If you did not buy your DS18B20 from a reputable source, it is probably a clone, and may have issues...

[petergebruers 1,265]

When I started my HA system (around 2013) I bought quite a few first generation D/W sensors and added the venerable DS18B20... I bought them from respected distributors and never spent much thought about them. Fast forward to 2018-2019. I got interested in doing more experiments with temperature measurement and control so I started looking for "alternative sources". Price point of a Dallas/Maxim version is about 1.5 tot 2 EUR in small quantities and as the say "if it sounds to good to be true then it probably is too good to be true..."

 

So I started buying sets of 5 or 10 from random sellers on AliExpress, around 0.5 EUR/piece... For fun and giggles and hoping to salvage some to do some real measurements

 

What I found out...

 

• Some seemed reasonably accurate, but there were outliers, even at room temperature. Some seemed to be out of spec (2 °C) near freezing point.
• Some seemed "fixed" to doing 12 bit conversions only.
• Some did not work 2-Wire aka phantom powered aka parasite powered.

 

I never posted about this because not many people talked about this sensor anymore and many Z-Wave devices report temperature nowadays.

 

But then I stumbled upon the work of "Chris Petrich", published April 2020:

 

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He decided to get to the bottom of this and ordered a bunch of those chips in2019, wrote a test scene (for Arduino) and this is his conclusion... I quote:

 

"We bought over 1000 "waterproof" probes or bare chips from more than 70 different vendors on ebay, AliExpress, and online stores -big and small- in 2019. All of the probes bought on ebay and AliExpress contained counterfeit DS18B20 sensors, and almost all sensors bought on those two sites were counterfeit."

 

He noticed similar "deviations" from the original DS18B20 as I did, and points out that not all devices are bad, but it certainly is a lottery...

 

If I wanted a temperature sensor, the DS18B20 might not be my first choice in 2020 -  but the 1-wire bus can easily do 10 meter and phantom power does have its charm

 

BTW I was able to triage my clones because I have different sensors eg BME280 or SHT-21 and Platinum RTD from reputable sources. If you only have clones, it might prove difficult to find out which sensors are accurate and which are not...

 

Edited April 20, 2020 by petergebruers

[10der 832]

4 hours ago, petergebruers said:

eg BMP280

btw BME more a cooling IMHO. 

[petergebruers 1,265]

8 minutes ago, 10der said:

btw BME more a cooling IMHO. 

Yeah, I have both, and I always mix those part numbers. BME280 is more interesting than the BMP280, it added humidity, so does pressure/temperature/humidity. I've edited my post.

[petergebruers 1,265]

I've finished checking my sensors buy running this sketch

 

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It is a copy of the original sketch but with "OneWire library" vendored in...

 

As expected, the devices from TME, a reputable seller, are genuine... 

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And as expected, the devices bought on AliExpress are all reported as fake. Some are usable, but with the caveats discussed above and on github - and I did not test any sensor above 25 °C and only some near 0 °C so I cannot really compare performance.