Loft conversion

Posted by Mark Taylor on 06/11/2004
Nice and cosy. Photo: Rab.

Seasons are out this season. And a German mannequin is ruffling feathers in the sleeping bag world. Mark Taylor takes a look.

The right sleeping bag can make or break a trip. It's the difference between a cosy nights rest and ten hours of uncontrolled shivering. But choosing the right bag can be a tricky business, and it's not been made any easier by a new EU standard.

Inner workings
Sleeping bags can be highly technical bits of equipment, but their job is essentially a simple one - to keep a heat balance by providing enough insulation to equalise your heat loss with the level of production. We're constantly losing heat to the environment in four ways; conduction (touching colder surfaces), convection (air currents drawing heat away), radiation (like an electric fire), and evaporation (through breathing and sweating). If you lose heat faster than you produce it then you'll enter a heat debt, get progressively colder and eventually die of hypothermia. Not much fun at all.

Sleeping bags are designed to reduce the losses from conduction and convection to a minimum. They achieve this by attempting to maintain the best barrier to conductive heat loss; an air gap. Not too big though, since that would encourage convective currents to begin their insidious work. This gap needs to be filled with something to trap still air yet prevent conduction, like down and modern synthetic sleeping bag fillings. No attempt is usually made to prevent heat loss by evaporation as this would be uncomfortable (ever slept in a plastic bivi bag?), or radiation since at low temperatures this is minimal.

Seasons
The age-old problem has always been choosing the correct bag for the intended end use. Hard at the best of times, more so when some manufacturers have been economical with the truth. In the past there have been a number of disparate regulatory standards, and it's been normal practice to label bags for customers by seasons. Slapping on a 1, 2, 3 or 4 season tag, plus a "5" season to indicate the highest rating for either altitude or arctic conditions. Originally the ratings were obtained by field-testing, but over time they were correlated with, and eventually replaced, by a British Standard tog test (a tog is the measure of thermal resistance used in the textile trade). Whilst these may be pretty arbitrary, at least they're intuitive, and retailers experience suggests that most customers buy above their requirements as an additional comfort factor.

You must conform
But to combat the apparent vagueness of the season tag, a new test was designed - the EN 13537. This had its root in Germany, a country famous for inventing complex test methods. A research centre had developed an expensive heated mannequin for testing safety clothing and to justify the cost needed to find other applications. Initially sleeping manufacturers supported the scheme, thinking it would simply put an end to high street shops slapping four season tags on anything that moved, but this would be at a high cost.

In the new test, the bag is placed on simulated ground consisting of a 12mm thick wooden board covered with a mattress with a thermal resistance of 8.5 tog. This ‘ground’ is held clear of the floor of the test chamber to allow air circulation and the magic thermal mannequin is laid on its back into the bag - neatly dressed in a two piece tracksuit and knee length socks! Hot air is then pumped into the dummy and measurements begin.

At first sight this test appears an improvement since it tests the whole sleeping bag / occupant assembly, however there are serious problems. Firstly the "ground" itself has a higher tog value than some lightweight sleeping bags, compounded by the sleeping mat used having four times the insulation of a standard foam sleeping mat. Trading standards are already receiving complaints that some lightweight bags are way overrated.

Secondly the results can be seriously misleading for variable insulation bags, i.e. those where the insulation is thinner in the base than the upper section. In fact having no insulation at all in the base (such as the Rab Quantum Top) won't even change the figures. And unlike existing methods there has been no attempt to correlate the results with user trials.

The expense of the whole procedure is also considerably greater than the simple tog measurement under the BS system. One manufacturer recently tested their entire range of bags to the British Standard for less than the cost of a single test to the EN. Perhaps less of a problem for larger companies, but if complied to would crush development in the small ones. Costing you, the consumer, in more ways than one.

Finally, and most importantly, the test can result in dangerously misleading labelling being applied - ironically exactly what it aimed to eliminate in the first place. The end results of the EN testing are two comfort ratings (a maximum and minimum) and an extreme rating for each bag. For example; +20/-5/-22. But there's been no attempt to link this to real life; these upper and lower "comfort" limits cover the potential for death from hypothermia or hyperthermia. And the extreme temperature is defined as the temperature "below which serious injury will occur" - that is, you'll die.

The future
With the introduction of this standard, all that's been achieved is another layer of confusion into the buying process. How cold is it at night in a Scottish mountain hut? How chilly does it feel during an Alpine bivi? Difficult questions, since it's all very subjective anyway - are you a hot or cold sleeper? When did you last eat? One man's cosy night is another mans nightmare. Confronted by confusing figures, comparison graphs and incorrect ratings, buyers are simply going to need an easier way to visualise the bags function. Like grouping them by seasons?
 

EN TEST FACTS 

-  To get an idea of what was "cold", they did field experiments on German soldiers. Using macho fit lads probably wasn't the best idea since all their test results now come out too cold.

- Originally they had two mannequins, a male and female. The woman dummy consistently needed an extra 5'c insulation.

- A major manufacturer sent off their entire range for testing and got different results from two different test houses. Not very "standard".

- The new test keeps the mannequins big toe at exactly 35'c.

- For sleeping bags a cheap cylinder would do the same job as the mannequins

- The EN test can't cope with bags designed to operate below -20'c. But four season bags go down to -25, and five season to -40.

- The test costs £1000 per bag.

- One major retailer has already scrapped EN labelling after being prosecuted by Trading Standards.



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