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HOW IS FILL POWER MEASURED?

Firstly there is no such thing as a 'World Standard Fill Power Test'. Each country uses its own system, UK, USA, Japan, Sweden, Germany, etc. The basic elements of the tests are, however, common to them all. Down fill power is determined by the number of cubic inches a small amount of down occupies. The down is fluffed up by an air blower, then kept at a controlled temperature and relative humidity (21°C and 65% humidity to prevent static from giving a false reading) for a minimum of three to five days. Then a small sample is taken, which varies between 20g, 30g or 1oz depending on the country.

This down is then placed into a clear tube (24cm x 60cm in the US) and agitated. A weight is then placed on to the down and allowed to settle. The poorer, denser feathers will compact more than the finer, loftier feathers and so a measurement can be made to find the cubic inches per square ounce (i.e. 500, 600, 800 etc). The test is only accurate to within 5%, so your bag may not be quite as lofty as it appears unless the tag claims a minimum fill power. The problem is that because the size of the tube, sample of down and lid weight vary from country to country; one man's 800-fill will be another man's 650-fill. The US test quoted by most US companies gives a result 4% higher then the traditional UK 'Lorch' test. Most serious outdoor down manufacturers do extensive in-house batch testing to make sure what they say on the swing tag is what's stuffed in your bag.

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