Subjective (Rucksack) Liter


Manufactur­ers measure capacity of a rucksack in both cubic inches and liters. The capacity in liters sounds weird, as the sack neither holds liquid nor gas. A rucksack contains solid substances, including dresses, a toiletry kit, camping bed, camera… The volume of solids is convention­ally measured in the said units. Liter is the name for quantity of gas and liquid yet it is a standard unit in the rucksack industry. The capacity is also measured in terms of trip duration when rucksack would be employed. Even so, both units liter and trip duration are subjective. All rucksacks of the same capacity irrespecti­ve of brand may not actually hold the same amount of stuff because of different size-capacity ratio. They prefer liters to cubic inches because liters are rounded off for easy reading and memorizati­on. The ease reduces accuracy of the capacity usually expressed in 2-digit figures (20L, 30L, 50L…). Longer cubic inch figures are difficult to remember. The size represents linear dimensions: length, width, and height. The ratio implies relationsh­ip between the size and the volume. A rucksack may look big but may not have large capacity or vice versa. How do the manufactur­ers arrive at the capacity in liters? The industry yet does not agree on a single standard method. But engineers are still developing the method(s) to ensure uniform capacity for the same size rucksacks, like one-liter liquid holds the same amount across the world. The rucksack capacity measuremen­t method standardiz­ation would simplify the comparison of rucksacks of different brands, ensuring fairness. The manufactur­ers stuff a selected substance of a fixed capacity, a fill, into the rucksack to measure the capacity (volume) that means how much (quantity) a thing can contain. The fill could be anything: air (gas), dried beans, ping pong balls… Since air is also measured in cubic inches, many manufactur­ers measure the rucksack volume in cubic inches and convert it into liters. Liter is also a preferred capacity unit for irregular spaces and the objects with irregular shapes because applying cubic inch unit is difficult. A gas is shapeless; and acquires the shape of its container.

Other fills have drawbacks too. Space between ping pong balls stowed in the rucksack is large. So, free spaces between the balls impact capacity accuracy, especially of, pockets and other small compartmen­ts. Ping pong balls are less dense and do not stretch the fabric. The American Society for Testing and Materials (A.S.T.M.) Internatio­nal developed a standard test method to measure the capacity of a backpack: ASTMF2153. The society defined a test method as “A definitive procedure that produces test result.” The society considered various types of fills, including buttons, chopped plastic tubes, disposable glass micro-pipets, dog food, dominoes, dried beans, foam-packing peanuts, golf balls, 20-mm hollow polypropyl­ene balls, ping pong balls, plastic washers, and tennis balls. It selected denser 15-20-mm hollow polypropyl­ene balls that imitate the density of the actual substance filled in the sack. The society recommende­d the following criteria for selection of standard fill: (1) non-compressib­le, (2) easily available, (3) fixed density per 1,000 cubic inch, and (4) able to cover entire area enclosed by fabric. This method has limitation­s too. Final capacity figure may vary for the selected fill and method applicatio­n. The method is unsuitable for bags with the capacity less than four liters. Some American companies, like Recreation­al Equipment Inc. (REI) and Tom Bihn, follow the ASTMF2153 to measure the rucksack capacity. REI fills a rucksack and all its pockets with plastic balls of the size of a gumball. The balls are transferre­d from the rucksack to a graduated measuring cylinder to find total liters. REI measures both extended and unextended capacity of the rucksack. Extra extendable features of some rucksacks add to the normal capacity. Extended capacity means sum of the additional feature capacity and the normal capacity (unextended capacity) when the rucksack is expanded to its full useable volume. Some manufactur­ers do not test capacity of water bottle holders, mesh pockets, and other open areas, adding further subjectivi­ty to liter. They prefer liters to cubic inches because liters are rounded off for easy reading and memorizati­on. The ease reduces accuracy of the capacity usually expressed in 2-digit figures (20L, 30L, 50L…). Longer cubic inch figures are difficult to remember. The car industry shares similar liter subjectivi­ty issues in the absence of a standard capacity measuremen­t method. The observant drivers may know that not only luggage capacity of two boots of same dimensions irrespecti­ve of brand differ but also that of the same model vary considerab­ly. Generally, the capacity is assessed using German Liters VDA (Verband der Automobili­ndustrie); and American SAE (Society of Automotive Engineers) methods. The former method involves stacking 1-liter blocks of 200x50x100­mm in the car luggage space. The space capacity (VDA number) equals total number of the blocks. The number may vary with a change in the preferred block configurat­ion. The latter method uses boxes (shoe-boxes, suitcases…) but again box stacking order would decide the volume. The results of these two methods do not match. In fine, subjective boot and rucksack liters require trial and error before arriving at its maximum capacity until a reliable standard emerges.

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