 Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me. |
Forum Index : Microcontroller and PC projects : 18B20 sensor vs others - who to believe.
Page 1 of 2   |
|||||
| Author | Message | ||||
Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
Howdy.  I just want to check I am not chasing my tail here. 18B20 sensors give a reading - such as 23.5'C, but the mercury thermometer beside it says 20'C. A digital thermometer beside them both agrees with the mercury thermometer.(says it is also 20'C) I have tried about three different 18B20 sensors, and they all return the same result, so I don't think I just have a crook 18B20 sensor. If I put an AM2302 humidity/temp sensor on another pin(with pullup etc), and read IT, I get a reading of 23.8'C, so it is MUCH closer to the 18B20 result then either the mercury one or the external digital thermometer - only 0.3'C difference, and I can live with that just fine, but I DO need 0.5'C or so accuracy. 3'C is way off when you want accurate measurements. I'm just wondering which one is correct? Is it safe to say that the 18B20 is a logical reference point for accurate temperature, and thus, I can pretty much ignore the results from the mercury and external digital units? I would have thought the external digital thermometer would have been accurate though, but it appears to be about 3'C out, which is a lot for a digital unit I would have thought.....  However, the digital one is just a cheap NOT-LAB-CERTIFIED one I got from the local hardware store, so perhaps.....? Smoke makes things work. When the smoke gets out, it stops! |
||||
TassyJim Guru  Joined: 07/08/2011 Location: AustraliaPosts: 6283 |
A long time ago, when I needed a reasonably accurate thermometer, I went to the thermometer shop and looked a 10 thermometers, all placed close together. I chose one that read in the middle of the range. You could raid the freezer and make an ice-bath. I remember doing that back at high school. I don't remember much from then... Jim https://globaltemp.com.au/calibrating-your-thermometer-fact-sheet The temperature of boiling water depends on atmospheric pressure. Something the link above fails to mention. VK7JH MMedit |
||||
| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
Good morning Grogster How confident are you in the mercury thermometer? I think you should start with a guarenteed glass device and rely on that because things like self heating, software funnies, you name it, can complicate things electrical. I am still waiting for some BME280s to arrive and I'm sick of listening to Tom Petty. Good luck. Peter |
||||
| JohnL Senior Member  Joined: 10/01/2014 Location: SeychellesPosts: 128 |
Grogster, I posted something similar in one of your other topics, but deleted it after I thought that it is not my business to tell you how to learn. Tell me if its out of order and I will shut up. You need to learn some fundamentals before gasping for "how long is a piece of string" answers. How much research and reading have you done to understand how 18B20 sensors work? Most of you guys are hobbyists with relatively little technical knowledge, but there is a point where you will have to learn rather than just keep on guessing and gasping for answers from other people. https://datasheets.maximintegrated.com/en/ds/DS18B20.pdf This is a technical data sheet for the sensor that will tell you everything you need to know. But if you don't understand and don't know how to read data sheets then I strongly suggest that you roll your sleeves up and start learning. Ohms law, basic DC and AC theory, at least basic digital electronics. Would strongly advise that you also learn basic 8 bit microcontroller fundamentals and eventually move to 32 bit. All of this stuff is available online these days with video/youtube tutorials. I became an engineer by having a passion for electronics and microcontrollers, but also had to do the hard yards and learn all the theory for a very good reason. With evolving technology, learning is an ongoing process. Other questions you need to ask your self: 1. Is it a genuine product from reputable supplier? If yes then data sheet from the original manufacturer will tell you everything you need to know. I hope that you are not offended by advice. Cheers. |
||||
| JohnS Guru Joined: 18/11/2011 Location: United KingdomPosts: 4044 |
I wonder if the software conversion of the data from the sensors is wrong? Can you get the raw values? (And post the value the non-sensors show.) John (who is thinking Grogs can read a datasheet rather well...) |
||||
| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
 Peter |
||||
| TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6283 |
DS18B20 claims +-0.5 degrees HTU21D claims +-0.3 degrees BMP180 claims +-1.0 typical up to +-2.0 at 3.3V AM2302/DHT22 claim +-0.5 typical up to +-1.0 I have 4 running in close proximity. I don't use parasitic power on the DS18B20 All sensors from eBay so quality unreliable. Jim VK7JH MMedit |
||||
| Paul_L Guru Joined: 03/03/2016 Location: United StatesPosts: 769 |
Get a bunch of ice, preferably shaved or crushed, if it's cubes shave it if you have a high power blender, get an insulated coffee cup, pre chill it with ice water, dump it out, pack it full of shaved ice with the pre chilled sensors imbedded, keep the sensors above the melt water accumulating in the bottom, (it's warmer than the ice), pour off the melt water as it accumulates, wait 15 minutes, read the sensors. Get some distilled water, forget about altitude correction - your house is about 40 feet above sea level, bring it to a boil, dip the sensors in for at least 3 minutes and read them. You now have a two point calibration. The band gap sensors depend on the ratio of the area of two sensors both of which are silicon pn junctions. Their linearity is absolute and predictable, depending on fundamental physics and can be calculated with the inclusion of Boltzman's constant. The thermistor in the AM2302 is subject to linearity variance due to impurities in the resistive element. It is less trustworthy than the fundamental physics of the band gap. The precision of the capillary tube in the glass thermometer is a complete unknown. I have no idea about the thing you bought in the hardware store. After the two point calibration trust the band gap sensors! Paul in NY |
||||
| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
Lots of replies - thanks chums. @ PeterB - No idea on the accuracy of the mercury thermometer at all. That is part of the problem. What I need is an accurate reference point. @ JohnL - Yep, I have the datasheet, and have read through it, but although it says it has a 0.5'C accuracy, the results I get from IT are not within where the other ones say the temperature is, so it got me wondering if the 18B20's are actually as accurate as the datasheet says they are, when other thermometers show quite significantly different temperatures. Yep, I do know how to read a data sheet, and did my Electronics theory exam years ago with Ohm's law, Lenz's law and all those other laws. I must admit that some of it has probably slipped a bit, but on-line calculators exist for all those formulas these days.  Not offended by advise. You can't learn, if you can't listen. However, the datasheet saying that it has this accuracy, but that not jiving with other thermometers placed in the same place just makes me wonder how trustworthy the results REALLY are. I cannot remember where I got those 18B20's from, but they are quite possibly clones from eBay, so I will make sure I get some genuine ones from element14 in my next order. Perhaps that is actually the problem - who can say. @ TassyJim - No, I am not using parasitic power on my 18B20's. I had an issue with that once before, and since then have decided to power them with 'Real' power rather then parasitic power. What makes me wonder, is that the mercury thermometer(an unknown) and the AM2302 humidity/temperature sensor(an unknown) BOTH show about the same temperature, but the 18B20 shows a different temperature by a few degrees, so which one is right? I think at this point, I need to test some GENUINE 18B20's, so I will order some to make sure I have the genuine one and not a clone. @ Paul_L - Good advice, thanks. I will try that in the next few days. Smoke makes things work. When the smoke gets out, it stops! |
||||
palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 1993 |
I'm wondering about that, just saw a news item that stated IKEA sell a map of the world with New Zealand missing. Though it may have sunk. "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
||||
| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
Are you still above sea level? Peter |
||||
| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
Well, DigiKey list New Zealand as part of Australia - so whaddya think of that?!  Smoke makes things work. When the smoke gets out, it stops! |
||||
| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
I meant is palcal above sea level? They have been a bit greedy with the rain of late...........typical  |
||||
| JohnS Guru Joined: 18/11/2011 Location: United KingdomPosts: 4044 |
Maybe they're listing by continent? John |
||||
GoodToGo! Senior Member Joined: 23/04/2017 Location: AustraliaPosts: 188 |
Naaa, New Zealand is just another suburb of Sydney...... Cheers, GTG!  ...... Don't worry mate, it'll be GoodToGo! |
||||
| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
@ everyone outside of Australia and New Zealand reading these last few posts - we like to poke fun at each other, and there is no malice in it, it's all in good fun. So don't worry. Smoke makes things work. When the smoke gets out, it stops! |
||||
| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
G'Day Paul_L If you can get a bucket of water from the local creek, bung it in the fridge until it goes lumpy and say "yep 0" Then put the same water on the stove, boil the crap out of it and say "yep 100" Then take those two points and plot them on the bit of paper your lunch came in, find 20 0r 30 and claim that that is more accurate than my 300mm reasonably expensive glass thermometer I will eat my hat. Peter |
||||
| palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 1993 |
@ PeterB Sun out for first time in two weeks yesterday, the total I got here 60KM. north of Townsville was 1,544mm. "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
||||
| Paul_L Guru Joined: 03/03/2016 Location: United StatesPosts: 769 |
The boiling point will change considerably depending on the amount of salts and other minerals in solution with the water. The local creek will provide water sourced mostly from rain -- if you filter out the organic compounds it picks up from decaying leaves and the like it will be pretty close to distilled water. Well water will be more likely to have absorbed minerals. The freezing point of creek water will change very little with the inclusion of organics. Well water can have its freezing point depressed when salts go into solution. The solution to all this is to go to the nearest auto parts store where they usually sell jugs of "distilled" water to replace water that has evaporated out of lead acid batteries. Nowadays that stuff is usually produced by osmotic filtration but it's good enough. You could also collect some rain water, but then I guess you guys down under have forgotten what rain is. Of all the various physical standards the freezing point of water is the one that is most easily established precisely. The chief difficulty in establishing the boiling point is pressure altitude ... but Grogs doesn't live up the side of a mountain. Paul in NY |
||||
| Paul_L Guru Joined: 03/03/2016 Location: United StatesPosts: 769 |
Just start up Google Earth, type in Mosgiel, NZ, and it displays Grogster's town with the altitude of the cursor. It's close enough so that you can see the altitude of each end of my driveway which is 100 feet long. Paul in NY. |
||||
| Page 1 of 2 |
|||||
 Print this page |
 |
The Back Shed's forum code is written, and hosted, in Australia. | © JAQ Software 2025 |
