As the title says I am seeing odd reading from the Weather Shield(Si7021 sensor) running on 6 Moteinos inside a large glass jar. Everything is running with the same weathermote example code on a mix of R3 and R4 Motenio boards powered by AAA batteries. I first suspected something was wrong when my dehumidifier was reporting about 20% less humidity than one of my nodes, but I assumed it was the sensor on the dehumidifier. However, after collecting every node deployed on that floor of my house and putting them into a large jar I was very surprised at the the results.
My test includes two calibrated LCD screen meters which both read 63% at the time the attached screen grab was taken. I have read over the datasheet for the Si7021 about how it has a film that can become dried or saturated and throw off the readings. That seems to be what is happening here, but strangely none of these sensors were exposed to extreme humidity. I do not have the equipment to perform the bake process from the datasheet to dry them out, so I will be leaving them in this jar hoping they correct themselves over time.
My questions at this point are:
1. I own 11 Weather Shields with the Si7021 sensor. Has anyone that deployed around that number or more experienced this issue?
2. Do the new Weather Shields show any early signs of this humidity drifting with the other sensor?
3. Has anyone found a simpler way to "bake" the sensors and dry out the film?
Funny you post this today - I don't have weather shields, but a couple of motes built around the same si7021 chip. I, too, was getting some pretty unlikely results so I did a test yesterday with 4 si7021 based nodes, 2 dht22 and an honest to god human hair analog hygrometer.
Results were the si7021 showed about 10% more relative humidity than the hair hygrometer and the dht22 about 10% less. All si7021 were in a +-3% range of each other.
I have a feeling the si7021 are off. Maybe the si7021 is not as good a sensor as I once thought.
Joe
IIRC, the datasheet says that it's sensitive to how it's reflowed.
[Edit: also, there's required a rehydration step after reflow, and I wouldn't be surprised to learn that many forget to do it or simply omit it. From the datasheet:
"The measured humidity value will generally shift slightly after solder reflow. A portion of this shift is permanent and
is accounted for in the accuracy specifications in Table 4. After soldering, an Si7021 should be allowed to
equilibrate under controlled RH conditions (room temperature, 45–55%RH) for at least 48 hours to eliminate the
remainder of the shift and return the device to its specified accuracy performance."]
There are certain salt solutions that can produce an accurate humidity level inside a sealed jar. I don't know whether any of the common ones produce a 45-55%RH. If so, that would probably be the easiest way. Regardless, they would offer an accurate standard for testing the accuracy of your humidity readings once your sensor goes operational.
The datasheet also has some vague warnings about the sensor loosing accuracy, some of it permanent, if it's kept at too high a humidity for too long.
Here's an article showing the weaknesses of the si7021. The author also finds the chip systematically measuring too high a relative humidity.
http://www.kandrsmith.org/rjs/misc/hygrometers/calib_many.html
@joelucid,
I've seen that article in the past, it's a really great analysis.
The BME280 is declared the winner:
QuoteWithin the data obtained here, the BME280 is the undisputed winner. It is the most accurate at the specified 25°C and it shows the least dependence on temperature.
Quote from: joelucid on October 03, 2016, 02:44:17 AM
Here's an article showing the weaknesses of the si7021. The author also finds the chip systematically measuring too high a relative humidity.
http://www.kandrsmith.org/rjs/misc/hygrometers/calib_many.html
At least for me that's a useful paper because I have some si7021 breakout boards that look just like the one that he tested. On the other hand, they're cheap boards, and I have doubts that the manufacturer did the rehydration cycle after making them. Who knows whether the breakout board manufacturer stayed within the bounds of the sensor's reflow profile. In general, though, the impression I get is that by default most of the ultra-cheap generic board assemblers know nothing at all about the parts they are putting together aside from "part A" goes into "position B" with a certain orientation.
The only (meager) datapoint I have to offer is that the humidity reported by a si7021 breakout board was in large disagreement with my Lennox thermostat, which does humidity readings so that I can set it to a "feels like" temperature rather than an absolute temperature.
I would expect the sensor to give readings within the accuracy tolerance of each other if the nodes are identically built and tested. Not sure if how you place them in the jar matters, or if it can be assumed the humidity is spatially uniform in that jar? Just a thought...
If I recall correctly, for humidity measurements there's a heating element on the die to heat up, maybe that's something to look into to stabilize the readings.
Anyhow the new WeatherShields have the BME280 sensor now.
Odd. All the motes I've had/have that use the Si7021 (approximately 10-12) have had plausible humidity readings. They might be off by as much as 5%, but nothing along the lines I've seen cited here and typically within reasonable range of other sensors in the similar vicinity (including different components, ie HIH-6120).
I wonder if it has to do with possible contamination of the sensor. I've been very careful to keep all liquids, including 'noclean' solder flux away from the sensor port, even when cleaning the board with IPA, I've stayed away from the sensor. Given that 'noclean' flux is hydroscopic, ISTM that this wouldn't be a good contaminant.
I don't know and I haven't tested hundreds, but, as I say, I haven't had an issue with this sensor.
Tom
In fact, there may be nothing wrong with the sensor per se. Silicon Labs is a reputable company, and I have no reason to suspect that the sensor falls short of what's promised by the datasheet. Rather, I was just pointing out that the paper joelucid cited seems to confuse the sensor itself with an inexpensive breakout board (BoB) that appears to have the sensor on it. The difference is that the BoB manufacturer may not have properly adhered to the precautionary directions contained in the datasheet during the course of making the BoB.
Yeah the issues I'm having with the sensor obviously could have to do with my homegrown brachial assembly methods. Still if the bme280 is more robust against this type of abuse it might be a good one to try out. I think I'm going to order a couple.
What definitely doesn't work is the dht22. I don't have one that has survived a year with still valid readings.
Joe
Quote from: joelucid on October 04, 2016, 01:57:52 AM
Yeah the issues I'm having with the sensor obviously could have to do with my homegrown brachial assembly methods. Still if the bme280 is more robust against this type of abuse it might be a good one to try out. I think I'm going to order a couple.
Joe
I've never heard the term "brachial assembly method" before, but I'm guessing you are referring to manual assembly?
@Felix
When you previously did your weather shield node using the si7021, did you do the rehydration cycle, and if so, how were you controlling your humidity during the rehydration phase?
BTW, I'm reasonably sure that humidity within a jar is pretty much uniform, due to Brownian motion. I say that because I've moved some pre-made humidity sensors around my house to a variety of different locations, and they seem to register pretty much the same dewpoint regardless of where they're positioned. RH might be different, though, because that is influenced by temperature, which may in fact vary from one location to the next inside the house, often influenced by the location and amount of heat load that the sun is impinging on that area of the house.
Quote from: WhiteHare on October 04, 2016, 05:54:20 AM
@Felix
When you previously did your weather shield node using the si7021, did you do the rehydration cycle, and if so, how were you controlling your humidity during the rehydration phase?
No, I did not do anything special afterwards. My lab is a pretty constant 50-60% humidity though and that would be plenty of opportunity to stabilize while they wait for depanelizing and testing.
Quote from: Felix on October 04, 2016, 08:22:05 AM
Quote from: WhiteHare on October 04, 2016, 05:54:20 AM
@Felix
When you previously did your weather shield node using the si7021, did you do the rehydration cycle, and if so, how were you controlling your humidity during the rehydration phase?
No, I did not do anything special afterwards. My lab is a pretty constant 50-60% humidity though and that would be plenty of opportunity to stabilize while they wait for depanelizing and testing.
Have you tested to see if your old si7021's are in good agreement with your new BME280? If so, maybe your lab humidity--which sounds favorable for rehydration--would explain why.
@JoeLucid
Do you happen to know what the humidity level at your lab was in the 48 hours after your si7021 fabrication?
Quote from: WhiteHare on October 04, 2016, 12:29:13 PM
Have you tested to see if your old si7021's are in good agreement with your new BME280? If so, maybe your lab humidity--which sounds favorable for rehydration--would explain why.
No, I only have a handful of si7021 based nodes and they are deployed throughout the property and I would not consider them test worthy.
FWIW now with this si7021 post-reflow requirement discovery (I was not aware myself, and I have not seen anything like this post-reflow special rehydration in my otherwise limited experience manufacturing electronics) I am quite glad to depart the 2 sensor design anyway.
It would seem I kicked off a fun little chat with this post :)
I've been a bit slammed with work, but wanted to attach a newer shot of all my humidity readings. I am very confident that the air is/was uniform in the large glass cookie jar I am using. To verify this I briefly added a mini fan inside the jar and while the temperature did slightly rise from the running fan it seemed to confirm my thinking and has since been removed. Four of the nodes under this test have an identically build as Felix was wondering. I have also checked for any splashing or damage and found no detectable signs with my simple USB microscope on the sensors in question.
After reading the link posted by Joe and the input by WhiteHare I believe my issue is the "wetting" of the polymer inside the sensor. Looking at my newest readings you can see my sensors are lowering their reading to get more inline with the true 63% reading. The main observation from this is that the sensors reading the highest humidity and the slowest to recover all came from higher humidity sections of my home. I had the the 2 worst sensors in the basement near my fish tanks which can drive the humidity up to 70ish% on bad summer days.
TomWS, what is the average humidity where you have your sensors deployed? I have a sinking feeling that these sensors work perfectly if the humidity stays in the 45-60% range and anything above that will cause them to creep up. These sensors were used to help avoid mold from extra moister by my fish tanks, but how fast are they creeping up? I will continue looking into this and try to keep anyone else interest updated on my findings about this humidity reading creep I seem to have.
As for the last chapter of this way too long post... I have just placed an order for 3 of the new weather shields to test along with some of the trace antenna moteinos(been waiting to try them out) hopefully this will give anyone in my situation more helpful info. I would love to keep using the old si7021 boards, but until I can get a handle on how fast the higher humidity effects the overall readings they will have to be benched for unreliability.
Thanks for all the input everyone,
-Phil
Quote from: CallOne_Phil on October 04, 2016, 02:22:50 PM
I will continue looking into this and try to keep anyone else interest updated on my findings about this humidity reading creep I seem to have.
Mark me as interested. I look forward to hearing what you find out.
The last time I went looking for a really high accuracy humidity sensor (around 3 years ago), the one I settled on was from this company: http://www.ist-usadivision.com/sensors/humidity/
They're not cheap though. They also have some really high accuracy temperature sensors too that, at the time anyway, seemed excellent.
I mention here just in case someone wanted a reference for testing their other sensors against.
Quote from: CallOne_Phil on October 04, 2016, 02:22:50 PM
TomWS, what is the average humidity where you have your sensors deployed? I have a sinking feeling that these sensors work perfectly if the humidity stays in the 45-60% range and anything above that will cause them to creep up. These sensors were used to help avoid mold from extra moister by my fish tanks, but how fast are they creeping up? I will continue looking into this and try to keep anyone else interest updated on my findings about this humidity reading creep I seem to have.
The space where the most recent units were deployed were two rooms in my workshop. The workshop had been idle all summer long and the humidity was between 75-78% when I arrived back from vacation. This measured by one of my WiFi sensors with the HIH-6120 TH sensor.
The Si7021s were the last two of a batch I purchased over 6 months ago and had spent the summer in a garage that was even more damp than 75% (although I didn't have a sensor in there). The units were housed in an MS3 foil envelope, but the desiccant had long been past its useful life and the envelope was only taped shut with masking tape.
When I deployed the two motes in the rooms adjoining the workshop they immediately registered around 78%, a bit higher than the workshop, but not extremely so. I then installed a dehumidifier in the workshop to get the humidity down to a proper level and, as the readings dropped in the workshop, the readings tracked in the two rooms with the motes. The readings are now:
Workshop 76.3°F 50%
Toms Office 78.08°F 48% RH
Finishing Room 74.66°F 51% RH
In another location, where I have a mix of WiFi HIH-6120, Tiny TH Motes with Si7021, and one Mote with DHT-22 & DS18B20 (for outdoor temp), all the indoor rooms are within +/-2% of each other, the Motes with Si7021 the highest of 4 rooms. This location has very little in the way of conditioning and the indoor rooms in the spring and summer will easily reach into the mid to high 70s RH. In the winter, however, the temps will drop into the teens (degrees F) :)
Net: These sensors work for me even when stored in less than ideal conditions.
Quote from: WhiteHare on October 04, 2016, 05:41:30 PM
They're not cheap though.
Much cheaper by the pack of 50... ::)
QuoteDo you happen to know what the humidity level at your lab was in the 48 hours after your si7021 fabrication?
It was in the 50-60% range I'd say.
Reading the si7021 datasheet is a little unsettling: after reflow rehydrate, then whenever it's too dry or too wet for longer periods of time bake and rehydrate again. Finally if it's really humid remember to turn on the embedded heater to avoid over-hydration. And it really helps to say you love them and bring flowers from time to time ...
I don't know about you, but to me these chips seem a bit high maintenance :)
I am probably just blind, but is there example code for the new sensor on the R2 boards? I saw there is some bme related code inside the motionmote example, but nothing on the weather shield stuff that jumped out at me. It's not the end of the world if there isn't, but I figured I would try whatever I can to limit user error in my testing.
Quote from: CallOne_Phil on October 08, 2016, 06:19:22 PM
I am probably just blind, but is there example code for the new sensor on the R2 boards? I saw there is some bme related code inside the motionmote example, but nothing on the weather shield stuff that jumped out at me. It's not the end of the world if there isn't, but I figured I would try whatever I can to limit user error in my testing.
Did you look at these sketch's that Felix pointed to in the blog;
https://github.com/LowPowerLab/WeatherShield
https://github.com/LowPowerLab/RFM69/blob/master/Examples/WeatherNode/WeatherNode.ino
It seems the new bme280 R2 code never made it there and the newest code is WeatherShield_Si7021_only.ino from 6 months ago.
It turns out I have the same sensor boards as in the article that Joe referred to. So, for grins, I hooked up both the si7021 and the BME280 to see how they compare.
For the Si7021, using the Adafruit library, I'm getting:
Humidity: 48.93 Temperature: 21.72
For the BME280, using the Sparkfun library, I'm getting:
Temperature: 22.77 degrees C
Temperature: 73.00 degrees F
Pressure: 99002.00 Pa
Altitude: 202.13m
Altitude: 663.15ft
%RH: 47.00 %
So, actually, for humidity, they're fairly close for an off-the-cuff, rough-and-ready test. Maybe next weekend or thereabouts I'll try a more careful comparison.
Conveniently enough, the boards have different I2C addresses, so I suppose one could run both from the same sensor node and then average their results, if one were so inclined.
@CallOne_Phil, My apologies, I thought I uploaded the sketch to github. Let me get back to this asap, I will update the blog entry also.
There's the MotionMote example (https://github.com/LowPowerLab/RFM69/blob/master/Examples/MotionMote/MotionMote.ino) that reads the BME280 sensor.
Otherwise any other BME280 library would work.
QuoteThe main observation from this is that the sensors reading the highest humidity and the slowest to recover all came from higher humidity sections of my home. I had the the 2 worst sensors in the basement near my fish tanks which can drive the humidity up to 70ish% on bad summer days.
That is interesting. I use these sensors to determine by measurement of absolute humidity inside/outside whether to run a fan to exchange air as dehumidifier. Outside it's often close to 100% humid (e.g. during the night) - so this problem could easily spoil my approach, letting me miss good opportunities because the outside sensor lags.
I've thrown together a little project which uses the heater to keep the sensor humidity always below 80%. Obviously for once not a coin cell project ... I should still be able to measure absolute humidity provided both sensors heat up to the same degree.
Joe
Quote from: joelucid on October 10, 2016, 12:09:07 PM
QuoteThe main observation from this is that the sensors reading the highest humidity and the slowest to recover all came from higher humidity sections of my home. I had the the 2 worst sensors in the basement near my fish tanks which can drive the humidity up to 70ish% on bad summer days.
That is interesting. I use these sensors to determine by measurement of absolute humidity inside/outside whether to run a fan to exchange air as dehumidifier. Outside it's often close to 100% humid (e.g. during the night) - so this problem could easily spoil my approach, letting me miss good opportunities because the outside sensor lags.
I've thrown together a little project which uses the heater to keep the sensor humidity always below 80%. Obviously for once not a coin cell project ... I should still be able to measure absolute humidity provided both sensors heat up to the same degree.
Joe
At least in the places I've lived, usually absolute humidity doesn't change all that rapidly. One of the exceptions is a storm front coming through, but you could handle that as a separate case (usually identified by rapid changes in air pressure). So, although not a perfect solution, perhaps you could grab an ourdoor sensor reading late in the day when the RH is lower to compute the dew point and then make your air exchange decisions based on that during the night when the RH is no longer in the goldilocks zone.
Ahem... Well, to quote Simon Pegg from Mission Impossible, "I'm spitballing this. It's not all going to be gold." ;)
Quotet least in the places I've lived, usually absolute humidity doesn't change all that rapidly. One of the exceptions is a storm front coming through, but you could handle that as a separate case (usually identified by rapid changes in air pressure). So, although not a perfect solution, perhaps you could grab an ourdoor sensor reading late in the day when the RH is lower to compute the dew point and then make your air exchange decisions based on that during the night when the RH is no longer in the goldilocks zone.
Oh it changes quite rapidly here. But I think the problem with long term exposure to high humidity environments is not the single wrong measurement when it's wet. It's the long term drift to higher humidity readings that happens with such exposure. The datasheet says it can take substantially longer than the standard "couple of days" to return to normal afterwards and that indeed the error can become permanent.
That's why I think one should just not expose the sensor to >80% humidity by switching on the heater when it gets that wet. The datasheet sort of guides that direction, too, although staying clear of a general recommendation.
I built two weather motes yesterday and they still measure 10% too dry after reflow. Being the impatient type they are now rehydrating on the balcony at a heater controlled 70-75% humidity.
Joe
Quote from: joelucid on October 11, 2016, 03:01:24 AM
Quotet least in the places I've lived, usually absolute humidity doesn't change all that rapidly. One of the exceptions is a storm front coming through, but you could handle that as a separate case (usually identified by rapid changes in air pressure). So, although not a perfect solution, perhaps you could grab an ourdoor sensor reading late in the day when the RH is lower to compute the dew point and then make your air exchange decisions based on that during the night when the RH is no longer in the goldilocks zone.
Oh it changes quite rapidly here. But I think the problem with long term exposure to high humidity environments is not the single wrong measurement when it's wet. It's the long term drift to higher humidity readings that happens with such exposure. The datasheet says it can take substantially longer than the standard "couple of days" to return to normal afterwards and that indeed the error can become permanent.
That's why I think one should just not expose the sensor to >80% humidity by switching on the heater when it gets that wet. The datasheet sort of guides that direction, too, although staying clear of a general recommendation.
I built two weather motes yesterday and they still measure 10% too dry after reflow. Being the impatient type they are now rehydrating on the balcony at a heater controlled 70-75% humidity.
Joe
Does the BME280 have the same drawbacks? If not, it sounds better to just use the BME280 instead for outdoors. Indoors, the si7120 might be fine, because if your house is >80% humidity, then it's probably a mold forest and you've got much bigger problems than just inaccurate sensors.
QuoteDoes the BME280 have the same drawbacks? If not, it sounds better to just use the BME280 instead for outdoors. Indoors, the si7120 might be fine, because if your house is >80% humidity, then it's probably a mold forest and you've got much bigger problems than just inaccurate sensors.
They have the same reconditioning stuff in their datasheet. But other than the si7021 they claim it's only needed if operated out of fairly broad ranges e.g. after soldering. I've ordered a couple to try out. Plus some si7021 with cover in case I'm damaging the sensors during reflow.
Joe
I measured the standby current on the el cheapo si7021 breakout board (above), and it was around 8uA. Looking closer, I noticed it had a voltage regulator on it. After removing that and shorting across its Vin and Vout pads, I'm now measuring the roughly 60nA standby current that's in the datasheet.
Yeah it's a very low energy sensor. Has been running fine for years with my coin cell motes. BTW, humidity about 90% outside today and the si7021 needs heater level 2 to keep humidity at the sensor <75%. That's 15mA! If I end up doing this I'd probably use 4x Eneloop to power so it last a couple of weeks.
My two new si7021 motes now both measure 43% RH here in my office after two days of conditioning. Four other si7021 nodes in the same place measure 62%, 58%, 53% and 52%. None of the nodes have recently experienced high humidity.
Now some of them I've treated pretty badly - reflowed a couple of times after having shorts underneath the chip etc. But the two 43%, the 58% and 52% weren't mistreated.
Sigh
In comparing the el cheapo si7021 breakout board (BoB) to the el cheapo BME280 BoB, I notice that aside from the sensor chips themselves the board layout and other discrete components are nearly identical. So, for comparison, I unsoldered the voltage regulator on the BME280 BoB and shorted across its Vin and Vout pads also, and then did a standby current measurement using the Dave Jones Microcurrent in combination with my Fluke 87V (the same instruments I used to measure standby current on the el cheapo si7021 BoB above). Doing that, the standby current on the el cheapo BME280 BoB measured at 148na. So, not as low as the si7021, but I have no complaints with that.
QuoteMy two new si7021 motes now both measure 43% RH here in my office after two days of conditioning. Four other si7021 nodes in the same place measure 62%, 58%, 53% and 52%. None of the nodes have recently experienced high humidity.
Did a test over saturated nacl solution today and amazingly the two new si7021 motes are accurate. Everybody else is off by >10%. I could easily see this being the result of contamination by dust during their lifetime given I don't have covers on.
5 new Si7021 with covers arrived today. I'll replace the problematic chips and see if that leads to a stable th mote.
Tom, I think you mentioned your motes were stable. Do you have covers on?
Joe
I wonder whether your new si7021 nodes with covers are also more resilient against high humidity? e.g. imagine your particular dust/contamination is hydrophilic, so that once it absorbs moisture it resists letting it go. If that dust/contamination is in contact with your sensor--because it lacked a cover--then maybe it skews high for a long time after high humidity exposure, which is what you originally observed.
QuoteI wonder whether your new si7021 nodes with covers are also more resilient against high humidity?
It's possible. But for the outdoors installation according to datasheet I think one really should use the heater for prolonged rainy periods.
I've got the new sensors installed. They measure:
76.6%, 77.5%, 75.2%, 78.5%, 77.1% over NaCl (should be around 75%). So all but one are in spec. The 78.5% one measures 0.3 C lower in temp than the others so for dew point measurements it's closer.
I've looked at my long term charts and humidity has systematically been increasing since the sensors installed. Moral of story: you really do need a cover or a different type of dust protection.
Joe
I recently built a little experimental wireless temp-rh node (see attachment). This particular one has a detachable si7021 breakout board hovering over the RFM69HW, and on the back there's the usual atmega328p-au, an LED, a resistor, and a capacitor.
Originally, I hadn't really intended for the breakout board to be mounted "upside down." However, thanks to this thread, I'm starting to think it might actually be better that way, because it seems that dust will be less prone to settling on the sensor cover over time. Anyhow, thought I'd mention that in case there's merit to the idea, as it may turn out to be one of the "give backs" in the spirit of open source hardware to help improve things for everyone going forward.
I have not had a lot of time to do more testing, but you guys have been killing it with all these posts. Finally got the new BME280 sensors running with accurate results so far on an open air test of 66%(see attachment). Didn't have much luck drying out the si7021 boards naturally in my sealed 60%-63% container and I'll need to look into using the build in heater when time allows.
Time will tell if the BME280 sensors have the same drifting issue as we are seeing with the si7021, based on what the datasheet says I am thinking it is possible.
Quote from: CallOne_Phil on October 14, 2016, 01:06:01 PM
Finally got the new BME280 sensors running with accurate results so far on an open air test of 66%(see attachment).
Phil,
Thanks for the follow up!
Very encouraging results from the BME280, I feel quite good about it 8)
@CallOne_Phil
That does seem like very good agreement, especially for humidity sensors. Are you seeing similar close agreement on temperature and pressure?
I asked the guys at silabs about the max humidity level beyond which to switch on the heater of the si7021. Here's the response:
QuoteTypically for relative humidity sensing, it is recommended to use the heater with 80%RH or greater. However if the only point of interest is the dew point and if the power budget allows for it, running the heater with about 5C of heating at all times is recommended. The heater does not affect the dew point calculation in a significant way. Please refer to App Note AN607 for more details on the heater in high RH environments and dew point calculation.
Joe
Quote from: joelucid on October 17, 2016, 02:50:31 PM
I asked the guys at silabs about the max humidity level beyond which to switch on the heater of the si7021. Here's the response:
QuoteTypically for relative humidity sensing, it is recommended to use the heater with 80%RH or greater. However if the only point of interest is the dew point and if the power budget allows for it, running the heater with about 5C of heating at all times is recommended. The heater does not affect the dew point calculation in a significant way. Please refer to App Note AN607 for more details on the heater in high RH environments and dew point calculation.
Joe
I guess "5C of heating" means heat until it's 5 degrees C above ambient temperature? About how much current is that?
It might be worth asking as a follow-up what the benefit of the constant 5C heating is, or equivalently, what happens if you don't constantly heat 5C. I'm guessing sensor drift, but it would be good to know what the magnitude is both with and without the constant 5C heating, because then one can rationally decide whether it's worth doing or not.
[Edit: Actually, though, reading the apnote (https://www.silabs.com/Support%20Documents/TechnicalDocs/AN607.pdf) it may not be that. It looks like the main concern is whether the sensor has a lower temperature than the dewpoint (at which point dew will condense on it and thus foil the humidity measurement). Maybe simply keeping 5C above ambient mitigates against that happening (?). So, aside from the 80%+ RH case, maybe you can limit yourself to doing the 5C heating only when the sensor temperature is close to the dewpoint--extremely unlikely indoors in a habited space, for instance.]
My take is that even high humidity levels below 80% will cause shifts in the long term. Increasing temp by 5 c will decrease rh by about 25%, keeping rh readings always below 75%. That appears to be unproblematic.
It's just amazing how much due diligence is apparently required before buying even a simple humidity sensor these days. I think this sort of thing should be unambiguously and prominently disclosed in the SiLabs datasheet, in bold red and large font, not merely hinted at or left to us to discover by other means.
I guess whatever sensor I finally settle on, I'm going to make sure it's detachable so that it's easily replaceable and/or future upgradeable to some other I2C sensor. I hope the BME280 is more forgiving, but now I'll always have a nagging thought (regardless of the sensor type or the make or model) that there's a gotcha that I won't find out about it until after it's deployed.
Quote from: WhiteHare on October 17, 2016, 07:08:38 PM
It's just amazing how much due diligence is apparently required before buying even a simple humidity sensor these days. I think this sort of thing should be unambiguously and prominently disclosed in the SiLabs datasheet, in bold red and large font, not merely hinted at or left to us to discover by other means.
I guess whatever sensor I finally settle on, I'm going to make sure it's detachable so that it's easily replaceable and/or future upgradeable to some other I2C sensor. I hope the BME280 is more forgiving, but now I'll always have a nagging thought (regardless of the sensor type or the make or model) that there's a gotcha that I won't find out about it until after it's deployed.
I'm with you. Imagine how I feel after manufacturing with a sensor that has such a gotcha :-X.
But by default (assume you have no clue about this gotcha and are shopping for a nice TH sensor) would you rather trust SiLabs or a noname chinese boat fab that makes the DHT11?
As a cross-check on one another, let's compare notes. My reading of the BME280 datasheet (https://ae-bst.resource.bosch.com/media/_tech/media/datasheets/BST-BME280_DS001-11.pdf) is that between 0C and 60C, the sensor's operating range (from page 8 ) is just what one would want: 0%-100%. Outside that temperature band, the upper end of the humidity operating range starts to decrease, as per Figure 1. So, if you were to put it into a freezer where the air had very high RH, it's operating range might be exceeded. What happens then, though, isn't clear: does it just inaccurately report the humidity while under those conditions but return to "good as new" (well almost) when it returns to within its operating range, or is it ruined, or does it require a reconditioning as per Section 7.9, or... what?
It does give the caveat that for the given operating range, the environment must be "non-condensing," which I interpret to mean the sensor shouldn't have a temperature lower than the dewpoint. Well, again, what happens if that were to occur? Does it eventually return to normal, or does it require a bake/rehydrate cycle or... what?
I'm not trying to nitpick, but I am actually planning to put one in an ordinary home freezer that we've been having trouble with to better troubleshoot what's going wrong. So, at least for me, the answers are not purely academic. Likewise, in Joe's use-case of an outdoor TH sensor, his sensor might (?) encounter those conditions during the winter.
QuoteI'm with you. Imagine how I feel after manufacturing with a sensor that has such a gotcha :-X.
But by default (assume you have no clue about this gotcha and are shopping for a nice TH sensor) would you rather trust SiLabs or a noname chinese boat fab that makes the DHT11?
It seems that measuring rh is just inherently hard. You need a sensing element that's exposed to the air but also extremely sensitive to contamination - that's just a bad combination. I don't expect the other sensors to be much better.
I certainly draw one conclusion: with rh motes it's not worth to worry beyond a couple of years of battery life. I don't think technology is where you can expect sensible readings after more than that.
QuoteLikewise, in Joe's use-case of an outdoor TH sensor, his sensor might (?) encounter those conditions during the winter.
Oh absolutely. The sensor only needs to be a bit colder than your environment for condensation to occur at 100% rh. It's easy for that to happen since it will lag air temp changes. That's why I actually like the si7021 for outside. If I'd use the bme280 I'd have to include a separate heating device.
I've been running with heater for a month or so outside. It's working pretty well, the data seems sound. I'm running at heater level 1, which heats the sensor by about 5 celsius.
My old outside humidity sensor was a dht22 which I had just replaced 4 months back. That one is pretty much pegged to 100% humidity, showing the high humidity exposure drift.
Just now for example the dht22 measures absolute humidity of 7.89 g/m3, while the heated si7021 comes to 6.43 g/m3, a huge difference.
I found I had to correct the measured temp of the si7021 with heater down by 1 to get correct values for abs humidity. The app note from silabs also instructs to do something similar with heater on.
Joe