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Graph of actual performance of fake DS18B20 #17

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emulti opened this issue Aug 21, 2020 · 7 comments
Closed

Graph of actual performance of fake DS18B20 #17

emulti opened this issue Aug 21, 2020 · 7 comments

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@emulti
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emulti commented Aug 21, 2020

Hi,

Thanks for highlighting this fake sensor issue!

Unfortunately I found out about it after I wasted my money on a bunch of waterproof "DS18B20" sensors on eBay in 2019.
The ROM codes are of the form 28-tt-tt-43-xx-00-80-crc.

To highlight just how useless (and potentially dangerous if used in a temperature control system) these fakes are, have a look at this graph of the temperature output at 12 bits resolution from five sensors at identical temperature. There are four fakes, and one genuine item. Temperatures are being read using the standard DallasTemperature library on an Arduino.

You only get one guess which is the genuine one...

One genuine, four fake DS18B20

Building a precise temperature control system is practically impossible with this garbage. The readings vary randomly by so much, and in an asymmetric way, that even by applying heavy smoothing they are neither accurate to within a degree, or stable. The response time of the smoothing loop becomes uselessly slow for many applications.

Chris

@cpetrich
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One of these things is not like the others.
One of these things just doesn't belong.
Can you tell which thing is not like the others
by the time I finish this song?

...and thanks for sharing that the -00-80s were still around in 2019.
Any idea what the standard deviation of the data are?

@emulti
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emulti commented Aug 23, 2020

I looked up the vendor's page on eBay. They still have over 900 in stock if you are keen to buy!
No idea on the standard deviation- but it doesn't look like they are really 12-bit devices looking at the quantisation steps.
These are the 'waterproof' variety. I'm going to rip them apart today and substitute Maxim sensors for my application. It will be interesting to see the internal potting arrangement- probably that well-known thermal conductor "silicone rubber".

@emulti
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emulti commented Aug 24, 2020

I should have known better, but the inside of the 'waterproof sensor' exceeded my lowest expectations.
The fake sensor (which is of course marked 'Dallas' with a plausible looking batch code) is potted in a low-grade silicone-like material that looks like bathroom sealant.
The maker has gone to the trouble of fitting sleeves over the soldered connections, but forgot to push the sleeves up over the pins of the device. As a result, two of the pins are almost touching.
inside 'waterproof' fake DS1820B sensor
'Intelligent' sleeving- pins almost touching

I advise anyone looking for DS18B20 sensors not to buy them from eBay/China!

@heitorPB
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Nice plot! Would you consider adding it the the README?

@cpetrich
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@emulti Thanks for the pointer! I believe I've ordered some based on your hints and will follow up when I receive them.

@heitorPB Got to think about it. While I agree that their performance is garbage they seem to be old stock from a European distributor if I understood @emulti correctly, i.e. they perform much worse than the sensors one can expect to get from a Chinese distributor these days. Again, I'll think about it once I manage to buy some.

@cpetrich cpetrich changed the title Graph of actual performance of fake Chinese DS18B20 Graph of actual performance of fake DS18B20 Aug 25, 2020
@emulti
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emulti commented Aug 26, 2020 via email

@cpetrich
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To follow up: the probes I received today contain Family B2 sensors.

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