How We Test Washing Machines: Stain Strip Spectroscopy

A washer's job may not be easy, but its primary goal is simple: It just has to remove stains. So to test any given washer's performance, we have to run stained fabric through the machine and see how much of it is removed. That may seem like a straightforward process, but the scientific techniques involved are quite complex.

First of all, there's the problem of giving the exact same set of stains to each machine. Without standardization, our results would be worthless. Luckily, in Krefeld, Germany there's a factory that churns out standardized stains, applies them to patches of fabric, and sews them into 18-inch stain strips. Pick up any one of these strips and you'll see the exact same stains: a control patch, followed by sweat, carbon/motor oil, cocoa, (pig's) blood, and red wine.


The stain strips before and after washing.

The stains are so standardized that the variety of red wine is the same on every single strip, and stains were all chosen due to their known stubbornness. We have yet to come across a washing machine actually capable of completely removing all of them.

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Attaching it to a towel ensures that the stain strip follows a typical laundry item through the wash.

Our next challenge is determining just how much of each stain is removed after a wash. We could give the stain strips a once-over, but the human eye isn't nearly precise enough for accurate results. That's why we use a process known as spectrophotometry.

In its most basic form, spectrophotometry is actually a rather simple 2-step process: illuminating an object with a certain sort of light and measuring the light that the object reflects back. With these two pieces of information we can deduce a lot about an object, such as chemical composition, density, or, in this case, how much of a stain remains. The less light that's reflected back, the heavier the stain.

Before we put the strips through the washer, we record how dark the stains are by using a spectrophotometer, a device that records how much light reflects back from a surface. Then, we run the strip through the machine. To simulate actual laundry, we add a standard load of towels, pillowcases, and sheets, pinning the stain strips to folded towels so that they follow the passage of typical laundry.


The spectrophotometer determines the cleanliness of a stain by measuring the amount of light that gets reflected back.

When the cycle is complete, we let the strips dry and use the spectrophotometer again, this time to analyze how well the washer removed the stain by comparing initial and final light absorption readings. This difference gives us a numerical value of just how well a washer removed a stain, which all the information we need to make a informed judgement of performance.

Contributing: Ethan Wolff-Mann

TAGS: how we test home laundry

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