# Idea: How to Search for an Air Compressor



## WarthogARJ (8 Sep 2021)

Hi there,
My first post, and I hope it will be of use.
I needed to buy an air compressor for an upcoming project: nothing hard, just needed a mid range DIY type.
So do I buy what I need, pay a bit more and get what i need NEXT project, or rent?
Sigh.

There are a LOT to look at.

I decide the Abbac range is of reasonable quality, and has a big range.
VERY big, so big it's easy to get lost.

Comment:
Based on excellent feedback below, I've modified this a bit.
The assumptions here are:
(1) Buying a NEW machine.
Obviously a good deal on a used machine won't be an exact match for price
(2) Don't need to run it 24/7/365, as in this is home use, not industrial/commercial
(3) You only look at machines where you know they are of reasonably quality
(4) Noise is not considered because you quickly see that getting a quieter machine is pretty well impossible unless you can live with very low rates, or very high price
THEREFORE, as I mention in replies to comments, if you want LESS noise, then move the machine AWAY from you.
Use a long pipe, of the right OD, and put it where your NEIGHBOURS can hear it, but not YOU...:-}
(5) Yes, I know FAD is better to use than theoretical CFM, but you will struggle to get FAD values on the machines sold for the non-industry use 

I made an Excel spreadsheet of abut 20-30 of the Abbac models.
To get an idea on how the specs changed.
Then, based on basic physics, it's obvious how to rank therm, to see the trends of the "power/size/oomph" of what you can get versus price.
OK, "oomph" doesn't really have SI recognised units, but here's what i did.

In an air compressor, you want some combination of PRESSURE x SUPPLY RATE.
They generally have some link/correlation, so let's make life simpler: we'll multiply the PRESSURE (in Bar) x Supply Rate (in lpm).
Let's call THAT combo the "Oomph Factor".....:-}

Note: the value does have some physics justification, it's actually WORK (Watts), but let's keep it in units of Bar-lpm for now, and call it "Oomph" for fun.

Next factor is VOLUME of receiver tank.
Bigger is Better, in general (as my English ladyfriends say of Canadians...:-}
Assuming you don't have a space issue.

And next, well, PRICE is a big factor.

So what you do, is make a graph as follows:
(1) Break up the compressors into "Families" of Receiver Volume size: 6, 24, 50, 90, 150 litres.
(2) In each FAMILY, you plot Oomph vs Price.

See attached.
The point is, that the BEST area in this graph is Bottom Right corner.
As in, you want the BIGGEST "Oomph" you can get, at LEAST PRICE.
That's where the Arrow points.

That is, if you want a general purpose, do-all machine.
And THEN, in the same general range of "Ooomph", you would like to get as big a Volume receiver as you can.
As long as you can still afford it.
And you can still fit it in (really, no pun intended).

So the BEST region, in terms of "Oomph" vs Price is circled in Blue.
And if you pay a little bit more, you get a 50 litre unit, rather than 24 litre.

I bought that: Montecarlo 30P

If you have more $$$, and want more Oomph, go higher up.
But go UP, and to the RIGHT.
That's the best direction.
But you pay more.

However, as far as value for $$$, the Abbac Montecarlo 30P is the best pick.
If you can afford the $$$.

Otherwise, for LESS $$, move DOWN a bit, but keep RIGHT.
The best for low $$$, and Max Oomph is the Pole Position L20.

I am at present in the Land of Brexxit.....sigh.
So it's all in Pounds (Pounds in a METRIC Country? Ah, but guys still weigh themselves here in STONE. And the highway is in MILES....and on wrong side of road too...).

I say sigh, because I ENTERED it with a EUROPEAN Passport (courtesy of my English Father).
And after Brexxit.....it has decayed to UK only.
So I'm stuck.

You can use your own data if you live in TrumpLand.

Or in the Land of the Free, North of there (without all the gun deaths), you can put in whatever Canadian Tire sells...:-}

But regardless, the concept holds true.

Viva Ooomph!!

Alan
King of Ooomph
P.S. I posted the Excel spreadsheet if anyone wants it.


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## artie (8 Sep 2021)

I thought I was a sad case.


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## Trainee neophyte (8 Sep 2021)

Once you own a compressor you will rework the chart by plotting oomph against appalling cacophony. If you can hear yourself think, that is.


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## TFrench (8 Sep 2021)

If you want ultimate ooooomph and quiet, get a hydrovane. I got one at work, it's quiet enough that you can hold a conversation next to it. Cost me £350 and all I had to do was service it.


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## baldkev (8 Sep 2021)

Just work out what the most power hungry tool will be and buy a compressor that will more than cooe with it. I.e a 14cfm tool will need a good capacity of storage plus a high recharge rate so the machine isnt always struggling


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## sometimewoodworker (9 Sep 2021)

While @WarthogARJ s approach is certainly an excellent start, and much thanks for the work, it leaves out 3 important factors and looks to be a bit of a been counters one.
The first missed point is as has been mentioned sound level.
The second is expected lifespan. Probably not the easiest to discover, though duty cycle may help.
The third is avalablety of spare parts.
all 3 of those assuming you want 1) lowish 2) good 3) good will move you way above the the lowest prices.


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## Fergie 307 (9 Sep 2021)

Most important thing you don't appear to have considered is free air delivery. This is not the same as the rated cfm, it's always lower. FAD is a much more accurate representation of what will actually be delivered to the tool. Problem is most manufacturers highlight the theoretical cfm figure, based on displacement etc but a compressor that has 15cfm on this basis might only have a FAD of 10cfm. So if your most hungry tool needs 15 cfm then you need to make sure that the fad is higher than that. Then noise. As T French says a vane type is much quieter, but also more expensive.


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## WarthogARJ (9 Sep 2021)

baldkev said:


> Just work out what the most power hungry tool will be and buy a compressor that will more than cooe with it. I.e a 14cfm tool will need a good capacity of storage plus a high recharge rate so the machine isnt always struggling


Yeah, I started out doing that. But then as I said, I wasn't sure what I'd end up needing it for.
So this approach is more for trying to get as much as you can, and try not to spend too much $$$.

In any case, this way helps you organise a lot of different machines.


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## WarthogARJ (9 Sep 2021)

Fergie 307 said:


> Most important thing you don't appear to have considered is free air delivery. This is not the same as the rated cfm, it's always lower. FAD is a much more accurate representation of what will actually be delivered to the tool. Problem is most manufacturers highlight the theoretical cfm figure, based on displacement etc but a compressor that has 15cfm on this basis might only have a FAD of 10cfm. So if your most hungry tool needs 15 cfm then you need to make sure that the fad is higher than that. Then noise. As T French says a vane type is much quieter, but also more expensive.



Yeah, good point.
Correct me if I'm wrong, but it seems hard to get a reliable/consistent value for FAD.

And my feeling is that for a given TYPE (as in direct drive), especiallly for a given manufacturer, then the ratio of CFM/FAD will be pretty constant.

Do you have values for FAD for a large number of compressors?
If you do, point me to them, and I'll add it to the analysis.
I couldn't find many machines that provide FAD data though.


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## WarthogARJ (9 Sep 2021)

sometimewoodworker said:


> While @WarthogARJ s approach is certainly an excellent start, and much thanks for the work, it leaves out 3 important factors and looks to be a bit of a been counters one.
> The first missed point is as has been mentioned sound level.
> The second is expected lifespan. Probably not the easiest to discover, though duty cycle may help.
> The third is avalablety of spare parts.
> all 3 of those assuming you want 1) lowish 2) good 3) good will move you way above the the lowest prices.



Thanks.
Yes indeed, I don't really intend it to find the PERFECT machine.

I DID consider noise level, and to be honest, I myself would prefer a quiet machine, and would pay extra.
But, for a general purpose/home-DIY machine you seem to need to accept what you are given.

In any case, I would grab the sound level issue by the horns, and AVOID it.
As in, if it's a noisy pipper, why put it in the same room as YOU are?
Just add in larger bore pipe/tube to it, and put it where your NEIGHBOURS can hear it, but you cannot...:-}
Heh heh...or, in a more soundproofed area.

That's how it's done in industry, in any case. You put in a BIG air compressor and then pipe its output around as you need it.

Lifespan: yes, sure. But that's as you say a fuzzy concept, and I have addressed it to some extent by only graphing the Abbac range, which seems reasonable quality.

As far as duty cycle, that's not a relevant fact here, because all these things are going to be roughly the same range. If you DO want a given duty cycle, then PRICE begins to be much less important.

And in fact, the way to avoid THAT issue is also to AVOID it.
Most people don't understand this point, but the air they get is NOT from the PUMP.
It's from the RECEIVOR.
You use the PUMP to pressurise the receiver, and then once it's at set point, it turns off.
Then you draw down on the stored air.

Physics again: the PUMP has a SMALL piston, as in a fraction of a litre.
And its RPM is not very high: say +/- 3k I think I've seen.
So you cannot get your piston to supply the CFM (or FAD if you like) at the PRESSURE you need.
Not for any length of time.

So you store it up.

And if you need MORE "oomph", what do you do?
Well, the brute force way is to buy a bigger pump.
Expensive.
But I think you're better off getting a bigger RECEIVOR: a second one.

And if you want a REALLY nice system, designed properly, I'd do as follows:
- Standard pump feeding a standard sized receivor (start off with this)
- High pressure pump, feeding....a high pressure receivor
You then link them up.
And have them feed a big bore line, and put them where you don't hear them.

The high pressure pump doesn't need to be very big, it just needs to get to say 300 Bar.
Get a used (but still safe) nitrogen or argon cylinder, or better, get two.
Not so easy to find, but they do come up for sale.

This is not an off-the-shelf machine: you need to be able to design for the pressure, and do it safely.
Adding in pressure relief valves where needed.
And I'd get the used bottles retested to be safe: but that's not expensive.

As for spare parts, well, again, you only add in brands where you CAN get spare parts.
But to be honest, I don't see it as a big deal. These are SIMPLE systems, you don't NEED to buy a spare part from the dealer in most cases.

And you should modify your machine so it runs better.
Add in better cooling: if it stays cooler, it will last a LOT longer.

Is easy to add in another fan, and run it on PWM control, off a temperature sensor.
For the pump side, if it gets pretty hot during its work, improve the cooling on that too.
Add fins and a fan as well.


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## WarthogARJ (9 Sep 2021)

TFrench said:


> If you want ultimate ooooomph and quiet, get a hydrovane. I got one at work, it's quiet enough that you can hold a conversation next to it. Cost me £350 and all I had to do was service it.



Yeah, I think the majority of what's sold are pretty crude machines.
But £350 is a lot more, and I suspect that was 2nd hand price too, right?

And as I said, if less noise is a factor, why not move the MACHINE away from where you WORK?
If you do the calcs, you can soon see that you don't need to use incredibly big piping to reduce line pressure drop to very low.

Pressure Drop Calculator

There are lots available, but this is easy to use.
Put in however many meters of pipe you'd need to put it next to your neighbour's bedroom window.
And adjust the OD to get the pressure drop you can live with.

It's a bit quirky, the UNITS on the flow rate change themselves, and gives you an error.
You need to keep changing them back to what you want every calculation.

For the air properties, you can get values from engineersedge, or else use:
Air Temperature: 25 C
Density (kg/m^3) = 1.184
Kinetic Viscosity (10^-6 m^2/sec) = 15.62

For a bit fancier, specify it at 15 Bar, and inlet at 25C, outlet at 20C (to show some cooling as it flowx through a long supply pipe).

The intent is not a NASA grade calc, but rather to see that 6mm ID is MUCH too small, and 8mm, or 10mm is needed at the least for anything very long.

In fact in a REALLY long supply line, it itself begins to act as part of your receiver/storeage system.
And you might choose to keep it at a higher pressure than you need for your tools, and add a secondary pressure valve at the end.


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## WarthogARJ (9 Sep 2021)

Trainee neophyte said:


> Once you own a compressor you will rework the chart by plotting oomph against appalling cacophony. If you can hear yourself think, that is.


Aha, but there's an easy solution. Just buy bigger loudspeakers for your stereo!

Is same as if you have a car with annoying grinding or sqealing sounds: turn your radio up.
Voila, all that goes away.
If leaking oil begins to blur the windscreen, you can add a better degreaser in your wiper fluid to deal with that. 
If the smoke from the engine starts to obscure your vision, that's harder to address.
Usually the best solution to that is to increase insurance, and leave car parked with keys in it.
That solves everything.

But surely, I don't need to suggest this to a Russian/Greek, do I?


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## TRITON (9 Sep 2021)

Have you considered calling out mountain rescue. They are very experienced at organizing searches


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## Trainee neophyte (9 Sep 2021)

WarthogARJ said:


> Aha, but there's an easy solution. Just buy bigger loudspeakers for your stereo!
> 
> Is same as if you have a car with annoying grinding or sqealing sounds: turn your radio up.
> Voila, all that goes away.
> ...


I am followed mercilessly by by a troop of jibbering monkeys. Every time I buy a car, there is blissful silence and tranquility, until the monkeys work out I have changed vehicles and take up residence in the new one. It takes a week to ten days before the jabbering and screeching begins - subtly at first, as they don't like to admit to their presence, but I can hear them. After a few years you have to shout to have a conversation so eventually I give in and swap my monkey-demon possessed car for a new one, and all is peace and happiness for about a week.



...and the nature of Monkey was irrepressible.


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## sometimewoodworker (9 Sep 2021)

WarthogARJ said:


> Get a used (but still safe) nitrogen or argon cylinder, or better, get two.
> Not so easy to find, but they do come up for sale.


I would look at diving cylinders as, if you can buddy up with a testing shop they will tell you when they have cylinders that fail the inspection & if you guarantee that they will only ever be used at 150bar or under you may be able to buy them. You will be limited to 11 litre tanks but maybe able to get them at a reasonable price.

In fact there are a couple of 15 litre tanks on eBay that are out of test selling for £20 & £22. Ideally you would get them tested, that would give you a 5 year, in test, lifetime with the likelihood of them retesting again.


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## clogs (9 Sep 2021)

looking for a compressor.....
for general use and want it to last
1, pump needs to be cast iron if poss, twin cylinder...better cooling if a V twin....
2, always a belt drive, much quieter and if the motor or the pump goes easy to replace the item.....small direct drives are just so noisey....gotta think of the neighbours....like it or not....
3, biggish tank but thats a personal choice depending on room and mobility....
4, costing, but I would be thinking of around £400 plus.....unless u go used....
5, there are some excellent used comp out there, mostly because people buy to SMALL.....remember the big ones wont like working on an extension lead...

I was lucky to buy this monster for not much more than a new 14 cu/ft comp....but it's 3 phase.....Atlas-CopCo....very quiet .....check out the size.... compare the chair....put it on caster as once a year I totally empty my workshop and sweep n wash the floors....not for everybody tho....hence cost...


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## mikej460 (9 Sep 2021)

I bought one of these and it has been excellent, it's very quiet.
Wolf 100 Air Compressor 14cfm 3HP 150psi 10bar Twin-Cylinder Belt Driven 5060255482407 | eBay


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## Sandyn (9 Sep 2021)

There is a comparison method called multicriteria decision analysis which is really good technique for finding the most suitable choice based on your personal requirements. You first identify the parameters which are important to you and allocate a weighting factor and for each compressor, a score. Each machine gets a rating, which is the weighting factor multiplied by the score. Many levels can be nested if a parameter has sub-parameters. It's a lot simpler than my explanation, lol. You put in all the figures and the 'best' option gets the highest score.


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## Fergie 307 (10 Sep 2021)

WarthogARJ said:


> Yeah, good point.
> Correct me if I'm wrong, but it seems hard to get a reliable/consistent value for FAD.
> 
> And my feeling is that for a given TYPE (as in direct drive), especiallly for a given manufacturer, then the ratio of CFM/FAD will be pretty constant.
> ...


They really should quite it as part of the spec, but often you have to ask. Generally I would say places that specialise in compressors will give that information in their listings, places like Machine Matt maybe not.


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## Pineapple (10 Sep 2021)

WarthogARJ said:


> Thanks.
> Yes indeed, I don't really intend it to find the PERFECT machine.
> 
> I DID consider noise level, and to be honest, I myself would prefer a quiet machine, and would pay extra.
> ...


Thanks for your apparently sound advice. By way of helping other members to understand you:->>








Reservoir definition and meaning | Collins English Dictionary


Reservoir definition: A reservoir is a lake that is used for storing water before it is supplied to people. | Meaning, pronunciation, translations and examples




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## WarthogARJ (10 Sep 2021)

sometimewoodworker said:


> I would look at diving cylinders as, if you can buddy up with a testing shop they will tell you when they have cylinders that fail the inspection & if you guarantee that they will only ever be used at 150bar or under you may be able to buy them. You will be limited to 11 litre tanks but maybe able to get them at a reasonable price.
> 
> In fact there are a couple of 15 litre tanks on eBay that are out of test selling for £20 & £22. Ideally you would get them tested, that would give you a 5 year, in test, lifetime with the likelihood of them retesting again.


Hi,
Yeah, I was looking at a range of tanks: not all the following are GOOD options though, I'm just showing the possibilities:
(1) Scuba tanks: aluminium or steel
(2) Conventional gas cylinders; Nitrogen, oxygen, argon, air
(3) Fire extinguisher bodies:
(4) Propane gas cylinders:

I'm a Materials Engineer (started as Chemical Engineer), and have been involved in a wide range of industrial environments, many of which use pressure vessels. For instance, at age 22 in 1982 I was offshore as a Field Engineer, working with well head pressures in excess of 10k psi at times (H2S and CO2 as well).

For this, I would follow the general provisos of the relevant ASME Codes: for pressure vessels. And I think it would be a "Bad Idea" to use any pressure vessel that the scuba guys failed. Well, I suppose it would depend WHY they failed it, but their inspection/hydro test is pretty basic.

You do not want anything that is pressurised with ANY gas to suddenly decide to unzip itself. A liquid is a different story, but not compressed gas.

One issue is a compress air system tank is being continually pressurised, and then depressurised. Maybe not drained to zero very often, but it sees a lot of cyclical pressure. Whereas a scuba tank, or most of the other examples see very few pressure cycles. This will affect possible failure due to fatigue. 

I would approach the Design of a Receiver Tank as follows:
(1) Determine my Design Pressure and Volume: maybe a range, going from high to low
(2) See what existing vessels are in that range, and what applications they are intended for.
As well as their materials, and dimensions.
(3) Unless the vessel was intended for an air compressor, you'd need to redo the design calcs for it.
And look at safety margins, material toughness, ability to inspect it for flaws etc etc, and to test it
It needs to be designed as "leak before break".
(4) Anything I used, I would only use iof after a full inspection.
Do something at least as demanding as a scuba tank test (hydro test, and visual inspection of internals). And I'd hydro test it to a LOT higher pressure than my Design Pressure: in effect, Proof Test it.
I might well get it X-rayed: that's not very expensive.
(5) Materials: I would try NOT to use aluminium. Nothing wrong with it, but for this purpose, unlike a scuba tank, you don't need the low weight.
And a good pressure vessel's steel COULD and SHOULD be a LOT tougher.
(6) Look at what safety valves, connections and pressure gauges are needed

This might sound $$$, but if you can do the design yourself, it's not.

Notes:
Scuba Tanks: if you rule out aluminium, then just steel tanks narrow your potential down a lot.
And they are pretty small.

Gas Cylinders: these are made to be banged around, are good quality/tough steel, and are well inspected/QC'd at manufacture, as well as regularily afterwards.
And they are BIG suckers: 50 litres water volume up to 200 litres.

They are hard to BUY though, most companies only want to RENT them.
One way would be to get it from a company that does a deposit, and no rent.
A full N2 cylinder costs you £135 then (deposit that you don't get back if you keep it).
The gas costs you £55, so you want to have someone else use that first.

See the attached graph, where I used prices from a big UK supplier of auxillary tanks. And i put on the point for a Nitrogen 50 litre cylinder (rated at 200 Bar). It's a LOT better. I'm expressing volume in cubic meters at room conditions (1 Bar).

The off-the -shelf tanks are BIG, but not very high pressure (10 - 11 Bar).

You would need to add a little bit for a relief valve, and a few connectors. And be clever about how you dealt with water draining. Maybe put the cylinder upside down, with a 4 way-T piece. Drain ater from the middle one, and have a gauge on one, and the outlet/intel on the other.

(3) Propane Tanks, and Fire Extinguishers:
I've not looked at how these are designed/made.
Perhaps they could be suiktable.
But I think too thin a wall thickness.
And not high enough pressures.
You want to get to 200 Bar I think.

I'm not saying this is the BEST solution, but it shows the idea is interesting to pursue.


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## WarthogARJ (10 Sep 2021)

Sandyn said:


> There is a comparison method called multicriteria decision analysis which is really good technique for finding the most suitable choice based on your personal requirements. You first identify the parameters which are important to you and allocate a weighting factor and for each compressor, a score. Each machine gets a rating, which is the weighting factor multiplied by the score. Many levels can be nested if a parameter has sub-parameters. It's a lot simpler than my explanation, lol. You put in all the figures and the 'best' option gets the highest score.


Well, that's basically EXACLY what I did here.

(1) Identify important parameters: pressure, supply rate, price, receiver volume
(2) Why use an arbitrary "weighting factor" when Mr. Physics is there to help....I'm an engineer, we only use "Arbitrary Weighting Factors" when we need to guess. 
No guessing needed when Mr. Physics is your friend.

OK, if you REALLY want Noise as a factor, you are welcome to try putting it in.
But then PRICE becomes irrelevant.
Or else you are willing to buy a REALLY low power machine.

But as i said, if Noise is important, just put it further away.
A 10mm section of pipe is CHEAP.


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## WarthogARJ (10 Sep 2021)

Fergie 307 said:


> They really should quite it as part of the spec, but often you have to ask. Generally I would say places that specialise in compressors will give that information in their listings, places like Machine Matt maybe not.


Actually, the place I got my data from claims these values ARE indeed FAD.
For ALL these machines in my example.

But if you have a bit of data for ANY piston pump, you can do a pretty accurate estimate of FAD.
You need to estimate a few things, but can do it pretty accurately.

The big issue is not really FAD I think, but rather I suspect many (most?) people use too small a line from the compressor to their tool.
Something like 6.5mm ID costs you a LOT of Bar if you have anything longer than a few meters of it, and at anything bigger than say 250lpm. 

Can work it out on the link I sent.


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## WarthogARJ (10 Sep 2021)

mikej460 said:


> I bought one of these and it has been excellent, it's very quiet.
> Wolf 100 Air Compressor 14cfm 3HP 150psi 10bar Twin-Cylinder Belt Driven 5060255482407 | eBay


Yeah, good choice.

I saw something like it.
In fact I think it's just a rebranded Abbac one.

It's a belt drive, and I didn't want to go over £250 if i could get what else I needed.
It was hard to find lower priced belts to put on.

I wonder how loud it's rated at?
I saw a YouTube video looking at noise vs Direct and Belt: need to watch that.

I'll add it onto the chart, and a few other ralated points.
I think in general belt drives have better duty-cyle than direct.
Maybe are just better cooled.


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## WarthogARJ (10 Sep 2021)

TFrench said:


> If you want ultimate ooooomph and quiet, get a hydrovane. I got one at work, it's quiet enough that you can hold a conversation next to it. Cost me £350 and all I had to do was service it.


BTW, what are its details?
I can add it to the chart.

But it needs its price "normalised", as in was that £350 a 2nd hand price?
Because the others are new prices.
Obviously at 2nd hand they cost less.

But then, maybe pretty worn.


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## WarthogARJ (10 Sep 2021)

clogs said:


> looking for a compressor.....
> for general use and want it to last
> 1, pump needs to be cast iron if poss, twin cylinder...better cooling if a V twin....
> 2, always a belt drive, much quieter and if the motor or the pump goes easy to replace the item.....small direct drives are just so noisey....gotta think of the neighbours....like it or not....
> ...


Awesome.....am jealous.
Err.....Tell me where you live in Crete, please....I have a friend with a big boat....:-}


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## WarthogARJ (10 Sep 2021)

OK, so thanks VERY much for all the EXCELLENT comments.
They heped me.

I'm not trying to sound as if I'm a compressor expert, I was really just trying to describe a way I think is useful to decide what could work.

I shall add in the comments that guys have made:
- Noise
- Reliability
- Multi-Criteria Analysis (for the fuzzy parts) 
- Auxilliary tank
- Belt vs Direct Drive
- Materials: Cast iron


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## WarthogARJ (10 Sep 2021)

And NEXT, is for a WELDING MACHINE.
And THAT is a more complex subject.


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## mikej460 (10 Sep 2021)

I've read good reviews about these
Hyundai 50 Litre Air Compressor, 11CFM/100psi, Oil Free, Low Noise, Electric 2hp | HY27550 (hyundaipowerequipment.co.uk)


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## Sandyn (11 Sep 2021)

WarthogARJ said:


> Why use an arbitrary "weighting factor" when Mr. Physics is there to help....I'm an engineer, we only use "Arbitrary Weighting Factors" when we need to guess.


So why is this such a difficult choice for you??


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## sometimewoodworker (11 Sep 2021)

WarthogARJ said:


> (5) Materials: I would try NOT to use aluminium. Nothing wrong with it, but for this purpose, unlike a scuba tank, you don't need the low weight.


That is in fact a false idea. A steel scuba tank is typically 10% lighter than its aluminium counter part For the same volume. Aluminium tanks don’t suffer from the rust problems inherent in steel tanks if there is any moisture in the fill. aluminium tanks are also cheaper for the same volume. So I completely disagree since the cost of a high pressure dry compressor and filters isn’t cheap.

the light weight tanks are the SCBA not the SCUBA tanks



WarthogARJ said:


> Whereas a scuba tank, or most of the other examples see very few pressure cycles.


The ones I was using would see around 5 full cycles per day 7 days a week going from about 190 bar to about 20 bar whereas I question that you would be putting a compressed air tool tank through that kind of numbers of cycles.

The SCBA tanks would definitely not see anything remotely like that kind of usage.


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## sometimewoodworker (11 Sep 2021)

WarthogARJ said:


> Gas Cylinders: these are made to be banged around, are good quality/tough steel, and are well inspected/QC'd at manufacture, as well as regularily afterwards.
> And they are BIG suckers: 50 litres water volume up to 200 litres.
> 
> They are hard to BUY though, most companies only want to RENT them.


That would depend on your location and local conditions. Here you can’t easily rent, you have to buy but then exchange for full ones.


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## BillK (11 Sep 2021)

Couple of thoughts (Ok, I can't count) -

Doubling capacity via extra tanks might improve things. But say you double the capacity, the comp then runs roughly twice as long to refill. The more a comp runs the warmer & wetter the air. Might collide with duty cycle issues if you're caning the thing.
You can connect two comps and run them together if you rig up a Y-manifold type effort. This way you have the hp of two pumps going. But you have to juggle the pressure switch settings or one of them does far more of the work.

A typical 1/4" PCL connector with it's small port can do something like 40cfm @ 100psi, so personally I don't worry much about hose size - except make it at least 10mm, pref 12. 
A lot of tools are supposed to have a lightweight 'whip' line at the end, to help with handling and wear & tear on hands etc. Those aren't big fat lines.

see spec here, 43cfm @ 100psi - ACA2593 Standard Adaptor Plug R 1/4 Male Thread | PCL (pclairtechnology.com) 

I've got two 3hp/150 comps, one of them's is an Abac. It's the quieter of the two but builds full pressure slower than the other. I do like Abac, they carry parts, even though mine's 15 yrs old or more I can still get all sorts for it. They also used to have engineers you could book to come round for repairs, maybe still do.

Rough rule of thumb, 3cfm FAD per pump hp. It's enough to get an idea good enough to free you from possible dodgy specs/claims and save time poring over details. I don't get too hung up on things; for the budget, get the most hp as first priority, and look at tank capacity and is the seller reliable or accessible. If it goes bang, can you send it back (happened to me, brand new comp, exciting shower of fireworks emitted on turning on). Can you get parts.
Of course if it's just for occasional use then the seller & parts thing matters less, but I use mine virtually every day so it's high on my list.


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## WarthogARJ (11 Sep 2021)

Sandyn said:


> So why is this such a difficult choice for you??



I wouldn't know.
Why does it seem like a difficult one for YOU?

For me, it was easy: I looked a bit online, including this Forum, and to be honest, nothing really that helpful in narrowing the choice down from the dozens, actually HUNDREDS of compressors available.

Sure, there were a few good, but somewhat isolated, comments on the Forums.

So i do what i always do if I'm buying a tool I want to use:
- Research the subject if I don't know enough first hand to decide already
- List the characteristics of a fairly wide range, and see how much the various charactersitics that are important, TO ME, vary
- Then narrow down on the range that looks good, and concentrate on price/availability for ones that fall in that general range
- I do it in Excel: makes it REALLY easy to add things, and organise it
- And as they say, "A picture is worth a 1,000 words", which is what a GRAPH or CHART is
- And right away, the best compressor for ME, that satisfied MY requirement, jumps right out at you: Bottom Right Corner

It all took me about an hour, and I'm pretty sure i'm going to get something that I'll like.
I'm an engineer, I don't really believe in impulse stuff based on vague ideas, or a few inconsistent remarks from someone on a Forum who might well not know what he's talking about.

Sure, if you are on a given Forum for a while, you start to see who knows their stuff, and who just fires off silly comments....
This Forum DOES seem to have some guys who DO know their stuff.

There are always the guys who sat in the back seats of the class and didn't listen, caused dung in the class, and failed the tests/exams....best to ignore them.
But sometimes you do reply if it's worth your while and others.


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## mikej460 (11 Sep 2021)

ooo fight fight!! now where did I put that popcorn


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## WarthogARJ (11 Sep 2021)

BillK said:


> Couple of thoughts (Ok, I can't count) -
> 
> Doubling capacity via extra tanks might improve things. But say you double the capacity, the comp then runs roughly twice as long to refill. The more a comp runs the warmer & wetter the air. Might collide with duty cycle issues if you're caning the thing.
> You can connect two comps and run them together if you rig up a Y-manifold type effort. This way you have the hp of two pumps going. But you have to juggle the pressure switch settings or one of them does far more of the work.
> ...


Thanks: good points.

i MY case, I have NOT owned a compressor for 15 years.
I've NEVER owned my own, but borrowed them, or used one where I work, or study at.
So I wanted to get something good.
And yes, Abbac seemed OK to me (I'm happy you've found that too).
I think they are Italian, but have a UK presence to distribute them to suppliers.
You cannot buy direct: correct?

Yeah, I agree that you need to think out your design carefully if you are moving away from something "off-the-shelf".
As far as tank size, my view is that after a given size of tank, it ends up acting more as a "battery" for your pumping system.

You can work out for any system how much the compressor is going to need to work, based on:
- Your useage demand
- Pump specs: piston size, RPM, efficiency
- Receiver size
- Cooling of the unit (even if it's not ACTIVELY cooled by fans/fins etc, it is STILL PASSIVELY cooled, just by sitting in a colder room)
- Air supply (ambient temperature, pressure, relative humidity)

And your system itself can vary, because you can select:
- Primary pump size: piston size/number/type, efficiency, RPM (this gives you pressure output and volume)
- Secondary pump size: as above
- Primary Receiver: size, pressure rating
- Control system
- Thermal aspects: degree of active/passive cooling
- Humidity/water: you cannot really control the RH of your intake air for a smaller unit, and as you compress it, depending on the thermal aspects, the saturation level of the air changes

One aspect that's not been mentioned, is in some places the cost of electricity varies during the day.
And you might have some supply of your own: solar, wind etc.
I have not looked at it carefully, but in principle you MIGHT be better off having a BIG high pressure receiver that you pressurise when the sun is shining, the wind is blowing, ort rates are lower (say after 19h00).
The idea of using pressurised air as energy storeage IS used, and you'd need to see if you can get a big enough tank to make it worth your while.

If you have a bit of land, then you might well dig a hole, and pour yourself a big concrete tank, with a thin lining of steel, or fibreglass.
And then pressurise THAT to store in effect, cheaper electricity.
Not saying it's a good idea for all.

From what I've seen, the suppliers charge a LOT for a pretty basic pressure vessel.
I mean, a vessel that's made with low end structural steel, and is rated to 11 Bar is NOT a very difficult thing to make.
They get tube tubing/piping of the required OD/ID, and then weld on two dished ends (that are probably forged by the thousands in India).
Then weld on a few connections, and legs.
And slap some paint on.

The standard that most suppliers go by is:
_*DIRECTIVE 2014/29/EU OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 26 February 2014 on the harmonisation of the laws of the Member States relating to the making available on the market of simple pressure vessels*_

You can get a copy (free) here:
EUR-Lex - 32014L0029 - EN - EUR-Lex

It's NOT manadatory to follow it, which surprised me.

The design methods it specifies are pretty relaxed, basically:
- Design for 5 x Design WP
- No specific provision for corrosion
- Welds are not 100% NDE, but rely on welder qualification and batch tests
- Hydro/pneumatic test (with quite a relaxed requirement of 150% of Design WP)
- Material requirements pretty wide open, and some attention is given to toughness, but nothing very rigorous, and especially not to CONSISTENCY of mechanical properties throughout a BATCH of supplied steel
- Welds are NOT checked for mechanical properties, they rely on general certification, and in effect on the final hydro/pneumatic test

I'm not saying the STANDARD is poor, but that the QA/QC that goes with it is not very demanding.
And no attention to failure by fatigue.
So if a conscientous company follows it, what it makes could be good.
BUT, since those tend to be Europeran based, their LABOUR costs are high.

If it's been made in China or India, where labour costs are less, then I have a concern over the potential of failures from a bad weld, or out-of-spec steel.
And not all EU companies are so great either: I'm dealing with a Spanish one now that does HORRIFIC welds......

In my eyes, for a pressure vessel that sits in an area where people are, and sees fluctuating pressures (due to being pumped up, discharged somewhat, then re-charged), and is expected to have a lifespan of many years, even DECADES....well, I would like more attention paid to failure by fatigue.

For fun, you should work out the amount of ENERGY that's stored in a receiver tank of any size.
And compare it to a bomb.....
Your 90 litre tank has the equivalent of 30g of TNT: same as a WWII hand grenade does.
A 1,000 litre tank at 11 Bar has the equivalent of 0.3kg of TNT.

I think something that's sold for hundreds of $$/£££/Euros should have had a better NDE: like full X-ray or u/s of the welds.
And tighter specs on the steel used.

And done as PART of the EXISTING price.

Otherwise, sell it for LESS, a LOT less, and the end user can get it done.
I would.


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## WarthogARJ (11 Sep 2021)

mikej460 said:


> ooo fight fight!! now where did I put that popcorn


Well, it's not a very constructive comment, and he DOES remind me of the lame brains in high school class.
Pretty funny actually, the head lame brain made a smart-ass comment, and the Geograhy teacher, very cool guy, said "Greg (said lmne brain), you know in 10 years time you will be working minimum wage, if you are lucky, and have a guy exactly like Alan as your boss".

And you know what.....well, I wasn't his boss, but Greg DOES work min wage pushing an silly person stick around....
Nothing WRONG with that, but he sure doesn't like it.


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## mikej460 (11 Sep 2021)

Well if you come across as all high and mighty my friend expect people to poke fun at you. Keep it light, we are all here to help one another and casting dispersions like you've just made amount to personal attacks and you just end up letting yourself down. I'm sure you have a lot to contribute so just be kind.


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## TFrench (11 Sep 2021)

WarthogARJ said:


> Well, it's not a very constructive comment, and he DOES remind me of the lame brains in high school class.
> Pretty funny actually, the head lame brain made a smart-ass comment, and the Geograhy teacher, very cool guy, said "Greg (said lmne brain), you know in 10 years time you will be working minimum wage, if you are lucky, and have a guy exactly like Alan as your boss".
> 
> And you know what.....well, I wasn't his boss, but Greg DOES work min wage pushing an silly person stick around....
> Nothing WRONG with that, but he sure doesn't like it.


You are being quite condescending mate. I understand that's probably a side effect of being an engineer but you might want to calm it down a tad. As someone who sat at the back of the class with the lame brains I'd like to think I'm an outlier for your theory.


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## WarthogARJ (11 Sep 2021)

sometimewoodworker said:


> That is in fact a false idea. A steel scuba tank is typically 10% lighter than its aluminium counter part For the same volume. Aluminium tanks don’t suffer from the rust problems inherent in steel tanks if there is any moisture in the fill. aluminium tanks are also cheaper for the same volume. So I completely disagree since the cost of a high pressure dry compressor and filters isn’t cheap.
> 
> the light weight tanks are the SCBA not the SCUBA tanks
> 
> ...



Aha.....OK, I was in correct to say that an advatage of aluminium SCUBA tanks was less weight.
Sorry for that.
Although when EMPTY they FLOAT, whereas STTEL ones do NOT tend to float.

But it's irrelevant for our application.

My main point is that the mechanical properties of aluminium are NOT an advantage for pressure vessels for ar compressors.
They are WORSE.
For many reasons:
- Toughness
- Plastic deformation: aluminium deforms under all loads, whereas steel only deforms above YIELD
- Aluminium alloys, especially those used for SCUBA tanks are very touchy about impurities in the alloy 

As far as loading cycles, sure, the SCUBA and related tanks see a loading cycle that goes from high load, to low load.
But not that many of them, relative to the design of the cylinder (big thick walls, and that drops the stress down).
And it's fine: for THAT application.

And a compressor tank is going to see CONTINUAL series of loading cycles due to use.
I don't mean COMPLETE pressurise to depressurise, but pressure FLUCTUATIONS.
It's STILL a loading cycle.

These CAUSE fluctuations in the HOOP STRESS.
And THAT, if you have a big enough flaw/defect, and if material toughness is low enough, and with enough cycles CAN cause FATIGUE.

If your material is not tough enough.
And using a 2nd hand scuba tank, with a HISTORY of loading cycles due to use, and hydro testings doesn't help matters.

It's been identified as problem with SCUBA tanks, because a flaw that starts to grow, can remain sub-surtface, and thus not visible.
And can cause the dangerous case of "break BEFORE leak".
See attached papers (Page 54, Conclusions).

The aluminium alloys used for SCUBA tanks doesn't have an especially high fracture toughness.
Compared to what you can EASILY get in a low alloy steel.
Or even better, a stainless steel like 316.

In addition, the way you make an aluminium scuba tank causes its mechanical properties to be different in respect to the longitudinal and circumferential directions.

This is not an issue for SCUBA tanks, assuming you manufacture them properly, and hydrotest regularily, but it's NOT ideal for use in air compressors.

So to make it clear, if you design a compressor tank PROPERLY, with the correct material, and with goold QC in manufacture, ESPECIALLY of the welds, then fatigue should NOT be an issue.

However, I draw your attention to what USED to happen in industrial presure vessels before they started using the ASME codes for design.

Check the graph in Figure 3.
You can see when they started going by ASME design code ideas.

And I'm NOT saying the current compressor ar vessels are ALL unsafe, but I AM saying that the standard regulating them is NOT very rigorous. And if you start using the wrong tanks, you CAN go BOOM....
Or buy from a poor manufacturer.

But if you are competent, you COULD get a tank that is both well designed, as well as less expensive.


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## Sandyn (11 Sep 2021)

WarthogARJ said:


> I'm an engineer, I don't really believe in impulse stuff based on vague ideas, or a few inconsistent remarks from someone on a Forum who might well not know what he's talking about.





WarthogARJ said:


> Well, it's not a very constructive comment, and he DOES remind me of the lame brains in high school class.
> Pretty funny actually, the head lame brain made a smart-ass comment, and the Geograhy teacher, very cool guy, said "Greg (said lmne brain), you know in 10 years time you will be working minimum wage, if you are lucky, and have a guy exactly like Alan as your boss".
> 
> And you know what.....well, I wasn't his boss, but Greg DOES work min wage pushing an silly person stick around....
> Nothing WRONG with that, but he sure doesn't like it.



Hey, chill out.   The comment was meant in fun, but I sincerely apologise if I have somehow offended you. It's never my intention to do that. I'm Scottish so our humour is often based on being able to laugh at ourselves (and others). We love to observe the humour in daily life. I just thought, "well there's an 'Engineer' who seems to have all the answers, but he can't choose a compressor". I still think it's humorous. sorry.
I'm absolutely not offended in any way about your insinuation that I'm some lame brain and don't know what I'm talking about. You are probably correct. As for minimum wage! I wish I was smart enough to get the minimum wage!! I'm so stupid I have to pay my boss to go to work!! The really funny thing is, apparently I'm stupid, but it only took me 5 min to choose a compressor, so being really clever doesn't seem to be a great advantage?
You seem to know your stuff, so chill out, welcome to the forum and enjoy.


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## baldkev (11 Sep 2021)

WarthogARJ said:


> Is same as if you have a car with annoying grinding or sqealing sounds: turn your radio up.
> Voila, all that goes away.
> If leaking oil begins to blur the windscreen, you can add a better degreaser in your wiper fluid to deal with that.
> If the smoke from the engine starts to obscure your vision, that's harder to address.
> ...



Ahhh, you have owned a few fords


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## TRITON (12 Sep 2021)

.


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## TRITON (12 Sep 2021)

Sandyn said:


> Hey, chill out.   The comment was meant in fun, but I sincerely apologise if I have somehow offended you. It's never my intention to do that. I'm Scottish so our humour is often based on being able to laugh at ourselves (and others). We love to observe the humour in daily life. I just thought, "well there's an 'Engineer' who seems to have all the answers, but he can't choose a compressor". I still think it's humorous. sorry.
> I'm absolutely not offended in any way about your insinuation that I'm some lame brain and don't know what I'm talking about. You are probably correct. As for minimum wage! I wish I was smart enough to get the minimum wage!! I'm so stupid I have to pay my boss to go to work!! The really funny thing is, apparently I'm stupid, but it only took me 5 min to choose a compressor, so being really clever doesn't seem to be a great advantage?
> You seem to know your stuff, so chill out, welcome to the forum and enjoy.


I agree. nothing to get so upset about, it was a clear observation.
What I fail to see is the point of this tread. It appears to be all about the data, rather than any sort of need of a compressor.


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## Keith 66 (12 Sep 2021)

^^^^^ Exactly, First what do you need a compressor for? If you want to drive a little airbrush a tiny diaphragm one will do, if you want to sandblast you will need a monster! In between is a multitude of sizes.
Main thing is FAD, Second is Horsepower of motor, If you have 3 phase power happy days as you can then get a nice big Ingersol rand VTwin type 15 or 30 & have tons of air. If you only have single phase you will be limited to approx 3 hp & that means high revving noisy machines of about 12 cfm or a bit more.
As for getting failed test scuba tanks to use as receivers? If a tank has failed test they chop a hole in it, they have to. Would you trust joe blogs of the street not to sell it on ebay ? I wouldnt.
Buy the biggest compressor that will run of the supply you have!


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## Sandyn (12 Sep 2021)

I'm sure you have found SGS. already. They have a great range of compressors, at good prices. They are probably all badged. There is some useful information on the site as well.

As a peace offering, I have attached a manual on Multi-Criteria_Analysis  it's aimed at local government, but the principle applies to any decision making.


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## mcgranag (13 Sep 2021)

WarthogARJ said:


> ...............- Thermal aspects: degree of active/passive cooling...
> One aspect that's not been mentioned, is in some places the cost of electricity varies during the day.
> And you might have some supply of your own: solar, wind etc.
> I have not looked at it carefully, but in principle you MIGHT be better off having a BIG high pressure receiver that you pressurise when the sun is shining, the wind is blowing, ort rates are lower (say after 19h00).
> ...


I have two observations on the above regarding (i) Thermal energy, and (ii) fatigue

(i) Compressed air is not a good energy storage medium, unless significant care and equipment is used. Once air is compressed as well as reducing volume, it increases in Pressure, and in Temperature. Once stored in a receiver, the pressure might still be there the next day, but the temperature generally isnt. This loss of thermal energy to ambient can be high and is a significant obstacle tor using compressed air as an energy storage medium. Compressing the air to higher pressures become more difficult in machinery terms too (think seals, leaks) and also with respect to temperature increasing further (think diesel engine). These factors are why most everyday compressed air systems are around 5-9 bar. For energy storage or recovery, small systems are not worth it.

(ii) Fatigue is likely not an issue. If the receiver is designed for 5x WP as written above, then that probably means that internal stresses in the receiver reach a max of 20% of UTS (ultimate tensile strength) in the worst case scenario. Fatigue in ferrous metals is only an issue above around 40% of the UTS. Once the air receiver stresses are kept below that value within their designed working pressure, then fatigue will not occur regardless of number of cycles. Non ferrous metals are a different matter, where every cycle can contribute to fatigue failure.


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## mcgranag (13 Sep 2021)

WarthogARJ said:


> ........................
> - Plastic deformation: aluminium deforms under all loads, whereas steel only deforms above YIELD
> .........................................


I'm not sure what you mean by deform here? Surely both aluminium and steel exhibit elastic behaviour before the Yield point, and plastic behaviour after? In both metals, any deformation below yield pt will be temporary, and above yield pt will be permanent?


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