heat will be a function of how much air is being compressed (and efficiency). The Alkin does appear to be significantly quieter than the Coltri. My MCH6 came with a built in hour meter and aluminum filter housing (w/replaceable cartridge).
 
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@ Humdknger- the reason I ask about the keeping the compressor pressurized between fills is that is what sold me on this thing. When the seller turned the compressor off at 4300 on my tank thinking it was at 4500(because of the angle he was looking at on the gage), I told him he shorted me by 200 psi, and said no biggie and flipped the switch back on before I could stop him. It had been drilled in my brain that starting a compressor under pressure IS NOT a good thing, but this 2004 model Alkin just purred along without a stumble, cough, shake or anything. The owner laughed and said I’ll never hurt this thing. So, that’s what sold me on the purchase.

But to this day since ownership, I won’t dare try it, lol!



The previous owner had it set for with a female end on the fill whip to thread directly on to fireman’s scba tank. I switched over the end to a female QD Foster, and after discussing it with my local paintball owner that’s an Airhog distributor he suggested to me to go back to the threaded fitting and just buy an adapter to attach for DIN tanks, like my Airhog. He said QD’s are fine, but if the option was available, it’s best to go with a better lock down method from fill whip to tank that comes in the form of threading a fitting on. It takes away the fear or possibility of a QD gone bad and the fill whip now becomes a deadly device if it breaks loose off of a tank. Yeah, so I went back to how the original owner had it set up. 

I own two tanks- one is a SCBA fireman’s tank that I simply thread on the fill whip to, and on my Airhog I go from male DIN to male SCBA. Really like the set up. There’s two guys locally who I got into this hobby and had each of them buy the Airhog 88 cu ft whole tank kit with the leg saddles and the 5’ fill hose, and I top off their tanks anytime we get together. 

Takes exactly 5 minutes to top off from 3200 to 4500 psi. 
 
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The rest have been 4500 psi dive compressors shown here. I noticed while looking through photos that they were all mounted on the same yard cart. Each is shown in its order of purchase. The first was a 20 year old Bauer Utilus which I used for 3 years then sold to a paintball field. They use it to fill a bank of storage tanks. It was replaced by an Alpha-Carette compressor which ran trouble free for 2 years. It was sold to upgrade to a Daystate LC-110 used during the past 2 years. I missed the heavy duty Bauer Utilus 220V unit so I upgraded to a new Alkin W31 three months ago. It is well made and seems to have the same build quality as the older Bauer. I feel confident that my compressor trading days are over. The W31 is a pleasure to operate and should last a lifetime. It effortlessly fills a 6.8 liter tank from zero to 4500 psi in 20 minutes.
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Respectfully, what's the best cost-benefit analysis from your experience as it applies to duty cycle, time to fill, and fill volume versus wear. You have more experience than I on the various compressor systems so I am curious where the ROI to fill volume to time makes sense. For example, I purchased an AirForce E-Pump (E-Chump) rated for 4500 PSI and it destroyed itself filling a 90 cu-in bottle. In your experience with these compressors do you find that the rated, but often undisclosed, duty cycle exceeds the normal wear on the compressor for filling? In your experience what would you consider the minimum rated compressor for filling a 60 minute SCBA tank to 4500 PSI? Thank you in advance!
 
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