Topic: Davis 6323 FASA RH accuracy?

[Ken7]

What did your Kestrel read during that time?

Did neighboring stations (excluding airports) also drop in DP?

This is why I also have a backup station on property that is simply a temp/humidity station. Even though it's a good +3F in avg DP, I can still track trends and see if the offset between stations has moved.

Kestrel has been rock solid. It, like the area airports, has changed very little and the Davis is now within 1 degree on DP and 2% on RH relative to the Kestrel. So this is quite consistent with the area airports as it has been since I got the new Drop 2.

Interestingly the PWS near me haven't changed much. They are still quite a bit wetter than my Davis, the Kestrel and the area airports. Likewise, the non-FAA sites on the NWS map also have not changed and are still quite a bit wetter than the FAA sites.

[openvista]

If the Kestrel was relatively constant during the period when the VP2 indicated a sharp drop in dew point and other area stations (including airports) have been stable, then, yes, it sounds like your SHT31 humidity sensor is fluctuating unpredictably.

While I haven't seen that particular behavior in any of my SHT sensors (I have 5 of them), that certainly doesn't mean it didn't happen in your case.

I'm sure you'll keep an eye on it! Documenting it can only help clarify matters as time goes on.

[Ken7]

If the Kestrel was relatively constant during the period when the VP2 indicated a sharp drop in dew point and other area stations (including airports) have been stable, then, yes, it sounds like your SHT31 humidity sensor is fluctuating unpredictably.

While I haven't seen that particular behavior in any of my SHT sensors (I have 5 of them), that certainly doesn't mean it didn't happen in your case.

I'm sure you'll keep an eye on it! Documenting it can only help clarify matters as time goes on.

Yes, as of now I'll be watching out for that magic temp of 45F in the future. If it turns out that the 45F 'flip' happens in the future, then the fluctuations won't be 'unpredictable', but rather 'predictable'. Remember this sudden change has now brought it well within specs. When I look at the WU temp/DP plot, it's easy to see the 'precipitous' drop occurred precisely when the temp hit 45. It could be coincidental, but either way it's very odd.

So I'm assuming you've never seen this temp-DP/RH relation in terms of hitting a threshold for accuracy?

[openvista]

So I'm assuming you've never seen this temp-DP/RH relation in terms of hitting a threshold for accuracy?

Well, truth be told, my most recent SHT31 sensor has a "switch" at 55F. But it's only a 2% change in absolute humidity. In other words, in midrange humidities, below 55 deg the error is a pretty consistent +6%. Above 55 deg, it's +4%. That only amounts to a shift of 1 deg F in dew point between about 30% to exactly 83%.

HOWEVER, and it just occurred to me -- I don't know why I didn't think of this earlier  -- you might be seeing the onboard recalibration routine of the sensor after having exited a period of high humidity. If the actual humidity was above 80% for awhile (several hours at least) and it's been drier for a spell, that behavior does occur with one of my 31 sensors (the others are permanently out of calibration, including my new one). In fact, the sensor I'm thinking of can go from +10% to dead accurate within a matter of minutes if it's sufficiently dry (65-70% or less)! Does that sound familiar?

[Ken7]

Oh, that sure does sound familiar! So you feel, at least in your case, this ‘switch’ hinges more on a long period of dry humidity following a period of relatively high humidity, rather than a certain temperature.

I could buy that and it should be easy to verify or refute (in my case) as time goes on. It would be intellectually satisfying to know the cause, if not comforting.

[openvista]

Oh, that sure does sound familiar! So you feel, at least in your case, this ‘switch’ hinges more on a long period of dry humidity following a period of relatively high humidity, rather than a certain temperature.

I could buy that and it should be easy to verify or refute (in my case) as time goes on. It would be intellectually satisfying to know the cause, if not comforting.

For purposes of this discussion, we'll call the sensor that recalibrates itself "sensor B" (since it is the 2nd newest sensor I own at 1 year old). "Sensor A" is my newest sensor (2 months old) that is active on my website.

Sensor B is the one that goes through the recalibration from wet conditions to dry. This is something that Sensirion mentions in their datasheet on the SHT31 (see attachment). It's designed behavior, although they claim the maximum recalibration value is something like 3%. In practice, perhaps also as a function of age, the recalibration can be several orders of magnitude higher (~10%).

Sensor A has a "switch" that seems wired to 55 degrees at which point it shifts by -2%.

Sensor B appears to have no temperature-based non-linearity, at least not that I've documented.

[openvista]

Just be advised that there is NO computational/correction mechanism *within* the SHT31 devices...it's *in* the ISS firmware.

That's not true. See the datasheet I attached to my last post. The recalibration behavior is hardwired into the sensor.

Whereas, the temperature shift I see in one of my sensors is probably a coincidence. But it is very consistently observable at EXACTLY 55 degrees.

[Old Tele man]

Just be advised that there is NO computational/correction mechanism *within* the SHT31 devices...it's *in* the ISS firmware.

That's not true. See the datasheet I attached to my last post. This behavior is hardwired into the sensor.

You are correct...I deleted my post.

[openvista]

In practice, perhaps also as a function of age, the recalibration can be several orders of magnitude higher (~10%).

I should clarify my above statement.

Unfortunately, I don't have an out-of-the-box documented baseline for sensor B. However, my memory tells me that the recalibration swings became greater with time and also, it seems, that the time out of calibration grew longer with each high humidity event. At one point, it was out of calibration for 48 hours at something like +11%. Not long after that I decided to roll the dice on a new sensor.

In any event, sensor B was aspirated full-time by an A/C-powered fan that blew 2.2 m/s of air at it. That's over twice the rate of the stock Davis solar fan. As many of us know now from other discussions, that increased airflow accelerates the aging process.

[CW2274]

So here's a bit more data about the non-linearity of this RH/DP error. Within the space of the last 20 minutes, my DP dropped nearly 5 degrees. That, all of a sudden, brings my RH & DP right in line with the airports. My DP is now within a degree of the airports and my RH is bang on. This is now within specs. Now before anyone jumps to conclusions and thinks the change in my DP & RH readings are reasonable and could be an actual depiction of a change in air masses in my area, it isn't.

A look at the area airports for that same time period, shows a zero change in DP readings. In actuality, there's been little to no change in DPs at the airports for the last 7 hours. We've been experiencing relatively windy conditions with no change in wind direction.

So it's as if some threshold has been hit on the Davis, and DPs & RH are now shifted to a much more accurate state. Is it a change in temperature? It appears that once we hit 45F, the DP dropped 5 degrees in the span of 15 minutes. This is all interesting & frustrating at the same time.

Completely normal. When comparing, you gotta remember that ASOS's use an averaging algorithm that makes readings much more "stable" than ours. We as PWS owners do not have that, and if you're aspirated, that can compound differences even more.

[openvista]

Completely normal. When comparing, you gotta remember that ASOS's use an averaging algorithm that makes readings much more "stable" than ours. We as PWS owners do not have that, and if you're aspirated, that can compound differences even more.

Except he also pointed out that other nearby non-airport stations along with his check device were ALL relatively constant during the period of his VP2's sudden drop. The Kestrel sensor is quite sensitive to changes in humidity, more so than the VP2.

Dew point should not be wildly varying when it's brisk and mostly cloudy at 41 deg N latitude (low sun angle).

[CW2274]

Completely normal. When comparing, you gotta remember that ASOS's use an averaging algorithm that makes readings much more "stable" than ours. We as PWS owners do not have that, and if you're aspirated, that can compound differences even more.

Except he also pointed out that other nearby non-airport stations along with his check device were ALL relatively constant during the period of his VP2's sudden drop. The Kestrel sensor is quite sensitive to changes in humidity, more so than the VP2.

Dew point should not be wildly varying when it's brisk and mostly cloudy at 41 deg N latitude (low sun angle).

Never said there isn't an issue, just something that must be considered over time if ones going to compare with ASOS's.

[Ken7]

For purposes of this discussion, we'll call the sensor that recalibrates itself "sensor B" (since it is the 2nd newest sensor I own at 1 year old). "Sensor A" is my newest sensor (2 months old) that is active on my website.

Sensor B is the one that goes through the recalibration from wet conditions to dry. This is something that Sensirion mentions in their datasheet on the SHT31 (see attachment). It's designed behavior, although they claim the maximum recalibration value is something like 3%. In practice, perhaps also as a function of age, the recalibration can be several orders of magnitude higher (~10%).

Sensor A has a "switch" that seems wired to 55 degrees at which point it shifts by -2%.

Sensor B appears to have no temperature-based non-linearity, at least not that I've documented.

It would be interesting if I'm seeing a combination of the onboard recalibration coupled with a temperature 'switch'. The one thing I think we're not sure about is the actual maximum recalibration capabilities of the sensor. If Sensirion is only claiming a maximum of 3%, do we really know that the recalibration can go much higher with age? Wouldn't Sensirion mention that in the data sheets?

As an aside, since the switch and/or recalibration, I've been tracking amazingly close on every parameter to the airports & the Drop. It's like a different unit. It's too bad there isn't an 'on demand' recalibration that the user could initiate.

[Ken7]

Completely normal. When comparing, you gotta remember that ASOS's use an averaging algorithm that makes readings much more "stable" than ours. We as PWS owners do not have that, and if you're aspirated, that can compound differences even more.

I don't think so. Remember, I mentioned the air mass we're currently influenced by has been resident in our area for the last 36 hours. There have been no significant fluctuations in DP & RH at the area airports since a few hours after this air mass' arrival. Only the expected diurnal temperature changes have shown any significant changes. 

So the area airports, on the NWS website, have shown no significant DP fluctuation and certainly nothing like what my Davis experienced as it went through its significant drying. In fact, DPs were extremely stable at the airports for at least 6-7 hours.

In terms of the other locations on the NWS site, they too were all stable (although much wetter than the FAA sites as was my Davis until it decided to 'dry out'). So everything was stable, airport sites, non-airport sites and my Kestrel Drop 2. Only my Davis underwent a significant change. This makes it pretty clear to me that it was just my Davis that was experiencing this sudden drying/recalibration or whatever it was.

[openvista]

It would be interesting if I'm seeing a combination of the onboard recalibration coupled with a temperature 'switch'. The one thing I think we're not sure about is the actual maximum recalibration capabilities of the sensor. If Sensirion is only claiming a maximum of 3%, do we really know that the recalibration can go much higher with age? Wouldn't Sensirion mention that in the data sheets?

Sensirion only mentions that prolonged exposure to humidity above 80% may result in degradation of overall sensor performance. I doubt every sensor takes the same trajectory since each is subjected to different environments and potential contamination. But I think that statement by the manufacturer provides plenty of space for the imagination without making their lawyers sweat.    As I said, I only intuited the aging process was to blame because I noticed the problem seemed to worsen with time. But it's entirely possible that the length of time it was out of calibration was purely a function of varying weather patterns and not age. At a certain point I began documenting how long that error period lasted. The minimum was ~6 hours and the maximum was ~48 hours.

This is how it would work. Moist air would move in. It would stay in place for half a day or longer. Then as the humidity would start dropping, sensor B would stay pinned in the 90s rather than dropping into the 80s. If the actual humidity stayed in the 80s for a day, then the sensor would run wet for a day. But if it dropped into the 70s then it was only a matter of time until it returned to accuracy. The drier the air, the shorter the interval until recalibration occurred. Assuming a 75% humidity air mass (measured at 2 meters) hovers indefinitely afterward, my memory tells me it would take, on average, about 12 hours for the sensor to recover. YMMV.

As an aside, since the switch and/or recalibration, I've been tracking amazingly close on every parameter to the airports & the Drop. It's like a different unit. It's too bad there isn't an 'on demand' recalibration that the user could initiate.

Yes, when my sensor B is on, it's dead on. It's a night and day difference.

[Ken7]

My scenario was much like yours, but it seemed to me I was stuck in the mid-70% range for over a day after a prolonged period of >80% humidity. It's great having the accuracy back...for however long it is.

For sure I'll be keeping a close eye on this unit once we get into a prolonged moist air situation. Looks like that may occur as early as this weekend. If you see someone cringing in the corner this weekend, that will be me.

[CW2274]

Completely normal.

I don't think so.

Ok, let me rephrase, completely normal for my VP2 and where I live.