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No-load versus full-load test

Our industry is really foolish. Design engineers expect, and vendors furnish, output specifications at both no-load and various load conditions. Whether it is a basic op amp, a motor, or a line driver, engineers need to know how the device performs when it is idling as well as doing its job.

So why are we foolish? Because other industries have figured out how to set their standard of acceptability using only no-load performance.

Here's what I mean: our three-year-old latex foam mattress no longer had its “stiffness”; instead, when you lay down, you'd sink in much more than when the mattress was new. But when you got off the bed, the mattress restored to near-original flatness.

After some back-and-forth discussions with the store and the mattress vendor, they sent a representative to check out our complaint.and see if the mattress was defective. I foolishly expected some sort of instrument which would check out the mattress compliance as well as restitution.

Instead, the mattress technical stretched a string from head to toe of the mattress, then measured the vertical sag along the string line with a ruler. He told me that the mattress vendors all had established a simple pass/fail criteria: if the measured sag was more than one inch, the mattress was defective.

But the problem is that this is a no-load test. You can have a mattress which yields too much under weight, but which has high restitution to its original position. The no-load test only checks the restoration, and says little or nothing about the compliance coefficient.

Yes, the test is simple, visual, and reasonably precise, but it is very misleading. The technician said that most mattresses he checks pass this test. Gee, why am I not surprised?

Ironically, many years ago I worked for a company which made materials-testing equipment. Among the models they offered were ones for mattress manufacturers, which tested the mattress by applying controlled force values and measured the resultant vertical deflection. The mattress companies would simulate years of use on their products (using heavy rollers and up-and-down pistons), then use this machine to see how well the mattress held up, both under no-load and various load settings.

Although more complicated than a string and ruler, their in-house tests are an honest assessment of the product's performance. Too bad they don't use a simplified version for assessing their product in the field, perhaps by placing known dead weights on top of the mattress. But their simple method lets them claim that their product is OK, even though the test is misleading and too simplistic.

Our industry may have a lot of very smart people, but some other industries seem to be a little smarter in how they set up and measure their performance with respect to the end customer!

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