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jbeale
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Cheap USB temperature sensors

Thu Jun 21, 2018 3:05 am

I needed to read ambient temperature to do some temperature corrrection for a sensor. Rather than deal with a lower-level interface directly I thought the easiest thing would be to get a USB sensor. I found two of them, one called "TEMPer" on Amazon and another one called "Digital USB DS18B20" sold on eBay from Latvia. Both of these work on the RPi, and have nominal 0.06 deg. C resolution but one of these is not like the other! See graph below of both sensors in the same room over the same time period, about 10 hours long during the day. You might think I just got a bad "TEMPer" unit, but no- according to online comments they all work that way! Most people just don't pay attention to better than 1 C resolution so they don't notice, I guess.

Comparison plot: https://photos.app.goo.gl/btzc7bs36YD4DC4HA
Last edited by jbeale on Thu Jun 21, 2018 5:26 am, edited 2 times in total.

DirkS
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Re: Cheap USB temperature sensors

Thu Jun 21, 2018 4:26 am

Can't see the image (error 404 - not found)

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jbeale
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Re: Cheap USB temperature sensors

Thu Jun 21, 2018 2:24 pm

Previously had to be logged into my own google account to see, but I fixed the image link, I think.

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bensimmo
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Re: Cheap USB temperature sensors

Thu Jun 21, 2018 2:56 pm

Image works for me, the DS is a smooth incresae, the other is all over the place but gets there in the end.

It's interesting the DS is set/reporting to a higher resolution than the normal default 0.5C setting you get in the modules.


Image

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bensimmo
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Re: Cheap USB temperature sensors

Thu Jun 21, 2018 3:01 pm

Are they on an extension lead ?

The DS18B if on say a 3B+ would start reporting the USB temperature as the socket warms up, they get the temperature from the GND line running into the modules. That style module would certainly get warmer. My 3B+ sockets are all toasty warm after the Pi's been on for a short while just sitting at the desktop.

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jbeale
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Re: Cheap USB temperature sensors

Thu Jun 21, 2018 3:53 pm

This is the part I'm talking about. There is a long lead containing the actual sensor. I stuck the sensor end to piece of metal lying on the table (separate from the R-Pi) so the temperature changes pretty slowly, and independently of the USB socket.
https://www.ebay.com/itm/Digital-USB-DS ... 1318135484

The "TEMPer" product is a self-contained USB dongle and would track the USB socket temperature, unless you put it on the end of a USB extension cable as I did. It has a really bad transfer curve with large "flat spots" though, so can't really provide better than 1 deg. C resolution even though it sometimes seems to. https://www.amazon.com/Upgraded-Externa ... 07B4Z376K/ <= don't expect too much

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Ronaldlees
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Re: Cheap USB temperature sensors

Fri Jun 29, 2018 6:11 pm

What a strange curve that left one is! I might trust the smooth curve, but the funky one - no way! I think its firmware needs touch up.
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