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{ <br /> or the person will sit on top. If too soft, the cushion will bottom out. <br /> Neither cushion supplies the "Feel Of Comfort'. <br /> You will normally specify a certain firmness for a particular appli- <br /> cation. However, as you have all experienced, specifying a certain <br /> firmness without tying in some other factors will not give the desired <br /> results. <br /> A low density foam does not have the same "Feel Of Comfort' <br /> as a higher density foam having the same firmness. <br /> Strictly speaking, firmness refers to a particular measurement, the <br /> 25% ILD or RMA. In our opinion however, this measurement of initial <br /> feeling of firmness does not define the total "Feel Of Comfort' in a <br /> cushion. You must add the deeper feeling of comfort and the feeling <br /> of push-back the cushion supplies. <br /> These three factors are the total effect that gives us this term we <br /> are using "Feel Of Comfort." <br /> We define these factors as 25% ILD for the initial feel, Compression <br /> Modulus for the deeper feel and Hysteresis for the push-back feel. <br /> How do you measure the 25% ILD value, Compression Modulus <br /> and Hysteresis? All three values are based on ILD measurements <br /> taken at various deflection levels. <br /> The ILD measurement of urethane foam appears to be relatively <br /> easy. However, it is not simple. It is subject to error at all levels of <br /> deflection and very poor correlation between laboratories will occur <br /> if a rigid, uniform method is not practiced. <br /> The principal reason for this error is the presence of the hydrogen <br /> bonds that exist in all urethane foams. Since these bonds can easily <br /> be broken and do not quickly reform, it is important that the foam <br /> samples be treated uniformly before a measurement is taken. Low <br /> density seating grades of foam, with their greater number of hydro- <br /> gen bonds, are subject to the greatest error. <br /> The accepted method of reducing this chance of error is to pre- <br /> flex the sample before making the measurement. The same method <br /> of preflexing should be used for maximum reproductibility. <br /> After preflexing, the sample is allowed to rest for a period of time <br /> before measuring. It requires a device with a high degree of accuracy <br /> for determining both the height of the disc in relation to the foam <br /> thickness and for measuring the pounds of force applied to the disc. <br /> The use of portable measuring devices even in the hands of ex- <br /> perienced and trained operators can only give an indication of the <br /> firmness. <br /> Neither density nor ILD is uniform within a slab. <br /> 6 <br />