Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?

[dendrite]

69.1°F with 98% RH already and clear skies at 9pm EDT. This should be a good night to see if the new SHT75 can hit 99% or 100%.

[jgentry]

69.1°F with 98% RH already and clear skies at 9pm EDT. This should be a good night to see if the new SHT75 can hit 99% or 100%.

I do know that Jerry has hit 99% with his 75

Noticed that one of the Airport stations has already reached 100%. I’m sure that’s due to the calculation made from the dewpoint sensor.

But I’ll be watching your station tonight

[jgentry]

69.1°F with 98% RH already and clear skies at 9pm EDT. This should be a good night to see if the new SHT75 can hit 99% or 100%.

Sounds like some really FROGGY weather...you down in the Louisiana bayous?

He is from New Hamphshire. Here is his station...

https://mesowest.utah.edu/cgi-bin/droman/meso_table_mesowest_m.cgi?stn=C7324&unit=0&time=&hours=24&hour1=01&day1=05&month1=08&year1=2018&radius=25&past=0&order=1

[dendrite]

With the humidity we've had this summer it has certainly felt more like LA than NH.

We've had about 7" of rain the past 2 weeks. The water table is high (despite D1 drought status) and we have mushrooms growing everywhere. When the wind is under 3mph, the moisture builds up and pools near the sfc. I see quick temp drops and dewpoint rises with the 75 and FARS when this occurs. When the wind picks up (5-10mph+) the slightly drier air mixes down and the T/Td respond in the other direction. We usually have dew forming on the lawn 2-3hrs before sunset when we get into these wet, stagnant airmasses.

[CW7491]

Well, as much as I hate to say this, looks like I'll be jumping on the "bash the 31" wet bias bandwagon. After weeks of scrutinizing even more than usual, I've come to the conclusion that I'm suffering the same symptoms, maybe just not quite as bad as everyone else. As a generality, I'm running 3-4F in dew higher than my "official buddies". The thing is, neighboring stations tend to run wet also, so makes me dubious, but yes, most are Davis's. My 11 and 15 behaved the same way, which also makes me wonder. I guess I'll dial back the humidity 2% and see how that looks for now. Crap...

Welcome aboard and about time.
I can confirm Davis has made shipping changes but I'm skeptical that's the issue. Haven't figured out yet because the Acurite SHT31 version doesn't display the same high bias.  I'm also testing on my NEW just built 2 days ago outdoor shaded porch and it's running close to 4-5% lower humidity all the time except in the morning dew when they all come together around 97% today.  It's not the LCC version however and isn't embedded into plastic like the Davis either.

I’m seeing the same thing with the Acurite SHT31. It’s really very accurate from what I’m seeing.  The humidity doesn’t seem to have any bias. 4-5% lower than the Davis 31 outside the high humidity range is about what I’m seeing and that’s right on with the psychro dyne. I’m not sure what Acurite has figured out that Davis hasn’t. It’s frustrating and promising all at the same time.

[CW2274]

Well, as much as I hate to say this, looks like I'll be jumping on the "bash the 31" wet bias bandwagon. After weeks of scrutinizing even more than usual, I've come to the conclusion that I'm suffering the same symptoms, maybe just not quite as bad as everyone else. As a generality, I'm running 3-4F in dew higher than my "official buddies". The thing is, neighboring stations tend to run wet also, so makes me dubious, but yes, most are Davis's. My 11 and 15 behaved the same way, which also makes me wonder. I guess I'll dial back the humidity 2% and see how that looks for now. Crap...

Welcome aboard and about time.
I can confirm Davis has made shipping changes but I'm skeptical that's the issue. Haven't figured out yet because the Acurite SHT31 version doesn't display the same high bias.  I'm also testing on my NEW just built 2 days ago outdoor shaded porch and it's running close to 4-5% lower humidity all the time except in the morning dew when they all come together around 97% today.  It's not the LCC version however and isn't embedded into plastic like the Davis either.

It’s frustrating and promising all at the same time.

Yes.
The frustrating part is how long will the frustrating part last?

[dendrite]

Well we did it. We had a bit of fog this morning, but nothing too thick. You could see a hint of blue sky through it at sunrise. The 75 showed 99% RH beginning around midnight and about a half hour ago it started bouncing around between 99% and the elusive 100%. It seems to have conveniently missed the 5 min Mesowest obs though so you won’t see it there. It’s fluctuating between 98% and 99% now.

[dendrite]

Here's my 75 before I sealed it with the silicone conformal coating.

 [ You are not allowed to view attachments ]

[ValentineWeather]

Here's my 75 before I sealed it with the silicone conformal coating.

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So are there any how to make these 75's with clear precise instructions?. I've tried to follow but usually give up. 

[dendrite]

Here's my 75 before I sealed it with the silicone conformal coating.

 [ You are not allowed to view attachments ]

So are there any how to make these 75's with clear precise instructions?. I've tried to follow but usually give up. 

I just cut the sensor off of an old Davis temp/hum. You can see how the wiring goes in my pic. The connector I used required no soldering although you could get a connector with pins that you could solder the wires to. I just make a clean snip of the wires and insert them into the connector and then gently close it to make the connection. The 75 already comes with the 10K pull up resistor between pins 1 and 4 so you don’t need to worry about that. I sealed it with coating from that little bridge to the sensor down to the connector just to be on the safe side. The Davis cord is 6 wires, but the red and black don’t get used so you can just cut them away.

[Bobvelle]

OK, I like this the best.

Hot glue on three sides. Yeah, three sides is probably overkill, 2 sides is plenty, but I had the gun warmed-up and was on a role 

The melted area on the SF2 top edge was not from the hot glue. It was a failed attempt to melt a stainless wire to it (NOT while on the sensor board) Obviously it did not work. The material is not a typical plastic.
Anyway , just thought I'd share.

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[ValentineWeather]

Here's my 75 before I sealed it with the silicone conformal coating.

 [ You are not allowed to view attachments ]

So are there any how to make these 75's with clear precise instructions?. I've tried to follow but usually give up. 

I just cut the sensor off of an old Davis temp/hum. You can see how the wiring goes in my pic. The connector I used required no soldering although you could get a connector with pins that you could solder the wires to. I just make a clean snip of the wires and insert them into the connector and then gently close it to make the connection. The 75 already comes with the 10K pull up resistor between pins 1 and 4 so you don’t need to worry about that. I sealed it with coating from that little bridge to the sensor down to the connector just to be on the safe side. The Davis cord is 6 wires, but the red and black don’t get used so you can just cut them away.

Okay that seems simple enough.
Do you have a link to connector, I'll use one of my old sensor cables.

[dendrite]

http://www.newark.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&langId=-1&urlRequestType=Base&partNumber=53X2803&storeId=10194

I'd get a few. As openvista and I realized, they can be easy to break when you're first getting used to using them.

[ValentineWeather]

http://www.newark.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&langId=-1&urlRequestType=Base&partNumber=53X2803&storeId=10194

I'd get a few. As openvista and I realized, they can be easy to break when you're first getting used to using them.

Thanks I'll make up a couple and have ready for next summer for sure. I plan on continuing with the 31's this winter but may change mind if I find the 75's accurate enough as temperatures drop into -20 range. The 15's aren't great once temps go sub zero and specs show about the same is what worries me.

[jgentry]

http://www.newark.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&langId=-1&urlRequestType=Base&partNumber=53X2803&storeId=10194

I'd get a few. As openvista and I realized, they can be easy to break when you're first getting used to using them.

Thanks I'll make up a couple and have ready for next summer for sure. I plan on continuing with the 31's this winter but may change mind if I find the 75's accurate enough as temperatures drop into -20 range. The 15's aren't great once temps go sub zero and specs show about the same is what worries me.

Do a test of the 75 for this winter and see how it does and let us know.

[dendrite]

I never noticed an issue with my old 75 in near 0F temps during the winter I used it. Unfortunately we never had temps frigid enough (-15F or colder) to really test it out.

[ValentineWeather]

I never noticed an issue with my old 75 in near 0F temps during the winter I used it. Unfortunately we never had temps frigid enough (-15F or colder) to really test it out.

What were you comparing too? Only way to know is have a side by side comparison. You can't trust others around you, I can't even trust the ASOS which started running +2° warm March 2017 Madis even picked the +2° jump.
Our city council members have taken notice finally after my persistence and some including me are pushing to have KVTN ASOS removed off of city property.  It's actually dangerous we had multiple icing instances last winter and ASOS never reported ice formation because of warm bias.

[dendrite]

I never noticed an issue with my old 75 in near 0F temps during the winter I used it. Unfortunately we never had temps frigid enough (-15F or colder) to really test it out.

What were you comparing too? Only way to know is have a side by side comparison. You can't trust others around you, I can't even trust the ASOS which started running +2° warm March 2017 Madis even picked the +2° jump.
Our city council members have taken notice finally after my persistence and some including me are pushing to have KVTN ASOS removed off of city property.  It's actually dangerous we had multiple icing instances last winter and ASOS never reported ice formation because of warm bias.

I had a 6470 probe from my soil station in my old 7714 passive shield from my WMII days. It only read whole numbers, but it was usually +/- 1F on those winter mornings. And yeah, the normal biases that you get used to with local stations on a day-to-day basis were still on par on those cold mornings. I have nothing scientific to give you though...just empirical. CW7491 had it testing accurately at -8F IIRC.

[ValentineWeather]

I'll definitely test it for everyone this winter in one of the shields. -26° was coldest last January, -24° at airport with +bias. 
Year before we were down to -32°F in town and station just east town now annexed the SHT31 recorded -36°.
I worry the sht75 may not be accurate at those temperatures. 
 

[CW2274]

I never noticed an issue with my old 75 in near 0F temps during the winter I used it. Unfortunately we never had temps frigid enough (-15F or colder) to really test it out.

Our city council members have taken notice finally after my persistence and some including me are pushing to have KVTN ASOS removed off of city property.  It's actually dangerous we had multiple icing instances last winter and ASOS never reported ice formation because of warm bias.

The primary purpose of any ASOS is obviously to serve those who fly in and out of the associated airport. I assume by moving it off city property will take it well off the airport proper, no?
Not saying there isn't a problem, but the bottom line is my old employer will have the final say of where and even if it gets moved. And if it does, it certainly won't be off the airport proper.

[ValentineWeather]

Privately owned airport. Not sure who has the right to do what but if it was my property it would be gone.

[CW2274]

Privately owned airport. Not sure who has the right to do what but if it was my property it would be gone.

Well, I guess pilots could just rely on a windsock...

[dendrite]

Doesn't the ASOS freezing rain sensor work independently from the other sensors? If it's sensing icing, it should report it whether the temp is 32F or 34F. There's probably some algorithm that overrides it once you get above 35F though.

NDOT has a whole arsenal of roadside weather stations. I would think they have road surface temperature and condition sensors (wet, icing, dry, etc) along with ptype.

[ValentineWeather]

Privately owned airport. Not sure who has the right to do what but if it was my property it would be gone.

Well, I guess pilots could just rely on a windsock...

Why not? This airport ASOS currently has instruments giving false data, I consider more dangerous than a simple windsock.  A council member who also runs the weather coop is fed up too. We've been discussing this for a year now.
If it was up to me it would be gone already especially after last winter with multiple icing conditions missed by ASOS.
 
They know about it, both of us have reported the issue multiple times. It's ongoing they ignore it so next step is getting city involved.

Not sure about the freezing rain sensor if it does it failed too.

[CW2274]

Privately owned airport. Not sure who has the right to do what but if it was my property it would be gone.

Well, I guess pilots could just rely on a windsock...

It's ongoing they ignore it so next step is getting city involved.

Who's they? I know the city owns the airport, now the question is who maintains the ASOS? If it's the city, then you're obviously on the right track.
If I were you and you're still not getting satisfaction, I'd give these guys a call and tell them you're concerns. If they can't help, they should be able to tell you who can. GL!
https://www.faa.gov/about/office_org/field_offices/fsdo/?state=NE