Topic: Davis 6323 FASA RH accuracy?

[Ken7]

I'm just wondering how common an 'issue' this is...or is it an issue? My 6323 FASA is about 6 months old and recently, thanks to PMs exchanged with Openvista (who's been a great help), led me to a NWS site I hadn't seen. Openvista also suggested the Drop 2 to help with calibration concerns. So with this in mind, I'm wondering if the Davis has an issue with RH/DP in the mid-ranges.

In looking at the 2 relatively close area airports (relatively small ones, one about 12 miles east and another about the same distance west), both show DPs about 4-5 F lower with RH levels about 10-12% lower in the 55-70% region. Yet virtually all the PWS stations near me that I trust, as well as the APRSWXNET/CWOP, show readings very close to my Davis. The agreeing PWS units cut across a bunch of different manufacturers, including other Davis VPs, VP2s, Vues & one Rainwise as well as the non-FAA stations on the NWS site.

It's hard to believe that all these other stations, including mine, are wrong to this degree, with so many showing the same 'error' with RH levels 10-12% higher and DPs 4-5F higher than the 2 area FAA locations. I should mention that there is nothing different in terms of topography or proximity to bodies of water that would affect any of these locations to this degree. I see this delta from 'dry wind directions' too and relatively high wind speeds. Yet once we get in to higher moisture levels where RH can reach into the 90s, there is generally good agreement with all sites, FAA included.

What adds a bit to my concern though, is my Drop 2 tends to be in much closer agreement with the 2 FAA airport locations than my Davis (at least in this region of RH). So are all these other stations running wetter than the FAA sites to this degree? If so, 'Yikes!'.

[openvista]

Appreciate the kind remarks, Ken.

Just to clarify, in case anyone is confused, Ken7 has a Davis 24-hour fan-aspirated VP2 with the SHT31 sensor suite inside.

In answer to the question is everyone else wrong on dew point, the answer would be a qualified "yes". Other stations may be accurate for their particular siting which may or may not have readily apparent additional sources of moisture. However, that's the exception, not the rule. The airports, generally, have the best equipment, siting and maintenance. 

Mostly, what Ken's describing is due to sensors growing wetter over time.

Here's an excerpt from an article entitled Evolution and Accuracy of Surface Humidity Reports in the Journal of Atmospheric and Oceanic Technology published September 2013:

Capacitive sensor drift

Visscher and Kornet (1994) found that all but one of
the capacitive sensors they investigated drifted higher,
mostly by 1%–2% RH yr-1. Our experience (see section
3) is broadly similar. Richardson et al. (2000) found that
one capacitive sensor was 4% RH too high before their
intensive observation period and 6% RH too high af-
terward. The Vaisala and Rotronic capacitive sensors
tested by Skaar et al. (1989) drifted up by between 0.3%
and 1.9% RH over a year. Burt (2012) suggests that
for the worst systems, drift can exceed 5% RH yr.
Lacombe et al. (2011) found little or no drift, probably
related to the drier conditions in their trial. The drift to
higher values is likely to be related to episodes when
the sensor is saturated (excess water condensing on the
sensor)

Source: https://journals.ametsoc.org/doi/pdf/10.1175/JTECH-D-12-00232.1

So, there you have it from the experts. I would also draw attention to the posts from kcidwx (NWS tech who does maintenance on ASOS/AWOS stations) in the SHT31 reliability thread in which he echoed those findings. That's why he said it was imperative to replace capacitive humidity sensors regularly if you care to stay within the sensor specifications. For most, this is a non-issue and they only replace sensors when something breaks. Sometimes not even then. Depends on your priorities.

[CW2274]

he said it was imperative to replace capacitive humidity sensors regularly if you care to stay within the sensor specifications.

That's all fine and dandy, IF we could get one that's not more jacked than the one you're trying to replace. I've gone through four additional 31's and they were all worse than two 2+ year old ones I already have.

[Ken7]

So at least in my case, as well as the other PWS in my area as well as the APRSWXNET/CWOP sites, the error is even greater than the errors reported in the Evolution and Accuracy of Surface Humidity Reports that openvista provided!

As I mentioned, at least I can testify in terms of my unit's age, it's only about 6 months old. So here we're dealing with a 24/7 Davis FASA 6323 that's not even 1 year old. Yet if we are to trust the two area airports (and I certainly agree with openvista's assertion that they are better units with better maintenance), my error is far in excess of even the highest stated error of 6% in the extract from Evolution and Accuracy of Surface Humidity Reports. It would be one thing to feel fairly confident that this high RH error of 12% is just my unit, but good grief, almost all my surrounding PWS as well as most of the APRSWXNET/CWOP sites, were showing this same gross error, with wetter RHs of 9% or greater.

This is certainly not a minor error. It's driving me nuts.

[Old Tele man]

Now you know WHY it's called WEATHER GUESSING (wink,wink)!

[openvista]

If you look closely at the excerpt I posted above, you'll notice they say things like "1%–2% RH yr-1". Just so everyone's on the same page "yr-1" means per year (it should be "yr" to the -1 power, but the formatting got busted in the copy/paste). In other words, various capacitive humidity sensors became 1-2% wetter in absolute terms per year according to their study. Stephen Burt found it's more like 5% *per year* for the worst sensors. So, no, it's not at all crazy what you are seeing. Also notice the reference in that quote above to another study where one sensor started +4% from reality and drifted to become +6% just in the time it took to complete the study.

People (and I'm including non-life-essential organizations here) routinely leave their humidity sensors in place many, many years without so much as a 2nd thought. Most sensors aren't calibrated in the first place. The station owner, with few exceptions, has no reliable device to know either way so the stations are simply placed online. If they bother at all, most do what you initially did. They compare to other stations near them and conclude: "I'm in agreement." That's also the magic behind the "QC" processes of MesoWest and Gladstone. They sometimes flag the airport as wrong (seen it many times) because the bulk of their database is PWSs. Garbage in, garbage out.

I would expect the average departure for your surrounding stations will be less than +12% as the winter progresses, unless you have some severe outliers nearby. Who knows. If you haven't already, you might want to create a spreadsheet for tracking purposes.

[Old Tele man]

Yep...it's called "SET IT and FORGET IT" installations.

+1 on that "...you might want to create a spreadsheet for tracking purposes."

“When you can measure what you are speaking about, and express it in numbers, you know something about it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts advanced to the stage of science.” [ Lord Kelvin, May 1883 ]

[Ken7]

And what makes it worse, is that since the error is non-linear, calibration is out of the question...unless your software allows multi-point calibration. Very depressing.

BTW, we're currently experiencing temps in the upper 30s and the errors are still about 9% (me and some other PWS-69%, airports-60%). So with my unit being relatively new (6 months), it should be lots of fun to see what happens after a year or two.

Now, let's see, how much is that Rainwise unit?

[CW2274]

the errors are still about 9% (me and some other PWS-69%, airports-60%).

To more truly compare, you should pay more attention to the dew points. 

[Old Tele man]

the errors are still about 9% (me and some other PWS-69%, airports-60%).

To more truly compare, you should pay more attention to the dew points.

True, if one is attempting to back-solve what their RH values should be...BUT...not so, if one is comparing "apples-to-apples" data, because dew point is derived (calculated) from RH and Temp. Thus, for our home stations, dew point is a derived--not measured--value:  Tdp from T and RH.

[JCA433]

I may start replacing my temperature and humidity sensor once a month maybe twice a month.

[Old Tele man]

I may start replacing my temperature and humidity sensor once a month maybe twice a month.

Uh, that could get expensive after awhile.

[JCA433]

The average humidity here is 83 percent.

[CW2274]

the errors are still about 9% (me and some other PWS-69%, airports-60%).

To more truly compare, you should pay more attention to the dew points.

True, if one is attempting to back-solve what their RH values should be...BUT...not so, if one is comparing "apples-to-apples" data, because dew point is derived (calculated) from RH and Temp. Thus, for our home stations, dew point is a derived--not measured--value:  Tdp from T and RH.

True, but the extrapolation calculates the same. I think using the dew is a better form of gauging accuracy without the need of dealing with the temp thrown in as well gumming things up.

[Ken7]

To more truly compare, you should pay more attention to the dew points.

Yeah, been there done that. DP spread is generally in the area of 5F. Yet looking at the NWS map, non-FAA stations as well as the more ‘trusted’ PWS are often the same as mine or with a 1 degree delta.

I find it bizarre that almost all stations have these similar, very large, RH/DP errors relative to the 2 FAA sites. Only the FAA stations show significantly drier DPs & RH. You would think that errors among the PWS and non-FAA sites on the NWS map would be all over the place, but they tend to be more in agreement (regardless of manufacturer) than anything else. Of course there are several outliers, but I’ve always ignored those anyway. 

I fully understand the better equipment & maintenance of the FAA stations, but the above facts still have me scratching my head. Random errors, both wetter and drier, that cut across PWS manufacturers and differing equipment I can understand, but when so many reporting stations show similar, always wetter values, it’s just weird.

[Old Tele man]

To more truly compare, you should pay more attention to the dew points.

Yeah, been there done that. DP spread is generally in the area of 5F. Yet looking at the NWS map, non-FAA stations as well as the more ‘trusted’ PWS are often the same as mine or with a 1 degree delta.

I find it bizarre that almost all stations have these similar, very large, RH/DP errors relative to the 2 FAA sites. Only the FAA stations show significantly drier DPs & RH. You would think that errors among the PWS and non-FAA sites on the NWS map would be all over the place, but they tend to be more in agreement (regardless of manufacturer) than anything else. Of course there are several outliers, but I’ve always ignored those anyway. 

I fully understand the better equipment & maintenance of the FAA stations, but the above facts still have me scratching my head. Random errors, both wetter and drier, that cut across PWS manufacturers and differing equipment I can understand, but when so many reporting stations show similar, always wetter values, it’s just weird.

Could it be (wink,wink) that the error similarities are due to everybody using similar type polymer capacitive RH sensors?

[JCA433]

To accurately measure outdoor humidity a true professional humidity sensor is needed.  The Davis temperature sensor however does perform well measuring temperature and temperature is way more important than humidity.   Temperature is in fact the most important thermodynamic variable because molecular internal energy increases with increasing temperature and enthalpy is defined as internal energy plus pressure multiplied by volume.

[CW2274]

temperature is way more important than humidity.   

 

[JCA433]

I did not mean to imply humidity is not important.  It is very  important. 

[openvista]

I hear ya, Ken. Having spent an exceeding amount of time looking at this issue across the country, I can report that what you’re seeing is not all that unusual. It’s to your credit that you remain skeptical, as every science-minded person should, until having researched it to your satisfaction.

[JCA433]

In my location the humidity is normally very high but yesterday during the afternoon the humidity was in the 30 to 40 percent range.  The Davis humidity sensor was much higher than the nearest NWS station and the Dew point temperatures were about 3 to 5 degrees F higher.   Most of the weather stations on Weather Underground in my area showed similar wet bias.



For most of the PWS the temperature and humidity are measured and used to calculate the dew point temperature.  It would be nice if we could measure temperature and dew point temperature and then use the temperature and dew point temperature to calculate the humidity.

[Ken7]

Could it be (wink,wink) that the error similarities are due to everybody using similar type polymer capacitive RH sensors?

Absolutely. It shows you how meaningless the accuracy specs are. Unjustifiably or not, I get a bit skeptical when a manufacturer simply states an accuracy of +/-x without stating a range for that accuracy. One is then left to assume the errors are perfectly linear. Linear errors across the measurement range has been a rarity in my experience.

Even if you get certification, do you ‘really’ know that certification occurred? Ugh.

[Ken7]

I hear ya, Ken. Having spent an exceeding amount of time looking at this issue across the country, I can report that what you’re seeing is not all that unusual. It’s to your credit that you remain skeptical, as every science-minded person should, until having researched it to your satisfaction.

Yup. I’m beginning to see why you suggested taking the Drop (I exchanged the first one) to the airport. At this point I’m betting the Drop’s readings will be very similar to the airport’s readings.

[Ken7]

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.

[openvista]

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.

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.