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Re: [Phys-L] nasty deceptive scale





I see that John has discovered that smog reduction code
is not restricted to VWs.
I took out my little 200g x 0.01g scale which looks similar,
and found it could not always hit the additional masses
from a mass set on the nose.

Brian W


7/9/2019 3:26 PM, John Denker via Phys-l wrote:
Hi Folks --

Suppose you were asked to investigate the damage caused by an
accident. And then you discover that it wasn't an accident
at all. It was intentional. It was staged to look like an
accident.

The damage is bad enough. The dishonesty makes it much worse.

=========

Now suppose you bought a small electronic scale. The display
is readable down to 0.1 gram, and the scale is advertised as
being accurate to 0.1 gram, over the range from 0 to 3000 g.

You calibrate it with reference masses: 10 g, 20 g, 50 g,
100 g, and sure enough, the reading is spot-on every time.
So far, that seems pretty nice.

Then you discover that everything from 99.3 g to 101.3 g
reads as 100.0. So the nice-looking calibration data is
deceptive. Over most of the range the device rounds to
the nearest 0.1 g, but when it gets near a round number
like 50 g or 100 g, the roundoff error is at least 20??
worse than that. To say the same thing another way, there
is a wide plateau in the response function, i.e. a huge
dead zone, which causes strong nonlinearity and missing
codes. Here is some data:
https://www.av8n.com/physics/img48/nasty-scale-data.png

This is not an issue with the calibration. Calibration
cannot make dead zones go away.

When it is not near a round number, there is some
inaccuracy and quite a bit of noise in the low-order
digit, but this is NOT what I am complaining about.
I expect the low-order digit to be noisy. Rounding
to get rid of the noise would make the device less
useful. Guard digits are your friends.

I don't know how to identify this scale except by appearance:
https://www.av8n.com/physics/img48/nasty-scale.png

It does not appear to have a model number or manufacturer
name on it anywhere. The box says
SUPERIOR MINI DIGITAL PLATFORM SCALE
but that is probably just a description, not a brand name.
The sellers don't appear to list it under such a name. It
also says "MADE IN CHINA" but that's not much help.

In my opinion, I would recommend against buying such a device.
It's inexpensive, but even so, you don't get what you pay for.
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