Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 17439 times)

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Offline jerryg

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I have a question, this thread has been about the vp2 and i was just wondering if the vue has this problem.

Offline openvista

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johnd, I don't believe the issue is whether Davis is longer than the other short straws in its segment. It's that they market the station with tight specs. When it's demonstrated through a variety of field and lab tests that those specs aren't accurate or even close, they refuse to run any of their own tests or to stand behind their product. Instead, they explicitly put the onus on the customer and put their retailers (you) into the direct line of fire.

Since Davis lets its marketing department publish fantasy specs, this thread exists to investigate and publicize the real specs granting prospective and existing customers better information. Perhaps, through some ingenuity, hard work and luck, we can cobble together a viable workaround that produces better results. Perhaps not.

In any event, it is not unreasonable to expect a device to live up to its published specifications (+/- 2% 10-90% humidity) and calibration longevity (5 years). If we all passively accept that it's impossible to get even somewhat accurate technical data on scientific equipment, then WE become the problem.
« Last Edit: July 19, 2018, 02:09:02 PM by openvista »
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Offline ValentineWeather

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Last update on the Acurite sensor passive in shade still running 8% lower humidity. 
Acurite:82-45%
Davis 82-53%

I agree with the above on price point but for anyone living in high heat and high humidity regions getting heat indexes in excess of 120° when it's actually much lower doesn't matter what the price point without reliability.

For me it's more about accuracy that I can afford. In this case the sensor itself is just fine.. The problem may very likely be the way Davis molds chip onto board.
I wasn't prepared to come forward with this information I received a few days ago from unnamed source until now after seeing the Acurite performance. I believe it's a strong possibility sensor is being damaged in this process. 
 
And for many this issue may never arise like arid regions and much of the western USA when its hot it's normally dry no problem found.
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Offline openvista

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Last update on the Acurite sensor passive in shade still running 8% lower humidity. 
Acurite:82-45%
Davis 82-53%

I agree with the above on price point but for anyone living in high heat and high humidity regions getting heat indexes in excess of 120° when it's actually much lower doesn't matter what the price point without reliability.

This is really worth repeating. We're not talking about a small discrepancy here. No one is quibbling that the sensor runs +/- 3 or 4% at times rather than 2%. It's WAY out of spec in certain ranges. I've seen errors approaching 9% with one of my sensors that's less than 2 years old.

You add in Davis' careless handling of the sensor and this would seem to be a preventable problem, not one that exists because of its price point.
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Online johnd

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I don't believe the issue is whether Davis is longer than the other short straws in its segment. It's that they market the station with tight specs.

OK that's fine - by all means call Davis out for specs that aren't accurate - I don't have a problem with that at all and I genuinely wish you success in persuading Davis to improve their product or revise their specs. That would be an excellent result and, to be clear, I couldn't be more in favour of accurate specs.

But in reality I suspect that the only likely outcome is that the RH specs could get downgraded and the product will stay exactly the same and at the same price and users will still have exactly the same buying decision to make over price/performance - it's just that the RH specs in the background will be slightly different.

A few other random comments, since I've intruded on the thread this far:

* What actually is the Sensirion spec for RH drift over time eg 2 years with the SHT31?

* I can totally understand your concern with the RH spec in hot/humid parts of the world. Here in the UK and probably much of the world north of say 45N or something it's really a non-problem. Of course it's good to have data as accurate as possible - not suggesting otherwise for a moment, but here few users pay too much daily attention to RH or dew point. OTOH we feel about rainfall inaccuracy probably much as you feel about RH inaccuracy and that's our #1 issue with the VP2.

* The whole specs issue on these mid-price stations is a minefield. Davis do provide quite a bit more detailed information than some competitors, but it's still not good enough. Everyone pretty much quotes intrinsic sensor specs and not operational specs that take into account things like radiation shield performance, exposure, wind speed vs wind direction, rainfall vs rain rate vs wind strength etc, nor do they typically address expected drift with age. Specifically on temp and RH, Davis have taken the easy option AFAICS of simply requoting the Sensirion spec rather than spend quite a bit of time, effort and money generating their own data-set over eg 2-3 years in a wide variety of climatic conditions. I wish they had done that, but I can see why they haven't.
« Last Edit: July 19, 2018, 02:52:42 PM by johnd »
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Offline openvista

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* What actually is the Sensirion spec for RH drift over time eg 2 years with the SHT31?

Sensirion claims less than 0.25% per year in drift. Therefore, we shouldn't be seeing the kind of issues we are given these sensors having only been for sale for a couple years now.

Davis wouldn't have to spend a lot of resources at this point to begin investigating the problem. They could do what many of us have done and locate stations that they know contain the SHT31 sensor and that also happen to be within, say, 5 miles of an official station that measures humidity/dew point. Their staff meteorologist should be able to identify days and locations with synoptic rather than mesoscale patterns and make meaningful comparisons. Put the results in a spreadsheet. That would provide them at least prima facie evidence. Beyond that they could ask select trusted clients (universities, agricultural networks, weather service offices -- any place that possesses reference equipment) to send them data. They wouldn't even have to tell them why. They could just say they are committed to accuracy in the field and are aggregating data to verify the performance of their instruments... something to that effect.

I can tell you, I've pulled up as many VP2 stations as I could find within a couple hundred miles of my house and I haven't found one yet that doesn't have a significant wet bias compared to known, good NWS stations nearby. That's not a good sign. I've also got a device that's calibrated and guaranteed to be accurate within +/- 2% so I can verify my station's accuracy and the accuracy of stations in town.

I'm north of the 46th parallel. You don't have to get beyond 25C to really notice the issue. I was seeing it yesterday when it was less than 20C outside, with RHs in the 40-50% range. It's just not as obvious (3-5% departure vs 8+% departure when its north of 30C and above about 40% humidity).

The thing is, John, Davis doesn't seem at all interested in the truth. They only seem interested in ducking responsibility. Their public relations department must be on life support because they've handled this in about the worst way imaginable. I highly doubt they'll even bother to change their spec sheet because... ignorance is bliss.
« Last Edit: July 19, 2018, 04:36:46 PM by openvista »
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Online johnd

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I'm north of the 46th parallel. You don't have to get beyond 25C to really notice the issue.

I think I've said pretty much all I have to say here at present. But just to clarify on this one specific point: I wasn't suggesting that any RH shortfall wasn't noticeable hereabouts, it was more that the RH reading, accurate or not, isn't a major priority for our UK users in general. If I had to rank the standard VP2 parameters by importance (ie by the extent to which minor errors trigger support calls)  I'd probably say:

1 Air temperature
2= Rainfall
2= Wind speed
4 Wind Direction
5 Pressure
6 RH/Dew point

I do understand that those in different regions may have other priorities.
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Online johnd

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England itself doesn't have deserts, but the UK et.al. does for sure...in Australia!

Probably best stand back while one of the Aussies answers that one  :grin: .

But actually we're not far off desert here at present over much of England - longest drought since 1976 and maybe further back. Surprisingly enough maybe, we don't get a lot of rain in Eastern England - maybe 20-25"/year but for crops, gardens etc we do rely on it being reasonably distributed throughout the year at roughly 2"/month with no marked seasonal variation. It varies randomly of course, some months might be 1" and others up to 4", but 2"/month is a pretty standard average. Right now though it's been 6-8 weeks with no significant rain and so everywhere is unusually parched. 30% chance of thunderstorms tomorrow though, but back to 30° next week apparently.

Apologies for the OT weather report!
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Offline WheatonRon

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JohnD, you made some excellent points today (as you always do) in this thread, particularly regarding the economics of why a competitor would take on Davis in view of the size of the PWS market—not likely. The thing most troubling to me is Davis is apparently selling products that aren’t up to spec and part of this problem, based on what I read in this thread (that makes it the Gospel, correct?), is a poorly designed cap around the SHT31 sensor and (or) how Davis ships SHT31 replacements to resellers like you and Mr. Wilhour, and eventually to me, the end user, is packaging that does not meet its supplier’s recommendation—that I find very disturbing.

But you are right. There isn’t a good reasonably priced comparable product out there, so we complain, but still buy more Davis gear, and yes, if a VP-3 ever comes out, I probably will buy it too, because the PWS market won’t have any similar toys for me to buy!
« Last Edit: July 19, 2018, 05:39:14 PM by WheatonRon »
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Offline ValentineWeather

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This is very good news Davis is going to take a serious look.
From Davis tech support: 

Hello Randy,

 

This is of serious concern to us and our engineering team is actively looking in to this.  I really appreciate you pushing so hard on this issue.  It’s only going to make us better.

 

I need to give them time to quantify the problem/drift, determine root cause and come up with a solution.

 


Randy

Offline CW2274

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Granted, completely different issues, but I imagine y'all have noticed that WU is at least trying to get things straighten out. With that, just as I and others have predicted, crickets from Davis. Would it be too difficult to acknowledge there's possibly, just possibly, an issue to your devoted clientele?? Throw us a bone, say you're culpable or not. Say something.
Look boys (Davis), you're rapidly losing credibility on this forum, if nothing else but because of your silence. Granted, we're only one tiny segment of your sales, but comparatively speaking, we're a damned educated one and want answers.
Give us something, like at least you're "looking into it".
Balls in your court.....

Offline DaleReid

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I guess I don't live around 96+% humidity much, so I really don't care.

I know Davis stations are expensive, I'm still not upgrading until the 3 comes out or I die, and then my wife will put on on the marker when they do.

But face it, for all the functions they provide, at a premium, there are few alternatives that are integrated and are wireless and last up to 14+ years (like my original  VP) and still be darned close, what do you expect?

Texas Weather has some old stuff that keeps on cranking since they were built very well and tough and their temperature/humidity sensors are pretty close to every thing else.  The only thing that is very consistent and as far as I an tell repeatable but then again I don't have a humidity cabinet, are the Vaisala WXT500 series.  Their rain sensor is a bit weird and they caution that it isn't for research grade observations but I have a (used) 510 and a 520 that cork along putting out temperatures within a 1/2 degree of each other and the humidity is within a percent or two.  The highest I've had was 99% on one and 98.7 % on the other one.  The barometers track within a few 0.001 in pressure as often as I cross check them.  Get out a checkbook for a (new) one in the range of two grand. They are by far the most consistent tracking with themselves or the TWI or the RM Young stuff.  There are some lower teir professional stuff that runs into the two to five grand figures, with a temp/humidity HMP333 from Vaisala as part of that for a couple grand with the goodies on board.

I think that Davis is expensive.  But buying a couple hundred dollar grade school system a few times adds up, too.  My $1200 Rainwise MarkIII is now showing me 100% and when it is lower humidity, it is pretty close to the other stations, all located on my 5 acres of property.

I learned long ago that the rain measurements were a bit of a variable, too.  The (used) tipping buckets that run $300 to $500 new are never equivalent, even though within a few feet of each other on the same surface and height.  And compared to the open collector (without the wind dress around the outside to shield it, which aren't at all the airports, either) there is always a few hundreds of variation.

I guess that while seeing this is one of the most active conversations on this thread, and with some really cool comparisons by some dedicated observers, I can't see faulting Davis for using the best they can get at a competitive price and doing quite well.  If they have packaged or shipped their new replacement sensors incorrectly, then folks like Scaled and Ambient need to complain that their customers have discovered a problem with Davis' packing and shipping, along with notes from the dedicated users here who have done the testing.

But after I read the specs for RM Young, Vaisala and other pro grade devices, they all say there is a known drift per year, and give graphs showing how much it might be.  And they come (new) calibrated with a sticker saying when the next calibration is due, if you can afford it.  The Vaisala WXT510 and 520 I have would run about $500 each to be tuned up.  My Met One ultrasonic wind sensor would be a $150 charge to just ship it in, and then more on top of it.  So when someone tells me that a $20 chip won't stay in calibration for a year or so, I'm not too put out.

I wish they'd say how to calibrate them rather than sending them back, with which screw to turn to adjust the slope and which to adjust the intercept, but then you'd need a closed chamber with a large bath of various salts to jump back and forth between to do it right.  I don't want to have the wife mad at me for spending the equivalent of a brand new Davis or two a year just to get my stuff re-calibrated.

I do note that one of my Vaisala's is no longer in certification (by about a decade) and it is within a few percent most of the time with all the other humidity sniffers I have. Even though it isn't perfect, it remains a stable workhorse for me.

But I'm not a university or have a government grant so I'm thinking anything from Rainwise, to Davis to Texas Weather is very cool to have, heck even the Peet Brothers that I have compare far closer than I thought based on what I paid for them.

Sorry to seem a bit touchy, but you buy a $400 cam corder don't expect to get Spielberg or Stanley Kubrik production quality videos, but the kids and grandkids still look so much better on the HD from the cheap camera compared to the really expensive prosumer stuff.  And the prop blades on airplanes or helicopters still are bent.

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Offline CW2274

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Holly crap! Talk about timing!! :shock:

Offline SnowHiker

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... I'm still not upgrading until the 3 comes out or I die, ...
When/if the 3 comes out I would still wait some time until it has a proven track record. Just look at how a few days or so ago the SHT 31 was being pushed and praised as if it was one of the greatest things ever by the same people who are complaining most vocally now.  Even if the 3 comes out, why upgrade if the equipment you have is still working fine?  I'm not one who sees every change as being a must-have upgrade.

I guess I don't live around 96+% humidity much, so I really don't care.
My view may also be "fogged" by my cool, dry climate.

Offline Intheswamp

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Hmmm.  Ok, seems the Davis implementation of the Sht31 sensor isn't optimal and has it's deficiencies.  Rather disappointing to find out.  :-|  As I've mentioned elsewhere, I'm the king of procrastinators and have a 2-year-old Sht31 sensor still in the bubble wrap that I intended to install soon.  Now I'm wondering if I should just wait and see what Davis comes up with.  Over the last year or so I've noticed the heat indexes have "seemed" elevated more than I recalled them being when the station was newer.

Yesterday I measured (old circa 2013 sensor still in place) a high temperature of 91.5F, looking at a graph the humidity appeared to be 55%...this calculates to a heat index of ~100F.  My Davis station reported a heat index of 111.4F.  So, even going with the discredited SHT31 assembly, I suppose I should see "some" improvement regarding these numbers...??? 

My station is now a touch over five years old, so I guess it's expected that the temp/humidity sensor is questionable.  I'm one of those living in a high-temperature/high humidity area as are most folks in the southeast part of the USA where the heat index can be fairly important.

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Offline graculus

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This is very good news Davis is going to take a serious look.
From Davis tech support: 

Hello Randy,

 

This is of serious concern to us and our engineering team is actively looking in to this.  I really appreciate you pushing so hard on this issue.  It’s only going to make us better.

 

I need to give them time to quantify the problem/drift, determine root cause and come up with a solution.

 


Strange that in all the years Davis have been using Sensiron they would not have seen this problem with the stations they have on test at Hayward and out in the field  :?

Offline jgentry

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Hmmm.  Ok, seems the Davis implementation of the Sht31 sensor isn't optimal and has it's deficiencies.  Rather disappointing to find out.  :-|  As I've mentioned elsewhere, I'm the king of procrastinators and have a 2-year-old Sht31 sensor still in the bubble wrap that I intended to install soon.  Now I'm wondering if I should just wait and see what Davis comes up with.  Over the last year or so I've noticed the heat indexes have "seemed" elevated more than I recalled them being when the station was newer.

Yesterday I measured (old circa 2013 sensor still in place) a high temperature of 91.5F, looking at a graph the humidity appeared to be 55%...this calculates to a heat index of ~100F.  My Davis station reported a heat index of 111.4F.  So, even going with the discredited SHT31 assembly, I suppose I should see "some" improvement regarding these numbers...??? 

My station is now a touch over five years old, so I guess it's expected that the temp/humidity sensor is questionable.  I'm one of those living in a high-temperature/high humidity area as are most folks in the southeast part of the USA where the heat index can be fairly important.

If I were you, I’ll go ahead and replace it with your new sensor but replace the filter cap to Sensirion’s SF2 cap
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Offline ValentineWeather

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Strange that in all the years Davis have been using Sensiron they would not have seen this problem with the stations they have on test at Hayward and out in the field  :?
 

Maybe they just don't look, these aren't necessarily all weather geeks like us watching closely especially me and many on this forum do.
They were running about +3 DP last time I looked over airport and currently same temperature and + 10% humidity from nearby airport. Really classic example without the high heat. I really don't know exactly where both are located other than Davis is near the water.

Put this in high heat index content and they may have noticed.
Randy

Offline SLOweather

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I don't recall if all of this has been mentioned... Sensirion has some specific info in their literature and datasheets that is pertinent.

From the datasheet for my analog SHT3x

Quote
The sensor shows best performance when operated within recommended normal temperature and humidity range of 5 –
60 °C and 20 – 80 %RH, respectively. Long term exposure to conditions outside normal range, especially at high humidity,
may temporarily offset the RH signal (e.g. +3%RH after 60h at >80%RH). After returning into the normal temperature and
humidity range the sensor will slowly come back to calibration state by itself. Prolonged exposure to extreme conditions
may accelerate ageing. To ensure stable operation of the humidity sensor, the conditions described in the document
“SHTxx Assembly of SMD Packages”, section “Storage and Handling Instructions” regarding exposure to volatile organic
compounds have to be met. Please note as well that this does apply not only to transportation and manufacturing, but also
to operation of the SHT3x-ARP.

From their handling instructions PDF:

Quote
For exposure to extreme conditions with regards to humidity and temperature please consult the datasheet of respective product. Please make sure that exposure time of the sensor to maximum range of operating conditions is limited as given in datasheet. Exposure to volatile organic compounds at high concentration and long exposure time is critical not only in assembly but also in the field. Such application needs to be carefully tested and qualified.

From the same PDF re: reconditioning:

Quote
Reconditioning Procedure
As stated above extreme conditions or exposure to solvent vapors may offset the sensor. The following reconditioning procedure may bring the sensor back to calibration state:
Baking: 100 – 105°C at < 5%RH for 10h
Re-Hydration: 20 – 30°C at ~ 75%RH for 12h 1.

Their storage, handling, mounting, and cleaning instructions are also sufficiently strict that it's likely that not everything is is being adhered to during manufacturing. 

Offline dendrite

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Strange that in all the years Davis have been using Sensiron they would not have seen this problem with the stations they have on test at Hayward and out in the field  :?
 
Maybe they just don't look, these aren't necessarily all weather geeks like us watching closely especially me and many on this forum do.
They were running about +3 DP last time I looked over airport and currently same temperature and + 10% humidity from nearby airport. Really classic example without the high heat. I really don't know exactly where both are located other than Davis is near the water.

Put this in high heat index content and they may have noticed.
Here's the Gladstone for KHWD.

https://weather.gladstonefamily.net/site/KHWD

If you zoom out on the map you will see the cluster of about 5 stations to the SSW that are all at Davis HQ. It may be interesting to compare all of those. Of course they probably have some at 2m and some up on their roof.

Offline ValentineWeather

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I don't recall if all of this has been mentioned... Sensirion has some specific info in their literature and datasheets that is pertinent.

From the datasheet for my analog SHT3x


From their handling instructions PDF:

Quote
For exposure to extreme conditions with regards to humidity and temperature please consult the datasheet of respective product. Please make sure that exposure time of the sensor to maximum range of operating conditions is limited as given in datasheet. Exposure to volatile organic compounds at high concentration and long exposure time is critical not only in assembly but also in the field. Such application needs to be carefully tested and qualified.



Think it has been but worth mentioning again. The assembly may be key, I just don't know how they do it and not expose chip to bad stuff when molded onto board.

Fingers crossed they find issue.   
Randy

Offline dendrite

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For some reason only E4718 is showing on the map for me now. Anyway...

Gladstone has the avg daytime error for HWD running at -1.5F. The Davis E4718 station is +1.9F. Maybe Davis experiences more marine taint? Just throwing out possibilities. Of course it could simply just be 3% too high too. ;)

Offline ValentineWeather

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My last Acurite check early this morning max humidity at 97%, Davis 96% aspirated so both looking normal.

At 12:40 CT I did a passive shade check against new Davis sensor with 5mph breeze running 7-9% lower humidity so really nothing changed from yesterday.

Not keeping acurite saturated in high humidity outside 24/7 so can't really determine how much drift if any is taking place toward wet side.  Max exposure time has been around 2 hours in early morning allowing it to record highest humidity.
Randy

Offline jgentry

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My last Acurite check early this morning max humidity at 97%, Davis 96% aspirated so both looking normal.

At 12:40 CT I did a passive shade check against new Davis sensor with 5mph breeze running 7-9% lower humidity so really nothing changed from yesterday.

Not keeping acurite saturated in high humidity outside 24/7 so can't really determine how much drift if any is taking place toward wet side.  Max exposure time has been around 2 hours in early morning allowing it to record highest humidity.

Im guessing you’re like me. DPs are 3-4 degrees off being wet in bias
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Offline ValentineWeather

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My last Acurite check early this morning max humidity at 97%, Davis 96% aspirated so both looking normal.

At 12:40 CT I did a passive shade check against new Davis sensor with 5mph breeze running 7-9% lower humidity so really nothing changed from yesterday.

Not keeping acurite saturated in high humidity outside 24/7 so can't really determine how much drift if any is taking place toward wet side.  Max exposure time has been around 2 hours in early morning allowing it to record highest humidity.

Im guessing you’re like me. DPs are 3-4 degrees off being wet in bias

+4° dp when I did the afternoon check.  Thread has went about as far as it can go unless anyone has something we didn't cover. Let Davis do their thing and see what they come up with.
Randy

 

anything