Chances are both numbers were derived from the same test. Typically, tent makers used whats called a water column test." In it, a fabric sample is placed over one end of a tall tube of predetermined diameter. The tube is filled with water and the fabric watched for leaks. Depending on the material, it may leak under the pressure of 1,000 mm of water, or 2,000 mm, or 5,000 mm. Somewhat obviously, the taller the column of water before the fabric leaks, the more waterproof the material. Most manufacturers dont bother to go higher than 5,000 mm.
The water pressure from a column that tall is about 165 psi. But certainly its more understandable to hear 165 psi" versus 5,000 mm." So thats why Sierra Designs uses one rating, Mountain Hardwear another. In terms of function, there is no difference. Both tents are at the far upper end of waterproofness in tent floors. The North Face also uses the 165 psi standard. Marmot likes the 5,000 mm lingo.
By way of comparison, kneeling on a tent floors applies about 50-6o psi of pressure. So with these tents youre well past the point where you can force water through the tent floor with your body weight.
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