I'm new to the forum and am interested in layering a new mattress based on a 6" slab of latex from my existing mattress (which I disassembled).  It'd be really helpful to know the ILD of this piece of latex and I wonder if I can measure it myself using the definition.   I understand it's the weight, in pounds, required to push a 50 sq-in "foot" 25% of the way into the mattress.

It just so happens that I have a 8" Pyrex pie dish that is roughly 50 sq-in (and I can correct for it being off a little).  Seems like I can lay it on the middle of the piece of 6" foam, and add weights (round barbell weights) until I've mashed it 1.5" into the foam.  By using a 3ft yardstick as a straight-edge, I should be able to measure the compression.  Conveniently, the pie plate is 1.5" high, so I've added the right amount of weight when the top of the pie plate is flush with the bottom of the straight-edge, which is laid across so that its ends rest on the uncompressed foam.

Make sense ?

No one ?   (Bumping, hope that's kosher here ...)

It makes sense, but hard to say how accurate you will be.  I am not sure exactly how they do the calculation, so your results may not compare with how the industry does.  Do they use a 6" piece of foam in industry tests?  I think the thickness will affect the results.  It would probably be useful to have some foam that you know the ILD of to test, so you could benchmark your method.

sandman wrote:

I guess I just really want to get a ballpark.

The IFD measure will vary based on thickness.  I think the standard piece they use to test is 20" x 20" x 4".   So, I think you will have to adjust your numbers to be comparable to what other ratings are.  A 6" piece will show a higher IFD than a 4" piece, so the true 4" firmness will be less than you measure.  If I am reading this correctly, a measured IFD of 34.5 in a 6" piece will be equivalent to an IFD of 28.0 in a standard 4" piece.   You may want to read this.

 

 

4.3 IFD varies significantly with foam thickness. On the exact same foam, the IFD increases as the thickness increases, as the following chart illustrates:

All samples are 20" x 20" x stated thickness.

 

4---------------------------28.0

5---------------------------31.0

6---------------------------34.5

7---------------------------38.5

8---------------------------43.0

IFD values in the above table were obtained from testing actual foam samples. These values should not be used as anything but a guide. The actual magnitude of the IFD versus thickness change must be determined for each foam type. A simple "rule of thumb" on the degree of change is rarely accurate.

The IFD increases with cushion thickness as you read from left to right. For example, if the IFD of a 2 inch thick cushion is 6.7 lbs/50 in sq., one could expect the IFD of that IDENTICAL foam, if it were 8 inches thick, to be approximately 16 lbs/50 in sq.. However, the rate of IFD increase with increasing cushion thickness also varies with increasing IFD. Note in Figure 1 how the slope of the IFD-thickness line increases as the IFD increases, which shows that the foam thickness effect is essentially "compounded" at higher IFD levels. Please keep in mind that the values represented on this graph are approximate and are displayed here only for visualization of the concept.

Wow sandman, thanks for that info !   I guess foam does not act as an ideal spring.   For ideal springs in series, the deflection percentage should be constant with force applied, since each spring stretches or compresses the same amount.   So two pieces of 4" foam stacked are like two springs in series, so if 25lb compresses one piece 25%, it should compress two pieces stacked on top of each other by 25% each, compressing the whole thing by 25%.   

But the data you cite shows that this is not the case for foam.   Good to know !