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Forum Index : Microcontroller and PC projects : 18B20 sensor vs others - who to believe.
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Bryan1 Guru  Joined: 22/02/2006 Location: AustraliaPosts: 1483 |
I use a couple of DS18B20 temp sensors on my 30 litre fermenter. Those temp sensors are the one in a SS can with a 3 metre lead and I did wonder about just how accurate they were. So I used 2 of my multimeters with thermocouples attached next to the DS18B20 temp sensor by using duct tape to the side of the fermenter. One sensor is 2/3's the way up and the other is about 75mm from the base. I have a 100 watt light globe fed by a cheap 10 amp ebay sourced relay, the temp reading showed the DS18B20's to be within 0.2 degrees. Like when it was 23.4 on the thermocouple the DS18B20 went up a 0.5 degree value. Likewise on the lowering of the temp 0.6 degrees on the down slope the DS18B20 did change value. This is all controlled by my micromite so I now have a temp controlled fermenter for the apple cider I make. Cheers Bryan |
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| Malibu Senior Member  Joined: 07/07/2018 Location: AustraliaPosts: 260 |
Hi Grogs... Not knowing your circuit, but from what I can see in general for these devices, there's a 4.7k pullup to the PSU. I'm just wondering if accuracy of this resistor could add/subtract to the accuracy of the device? Maybe a 10% resistor might give a wider error reading than a 1% resistor? It's just a thought I had  Another thought is to get yourself a medical style thermometer - say, a home unit from a reputable company like Omron, and maybe that could provide a good base comparison point? (BTW, I've never used these devices, so I can't speak with any authority on it!  )John |
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| Paul_L Guru Joined: 03/03/2016 Location: United StatesPosts: 769 |
The devil is in the details. Notice that I said that you should imbed the sensors in packed shaved ice, pour off the melt water as it accumulates, wait 15 minutes and then read the sensors. The trick is to measure the temperature of the ice as it melts. The already melted melt water has already absorbed heat and is now warmer than 0°C. The sensors must not be in the melt water which accumulates in the bottom of the coffee cup, they must be in contact with the ice above the melt water. This is an ice bath, not a water slurry bath and will produce a calibration uncertainty of +/- 0.05° C!!! This was written by NIST back in 1990. Notice that it mentions syphoning the warmer melt water from the flask as it accumulates. https://www.nist.gov/sites/default/files/documents/calibrations/sp819.pdf NIST has recently produced this video which suggests that you use a water/ice slurry which is less accurate but easier than the shaved ice method above. Its uncertainty is probably within +/- 0.2° C. https://www.youtube.com/watch?v=KYOJayWqB3g Paul in NY |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
G'Day Paul_L My post was meant to be a bit tongue in cheek but also point out the difficulties involved in your method. Yes, it is accurate but must be done with great care. I still think a good quality glass thermometer would be more than accurate enough. And I will not eat my hat Peter |
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| Paul_L Guru Joined: 03/03/2016 Location: United StatesPosts: 769 |
How ya doin PeterB in Oz? Don't bite your tongue while it's in there.  Ice is probably cheaper than a good glass thermometer. It doesn't take a lot of care. Just use a clean coffee cup, freeze rain water, chop up the ice cubes in a blender, don't stick the sensors all the way to the bottom, pour of any excess melt water, wait for a stable reading. I don't wear a hat. I prefer eating kielbasa. Paul in NY |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
@Paul_L I didn't mean your hat I meant my hat which is made from rabbit fur, I think. You have come to the crux of the matter. Palcal is getting all our rain. We have had 1000 this year. That's spots not mm. We are the driest state in the driest continent. My grandfather talked about seeing rain when he was a boy. We have 2 rivers each with a plug to stop draining into the sea. So you will understand that clean water is a problem. If my BME280s don't start arriving tomorrow Ill get quite cross Peter |
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| Paul_L Guru Joined: 03/03/2016 Location: United StatesPosts: 769 |
Hi Peter, I could mail you some nice clean brand new rain water. We had 1000 spots earlier today in about 10 seconds. I can also post pictures of rain to refresh your memory if you want. Have you devised any efficient method of collecting your normal personal effluent and distilling it back into new water? NASA could probably use your special expertise in the space station. Paul in NY |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
G'Day Paul in NY Thank you very much. That would be nice. I cleaned the dirt out of our rain gauge yesterday and tomorrow a bloke is going to clean our gutters etc. All that means it will never rain again. And we still haven't solved Grogster's problem. But it has been fun. Peter |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1667 |
Hey Grog, Don't think it's been mentioned but you could look at the Sensirion SHT range. some are claimed to be +/- 0.2°C. Davis uses them in their weather stations. Mine was an SHT11, but they later upgrade & I followed to an SHT31. Have a look here, and here for a data sheet. Cheers. |
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
No, I did not know of them at all. Thanks for the heads-up. I am reading it's datasheet now. You can get the QFN 31's from element14 for about $10 each. Typical 1.5% humidity accuracy, and 0.1'C accuracy!  I will look into these, and perhaps get a couple when I order some 18B20's from element14 to be sure they are the genuine ones. Smoke makes things work. When the smoke gets out, it stops! |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
G'Day Grogster et al This morning 2 18B20s in stainless steel arrived T1 = 26.25 T2 = 26 glass thermometer = 25.5 A/C is off so not much air movement but temp. is dropping slowly. I'm still waiting for my BME280s. Peter |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
And again Two BME280s arrived yesterday. And they didn't work until I changed the address from 77 to 76. BME280 = 23.6 glass thermometer = 23.2 It looks like my glass thermometer is about 0.5 low. I can't remember what its claimed accuracy is but it is quite good enough for me. Would you rather have quite accurate temp. RH & atmospheric pressure or a cup of coffee ? Peter |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
I have found these, which are I2C temperature and humidity sensors in 8-pin SOIC, for NZ$13.80 each - US$9.41 each. Made by Honeywell, so a well known brand. I am looking at these now. I have one - the last one that element14 had in stock at the time. Link to PDF datasheet Smoke makes things work. When the smoke gets out, it stops! |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
But the BME280 is cheaper and can tell you if you are sitting down or standing up Your device has a time constant of 30 secs. I don't know about the others but that seems a long time. I think you can throw your mercury and cheap digital devices away. These modern digital things are cheap, accurate and easy to use when someone writes the necessary code. We may have to turn the heat on tonight.  Peter |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
I do seem to recall talk on the BME280. I will have a closer look at that. I only bought one, so no major loss. Smoke makes things work. When the smoke gets out, it stops! |
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