The Blue Water test method closely duplicates actual fire/rescue conditions using a realistic body weight. The weight of 80kg is abraded directly over 90° arc using a rotating stone. The angle produces an actual face pressure of approx 113kg (80x1.41=112.8)*. The rotation of the stone prevents buildup of abraded material which would alter the true results of the test.
The NSS test method uses a known grit stone just as the Blue Water method. However, the 80kg weight is first run over a pulley 15° lower than the stone. This change in angle produces a face pressure of just 21kg approx (80x0.26=20.8). That is only 21kg of the 80kg actually presses against the face of the stone.
The PMI method is similar to the NSS method in that the weight 40kg is first run over a pulley then across an abrasive surface. The angle produced by this pulley is roughly equal to 30°. This angle produces a face pressure of approx 21kg (40x.52-=20.8)*. Force placed upon the abrasive surface is hardly close to normal body weight.
* Source: American Hoist and Derrick Technical Manual
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