I'm going to have a go at making one of these to shield the sensor from direct sunlight.
They use a DS18B20, but I'm sure I can adapt it to accommodate the BME/BMP.
PS: A set of metal-cased DS18B20s arrived this morning - you know what my next job will be?
The DS18.. is still one of my favs in terms of accuracy and precision.
I have a similar problem with the BME280 (The one with humidity). The temperature is about 3 + degrees higher than it should be, especially when temperatures are above 18 degrees C.
But this doesn't appear to be anything to do with being cheap Chinese apparently.
The BME280 from Bosch is prone to 'Self Heating', it is documented in the spec sheet, the BMP's are apparently not so it could be related to the humidity part of the sensor. (Well, it doesn't say so in the spec sheet for the BMP's anyway).
From the data sheet....
"This temperature value depends on the PCB temperature, sensor element self-heating and ambient temperature and is typically above ambient temperature. "
I find this very strange as it is supposed to be more accurate, but it is in the spec. The DS18B20's are more accurate for temperature but with larger steps (lower resolution) and are very cheap. In future, as I expand my monitor, I will use these for temperature rather than the BME.
What is unclear is whether it effects the humidity readings or not as humidity uses temperature in its calculations which is why most (if not all) the humidity sensors also do temperature.
I have several alternative sensors for humidity and out of 5 different type/models of humidity sensor, no two gave the same result for humidity yet all gave very similar temperature readings with the exception of the BME280.
Edit.....
I have mine powered on continuously, so I wonder if it was powered down between readings or placed in standby if then it would not suffer the self-heating and improve accuracy!
I also read in case they are used with an esp or something, keep them on distance from the board, apperently the wifi chip interferes as well.
The key purpose of the temperature sensor is for temperature compensation of the humidity and pressure sensors, so the important thing is that it measures the temperature of the device accurately. For the compensation it is not important if it does not measure ambient temperature accurately. So the temperature reading is probably accurate and to spec, it is just not measuring what you want to measure.
That about the wifi I didn't know and as my BME280 sits directly above it although in a separate ventilated compartment, I may try moving it away from the ESP then.
Colin, a very good point about the temperature compensation for humidity, I hadn't thought about it that way.
Some of the other humidity sensors I have are sold purely as humidity sensors not as temperature and humidity although temperature readings can be taken.
The BME/BMP I'm using sits on one end of a 63-row breadboard - the Wemos is at the other end.
My weather station is powered-off (in 'deepsleep mode) for 10-minutes, then wakes-up, switches-on power to the sensors, takes some readings and sends them via MQTT to Node-RED (this sequence takes about 12-seconds), switches-off the power to the sensors and then goes back to sleep for another 10-mins
That is definitely the way to do it Dave. I made some tests with a Wemos and a SHT30 shield, if you mount the SHT30 too close to the Wemos you get skewed readings. I overcame this by using long headers to mount the shield and using deep sleep between measurements.
BTW it is possible to calibrate / check humidity sensors relatively easily using saturated salt solutions. If you, or anyone else is interested drop me a PM with your Email address and I can send you the PDF file containing humidity values for various salts and temperatures. I cannot upload it here.
Here is a site explaining the process:
I read about humidity testing with salt a while ago when I was checking my 5 different sensors, but never got round to it. However, that article looks very easy and an interesting thing to carry out.
Thanks for the pointer Garry
Nice, you'll find some of my examples dotted about the forum as screenshots. Here is a sample from one of my dashboards:
That is.... Totally Information !
I Like the BMP/BME-280's, the originals are from Bosch and they are both pretty accurate. I don't like that it is very hard to remember which is which.
I've a feeling that there might be a 260 range as well that doesn't have the pressure sensor.
Another nice sensor for temperature and humidity is the HTU21D, AKA SI7021 around the same accuracy as the BME280. These are the ones I mainly use now. Certainly avoid the DHT-11/22, etc as they are absolute rubbish, they are wildly inaccurate and drift over time.
When reading altitude, you must realise that this is calculated and not direct from the sensor which reads pressure. Note that, like humidity, altitude calculations are temperature sensitive because temperature changes the atmospheric pressure (some basic physics is always helpful!).
When measuring any of these things, do remember that while the sensors may claim "0.1% accuracy" and so on, that is really rubbish. You can get pretty accurate absolute temperature readings but humidity is massively sensitive to all sorts of environmental factors and a simple sensor with no mechanical airflow control will be lucky to get a reading within 10-20% absolute.
That is temperature only of course and generally comes in a waterproof housing. I have a couple lying around but I've never got round to using them "in the wild". Should be as accurate as the BMP/E-280 if not very slightly better because of the packaging.
The problem is that it is quite open so you would need to put it somewhere well sheltered to keep water off the sensor, I imagine that dew will impact the life and readings of a BMP/E-280 as well. Nice idea for the temperature readings though, clever.
I recently brought a rather nice rotary tool (like a "dremel") in an Amazon sale - under the Tacklife label, really well made and surprisingly useful, it would be ideal for making those kinds of projects.
This is quite common with many sensors. The way around it is not to over-read. Keep it powered off until needed, give it a second to get going, make a reading then turn it off again. Very little point in doing readings more than once a minute anyway, indeed once every 5 minutes is generally more than enough.
It absolutely does, humidity calculations are very sensitive to temperature as you can easily measure for yourself.
Yes, see my comments above.
You've got it 
Yes, this is why the Wemos DHT-22 board is doubly useless.
I was really making reference to some of the dodgy packages out there. I have had two fail within six months of use in "suspicious" circumstances. But I'm sure the sensors themselves do exactly what is stated in the datasheet.
Well, that's school holidays almost over. Looking forward to some updates.
Martin
Any updates, its all gone quiet.
Martin
Hi Martin,
I had a set of IN219A current/voltage sensing devices arrive this morning, so my (weather station) student will have the task this week to incorporate one of them in his circuit.
I'm going to try it out one of them on one of my own circuits tomorrow (just to make sure there are no problems).
Thanks for your continued interest in the project.
Regards, David.
Well, I'm home after 7 weeks away at work.
Time to play.
How's things coming along with the weather station?
Martin
Hi Martin,
Welcome back (home).
Lashed-up one of the INA219 sensors on a spare breadboard - worked like a dream with a Wemos D1 Mini.
The weather station is on hold at the moment as the school has an Open Evening towards the end of September and my students have offered to run some demos of their work within the Club to prospective parents.
I've also taken on a new set of students (as this is the start of a new school-year) so I'll need to start from scratch with them.
Thanks for your continued interest in the project.
David
So, what you thinking of for the new students ?
Just building on what the others did.
I have a lot of work to do on mine, lot of re-coding and change some sensors. Things haven't been too reliable while I have been away.
Here's an updated link.
https://snapshot.raintank.io/dashboard/snapshot/GD7gXzsUZHY0pUKww0CNG2sp32UWDEwk?orgId=2
Martin
My word that's an impressive dashboard.
My plan (for the new students) is to start at the beginning...
David
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