Pack Volume Calculation Doesn’t Add Up

[dan mchale]

So, is 12" x 8" the ASTM standard? I still think it's practical to have a quick method of comparing packs when somebody has an old pack and they don't know the volume of it. It can be a difficult thing to go find a bushel of 20mm balls and a tall box with a 12 x 8 cross-section. Having the standard still does not eliminate the problem for people that do not have the right tools and need to measure a pack, as in the case I outlined above. My current particular method is quick for people that want quick comparisons, without getting the beads out. The interesting thing about all this is that you can fill a cylindrical bag and for all practical purposes morph the cylinder/bag shape into something more rectangular and eliptical, without changing the contents of the bag. This has to do with the fact that the volume of the contents of most backpacks is not constant, because only a small percetage of the content is actually solid. I have always asked, "do you want a solidly packed or loosely packed 3,000 cuin", for instance, to really confuse people. This is why I prefer to use the cylindrical method since it represents the maximum true volume a pack is capable of rather than some concocted fantasy. Most packs are fairly constant in their dimensions. Lay it flat, measure across, double that figure and you get a circumference – no filling needed. Many people don't even know the circumference of their pack. A packs circumference can be just as meaningfull a number as cuin or liters. It's easy to break it down into a retangular shape on paper. What do you do with something that says 60 liters?

A 12 x 8 box 23" tall gives 2208 cuin.

A cylinder of the same outside dimensions (40") that is 23" tall is 2921 cuin ! This is where the illusion is; containers with these 2 different shapes would not have the same height, as others have said.

There was a time, back in the 70s when I remember when people started using cuin to describe packs. I remember not liking it – like we did not need it because we got along without it so well! Then companies started saying how many stitches per inch they put in their product, and on and on it went until today we have spreadsheets full of this stuff! HaHa!

[Franco Darioli]

Maybe this is made out to be more complex than it is.
If I get a 1L pot and fill that with polystyrene balls, I will have "1 liter" of balls.
Now no matter what shape container I put them in , they will still have a tot volume of 1 liter (about 61 cubic inches)
So if I keep putting balls into that pot and then transfer that to the inside of my pack eventually I will know the capacity in liters and cubic inches (well close enough…) regardless of the shape.
Of course I could be wrong…
Franco
BTW, I just had a look at some backpack sites to see how they translate from Liters to Cubic Inches.
Some are consistent, others, like Osprey are all over the place…
For example the Aerial 75 is correct but not so the 65
http://www.ospreypacks.com/detail.php?productID=189&colorCode=609&tab=specifications

[dan mchale]

You are wrong because packs can take on different shapes and different shapes have different volumes? That's the point of the thread. :>) The main problem is that you simply can't say a circle and a rectangle are the same because their perimeters add up the same. It really should not matter what you dump the beads into to measure though, as long as the object is easily measurable. If it's a tall skinny rectangle it should not matter, it's just not a practical pack shape. Sorry for editing this – I should have continued in the thread!

[Jerry Adams]

When you use the balls, do you calculate the volume of the pack as the volume of the balls?

What about the space between the balls? If you don't include that you're missing a bunch of the usable volume.

If it's a frameless pack, do you make the pack assume the shape as actually used? sort of flattened out against your back? Or do you just stuff in balls which will make it assume a cylindrical shape?

I think this would also happen, but to a lesser degree, with a framed pack.

I see what Dan is saying, the balls method of determining volume is not a clean solution to determining volume.

[Bob Gross]

The balls method will get more accurate if you use smaller balls. For example, marbles. Or, if you want to go crazy BB's.

–B.G.–

[Ken Thompson]

Why not use liquid? Plastic bag in pack and fill.

[Jerry Adams]

If you used liquid, you'de still have the problem that it would distort the pack into something more like a cylinder than the actual shape as used.

[dan mchale]

Since we don't know how to calculate the volume of an rectangular shaped ellipse we are exaggerating the effect of flattening the cylinder aren't we? The pack is never going to really have corners and it does not matter what shape we dump its contents into as long as it's not so extreme that it does not hold the balls effeciently and is easily measured. If the perimeter of the box we dump the balls into is the same as the circumference of the cylinder, the box will simply be taller. The volume is the volume, it just won't be the same height as the pack being measured. Sure, a 10 x 10 rectangle is 100 square somethings but if we have a 15 x 5 rectangle that also has a 40 something perimeter, it only has a 75 square something surface area so it has to be taller still. None of this matters for the measuring device. What are we even talking about anyway? :>) There's really no mystery here except that in assuming we can turn a circle into a rectangle and the corresponding heights of the finished product will be the same.

Fill those packs with beads. Dump them into a taller rather than a flattish box because the margin of error will be less in measuring the taller box.

[Wild Exped]

'Why not use liquid? Plastic bag in pack and fill'

This is good. For the point of the exercise, finding a universal measurement, it seems this would be a better, more simple method. Its better having it done than not.

There might be some variation in the distortion but really, what about the space between your little balls?…. The point is to compare one against the other with a general idea of what size you need and are buying. Not good enough for the pedants amongst us though obviously, but perhaps even satisfying for them in actual practice.

[dan mchale]

The space between the balls does not matter (literally!) It's the way the balls fill differently shaped vessels in a constant and predictable way that matters. Water molecules have space between them also.

Franco, you were right. I just had to work through where the illusion was in all this. Andrew had it in the second post, although he did not address the illusion. It was starting to look like volume could actually vanish into thin air…or was that fat air? :>)

[Mike W]

…

[Wild Exped]

Mike your talking about a better way to measure pack volume (regardless of distortion (and as was I) and applied in a simplified,real world scenario) I agree, but it doesnt really relate to the question at hand or the standards (as they stand).

I will say that I have four new Osprey packs here and it is strikingly obvious that they run small in comparison to any local or european purchase, no measurement needed.

[dan mchale]

Looks like we have a new definition for MYOG;

MEASURE YOU OWN GEAR

Clearly, filling the pack above with water creates a circumference/cylindrical type measurement for volume. There may be an astm standard but I think this community would be interested in simply learning to measure volume on it's own. Being able to walk into a store with a tape measure to measure circumference and length is much easier than walking in with a bushel of beads and a box. They won't let you in with the latter.

[Franco Darioli]

I really cannot see how (and why…) one would come up with a mathematical formula for an irregular and variable shape like a backpack.
If you are that keen to get a MYOG (Measure Your Own Gear) going with packs , there is a very simple way.
Sorry back to balls…
Get hold of a bag of filling material like polystyrene or starch pellets.
Fill a 2 -4 liter pot with that. Transfer that into a bag (a thin shopping bag will do)
(I will use the 4L version…)
Tape the bag up but not tight so that the material inside can change shape. Once you have 10x 4L bags you will have the equivalent of 40L of gear. (40×61=2440 cubic inches)
Here is where the anal retentive type will get stuck, but to most, if you have 15x 4 L bags inside a pack and a bit of space left you may just be able to work out that the pack can hold a bit more than 60L but not 64
This is also applicable having just a bit less than the space for 15x 4L bags giving you somewhere between 56L and 60L

As I stated in a previous post , the fact that manufacturers cannot do conversions does not help either..
Here is the example that no one bothered to look up :
Osprey Ariel 65 XS 3600 CU in 59 L
S 3800 62L
M 4200 65L
L 4800 68L
http://www.ospreypacks.com/detail.php?productID=189&colorCode=609&tab=specifications
now 59 L is about 3600 cu in, but 65 and 68 L are not 4200 and 4800 cu in….
XS
Franco

[Mike W]

…

[Wild Exped]

Same point I guess, just looked at from different perspectives with some ambiguous meaning. Personally, i'd rather see an unambiguous set of standards, moreso one that US manufacturers will abide by, however the method. Even vast inaccuracies matter little if one knew what to expect when making comparisons and choices.

[dan mchale]

I like being practical so when somebody brings a pack to me just to show how their old packs fits or something, and don't really know how to express how big it is, I don't get a bag of marbles or balls out to meausure it. I see what its circumference is – it's quick and dirty and tells me all I need to know to get started. It's very rare that a person has to know or demands to know cubic inches beyond a 500 cuin margin of error because most people want a pack that is fairly versatile anyway. It's the one measurement that does not lie or cannot be mistaken because it has not been converted from something else. It is a number than is very easy to relate to, just like cuin numbers. If somebody says I have a 42" circumference pack that can be just as meaningful for some as saying it's 4,000 cuin inches.

[Dale Wambaugh]

The European system isn't perfect, but it does give a consistent reference. I could see issues with how tight the balls are packed, how full extension collars are filled, and dealing with pockets– multiple small spaces should generate a more error, although I doubt the actual numbers would be significant.

Many UL manufacturers do a decent breakdown on their measurements– so much for the main pack body, so much for the extension collar, pockets, and so on. That seems fair enough for the consumer to make comparisons if done with some accuracy. Those packs are fairly simple designs, so getting a close calculation shouldn't be all that difficult. More complex designs need something like the European system to get a good feeling for the capacity. Evaluating the pack shape for efficiency of loading is a whole other can of worms.

I noted that the BPL review of the Osprey Exos 46 made the point that the pack seemed smaller than the stated 46 liter capacity and I agree— it doesn't seem to be a lot larger than a Black Diamond 34 liter pack I have (I can't tell you the actual difference). I bought it knowing that was an issue, so no rude surprises. If you are buying an item sight-unseen, I think it makes more difference to the consumer. With the product in hand, it is what it is– it works or it doesn't. If I were the manufacturer, I would expect some returns if I were off the mark more than 10%.

[Roger Caffin]

> BPL's article indicated that the pack I own is 9 liters larger than was
> advertised by the manufacturer. My measurement showed it to be 1 liter
> larger. So what happened?

My guess was that the production process may have changed slightly. What looked like small changes in shape and sewing can easily produce changes like this.

I remember measuring TWO Jansport packs that were claimed to be 65 L. They were both about 50 L. It turns out they were pre-production models which were just wrong in volume. The production version was changed to fix this and came out very close to the claim.

Cheers

[Roger Caffin]

> If the current standard doesn't meet my needs as a consumer, then I would
> encourage manufactures like Dan to come up with something more reliable.

You are assuming that the current standard does not meet your needs. I don't agree. The problem is that some manufacturers simply do not follow the standard. Some of them include outside mesh pockets which is not permitted as they cannot be sealed off. Some of them measure the different compartments separately and then add all the results together – which is a gross violation of the concept. When the main bag is full you can't get much in the outer pockets.

The standard is OK, but some manufacturers just don't follow the standard. Very simple.

Cheers

[Ron Bell / MLD]

Lawson- Sorry if I'm off the OP a bit- but I wanted to get this broad idea down on the BPL forums and here seemed as good as any- for those that want only the nut- skip to the last paragraph.

Back in 2002 a group of Outdoor Industry Association (OIA) members/participants met at the Outdoor Retailer show to create a pack volume measurement standard for ASTM.
Basically it is was an appointed committee of representatives from the the big pack companies and major retailers running the committee – the committee chair was an employee from REI. That is who belongs to the OIA and so that makes sense.
These folks were Hard Core since they had to meet at 7:30am in the morning!
My hat is off to them – I’d never get up at 5:30am on a business trip to get ready and trudge across town to go to a meeting – unless money was one the line or it had really awesome fun involved. (Please don’t read anything malicious into this. This is not a knock on them but you get the picture – they all had skin in the game- but nothing wrong with that.)
In 2002, what did most backpacks built by major companies sold in major retail stores look like? -Giant sacks with heavy frames, some shaped ever more interestingly (and hard to measure) , some featured various zippered pouches and lids on the outside. -So that standard was reasonable and probably works OK for those type design packs.
Remember 2002? How many UL packs were there with open top expansion outside mesh pockets? Not many.
Skip to 2012- How many of the UL and SUL packs have outside mesh expansion pockets – a lot- it’s a core feature in almost all the most popular UL packs due to function. But those large storage pockets can not be included in the overall pack volume because it does not fit the 2002 ASTM definition? -Plus many modern cottage and mainstream UL packs have other features that would not be covered in the ASTM/OIA pack volume standard.
So what is a SUL/UL pack maker like MLD to do??? Stick to the standard like it seems BPL requires and create a new problem or just do the best we can and try to make a good faith complete description with a good breakdown of the volumes. With one number that is strictly ASTM a week long size thru-hike style pack (Exodus) looks on a neat and tidy on a one number chart – but it also looks like it’s a day pack.
I’ve never had a customer send back a pack and tell us the website info misled them as to the packs size or function – so I think there is no problem with how we and most other SUL/UL pack companies do it now. (Every once and a while someone does decide they need a larger or smaller pack – but not from being confused by the published specs- at least any I know of.)
For me it makes sense to try to relate the function of any product to the published information to help customers as much as possible and not to stick to any official standard if it does not make sense and/or has not been specifically developed for our evolving and somewhat new sport of UL backpacking. Another MLD example is our published list of warmth using our quilts. It’s a bit tongue in cheek but funny how accurate it is as reported from many users in the field – that’s partially how we determined it. Printing any standard type measurement number just would not be as informative for that specialized product.

When any standard that was not developed for UL BP gear is applied to UL BP gear, no matter if it is for pack volume, waterproofness, torso length measure, bag warmth measurement, etc I always default to the multitude of reasonable user reports from the field and numerous customer feedback over any set number or lab test measurement andI believe that if any small UL gear company did not primarily listen to their customers first then their business model would eventually fail.

[Roger Caffin]

Hi Ron

> So what is a SUL/UL pack maker like MLD to do??? Stick to the standard like it seems BPL requires

My opinion only:

Quoting a pack volume without any explanation will always create confusion if the number is not per the existing ASTM standard. We know that right now.

So how about quoting a number per the ASTM standard and then quoting the volume of the extra bits, with explanations? Imho, explaining to the customer exactly what is going on will likely clear the air and engender some confidence as well.

After all, that is how sleeping bags are quoted: 'comfort rating of 25 F per the ENxxxx standard' and so on.

My 2c.

Cheers

[Hamish McHamish]

I used the BPL method to check the volumes of a number of packs I was looking at. The Black Diamond Speed 30 and Speed 40 packs (the old styles, not the bloated current ones) were very accurately labeled.

Most interestingly, the North Face daypack I checked had a claimed volume of 1850 cu. in. and a tested volume of about 1300. Considering the 2.5 lb weight, I was not impressed.

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