Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 58918 times)

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Offline jgentry

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I have the 75 almost ready to go. Here it is next to a Davis sensor for size comparison.

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Oh wow! You didnít have to solder right?  How are you going to protect the exposed wires?
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline jgentry

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Ran a little experiment over night and today, i put a sensor in a damp wash cloth an let it sit over night showing 98%. This am i removed it from the cloth and put it in a fars to see how it would respond, well it was running around 2% high to start and as the day went on the humidity on main station was running around 39 to 45% but the test sensor was running between 5 to 7% higher. So i removed the sensor and baked it for several hours and the error corrected itself and after awhile was checking with the other station. So what they say about prolonged exposure to high humidity sure showed up and with a fan running at night in high humidity you can image the error that shows up until it dries out. One thing it does correct itself but it takes awhile and your reported readings are going to be really screwed up for awhile. I had noticed for a long time that the humidity reading was slow to come down in the am after a humid night, sometimes taking two to three hours to get dried out.


This is with the 15, correct?
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Offline ValentineWeather

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Ran a little experiment over night and today, i put a sensor in a damp wash cloth an let it sit over night showing 98%. This am i removed it from the cloth and put it in a fars to see how it would respond, well it was running around 2% high to start and as the day went on the humidity on main station was running around 39 to 45% but the test sensor was running between 5 to 7% higher. So i removed the sensor and baked it for several hours and the error corrected itself and after awhile was checking with the other station. So what they say about prolonged exposure to high humidity sure showed up and with a fan running at night in high humidity you can image the error that shows up until it dries out. One thing it does correct itself but it takes awhile and your reported readings are going to be really screwed up for awhile. I had noticed for a long time that the humidity reading was slow to come down in the am after a humid night, sometimes taking two to three hours to get dried out.

Interesting Jerry.
I wonder if aspiration makes it worse? You just used a washcloth so maybe not.

Even in the written documentation it says sensor will need to be reset sometimes if prolonged to wet +85% conditions.
It's just not the right sensor for wet applications is our number 1 issue IMO.
Randy

Offline CW2274

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So what they say about prolonged exposure to high humidity sure showed up and with a fan running at night in high humidity you can image the error that shows up until it dries out.
I'm still trying to wrap my head around how a fan could possibly keep a sensor "wetter" than a passive shield. Evaporation or saturation is merely expedited by the fan, how could that not be desirable??

Offline jerryg

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I used an 11 that i have and i don't know why things work the way they do i just know in a side by side comparison the fars takes longer to dry out than the standard shield. I think the fars with the big chamber it has has a lot more area to retain condensed moisture than the standard shield with the sensor located at the bottom so is more readily exposed to the drier air with all the plates being open to the air. I took a fars apart early in the am while still high humidity and the inside of the sensor chamber was really damp with beads of moisture and it takes awhile for the fan to pull enough dry air through the chambers and tubes to dry everything out so no false readings by the sensor. Just my thoughts on the matter, i am no expert on this stuff but i believe what i see with my own eyes.

Offline CW2274

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I think the fars with the big chamber it has has a lot more area to retain condensed moisture than the standard shield with the sensor located at the bottom so is more readily exposed to the drier air with all the plates being open to the air.
I don't see it. All a fan does is take ambient air and draw it across the sensor, evaporating or saturating certainly more efficient than any passive shield.
Not saying your observations are wrong, but my physics don't see your physics. :???:
« Last Edit: July 27, 2018, 08:54:54 PM by CW2274 »

Offline dendrite

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I have the 75 almost ready to go. Here it is next to a Davis sensor for size comparison.

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Oh wow! You didn’t have to solder right?  How are you going to protect the exposed wires?
Which exposed wires? I have some silicone conformal coating I may put on the pin end of the 75 PCB and up to where it meets the connector. Maybe I'll put a little on the wire end of the connector too.

And no, no soldering. It's an IDC connector so slide the wires in all the way and close it up tight. I think the Davis wires are 26 AWG and the connector accepts 26-28 AWG.
« Last Edit: July 27, 2018, 09:35:25 PM by dendrite »

Offline Bobvelle

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I think the fars with the big chamber it has has a lot more area to retain condensed moisture than the standard shield with the sensor located at the bottom so is more readily exposed to the drier air with all the plates being open to the air.
I don't see it. All a fan does is take ambient air and draw it across the sensor, evaporating or saturating certainly more efficient than any passive shield.
Not saying your observations are wrong, but my physics don't see your physics. :???:

Interestingly, only half of the function of the fan is pulling air across the sensor as this 24hr FARS cutaway shows. The most interesting design feature is the 2 layer thermal curtain that surrounds the sensor thoroughly insulating the inner chamber.
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When I built mine from parts. I put the sensor low in the chamber, like this.
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 I don't know if this is how Davis does it when purchased as a complete setup, but its seems to me that being in that area actually keeps the full force of the air from blowing directly on the sensor, while at the same time still 'saturated' in the air pulled in from below.
« Last Edit: July 27, 2018, 10:48:43 PM by Bobvelle »

Offline openvista

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I used an 11 that i have and i don't know why things work the way they do i just know in a side by side comparison the fars takes longer to dry out than the standard shield. I think the fars with the big chamber it has has a lot more area to retain condensed moisture than the standard shield with the sensor located at the bottom so is more readily exposed to the drier air with all the plates being open to the air. I took a fars apart early in the am while still high humidity and the inside of the sensor chamber was really damp with beads of moisture and it takes awhile for the fan to pull enough dry air through the chambers and tubes to dry everything out so no false readings by the sensor. Just my thoughts on the matter, i am no expert on this stuff but i believe what i see with my own eyes.

I get your point about the differing masses of the shields. However, on nights/mornings when the air doesn't reach full saturation (quite common here), an aspirated shield responds quicker to drier air warmed by the morning sun. This is especially true when winds at 2m are below 5mph (most mornings here). For people like me living in cities and small suburban lots that don't get a lot of air movement and/or experience heat island effects (which artificially keeps temp above dew point sometimes), passive shields aren't normally going to read drier than a shield with a higher rate of evaporation.
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Offline CW2274

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I think the fars with the big chamber it has has a lot more area to retain condensed moisture than the standard shield with the sensor located at the bottom so is more readily exposed to the drier air with all the plates being open to the air.
I don't see it. All a fan does is take ambient air and draw it across the sensor, evaporating or saturating certainly more efficient than any passive shield.
Not saying your observations are wrong, but my physics don't see your physics. :???:
but its seems to me that being in that area actually keeps the full force of the air from blowing directly on the sensor, while at the same time still 'saturated' in the air pulled in from below.
I would certainly think the small area in the chamber would negate sensor placement. The filter also keeps the "full force" of the air off the sensor.

Offline jgentry

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I have the 75 almost ready to go. Here it is next to a Davis sensor for size comparison.

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Oh wow! You didnít have to solder right?  How are you going to protect the exposed wires?
Which exposed wires? I have some silicone conformal coating I may put on the pin end of the 75 PCB and up to where it meets the connector. Maybe I'll put a little on the wire end of the connector too.


And no, no soldering. It's an IDC connector so slide the wires in all the way and close it up tight. I think the Davis wires are 26 AWG and the connector accepts 26-28 AWG.

Good deal. Will probably end up doing that.




Between where you spliced the wires and the connector.

Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline Bobvelle

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I think the fars with the big chamber it has has a lot more area to retain condensed moisture than the standard shield with the sensor located at the bottom so is more readily exposed to the drier air with all the plates being open to the air.
I don't see it. All a fan does is take ambient air and draw it across the sensor, evaporating or saturating certainly more efficient than any passive shield.
Not saying your observations are wrong, but my physics don't see your physics. :???:
but its seems to me that being in that area actually keeps the full force of the air from blowing directly on the sensor, while at the same time still 'saturated' in the air pulled in from below.
I would certainly think the small area in the chamber would negate sensor placement. The filter also keeps the "full force" of the air off the sensor.

I disagree. The baffle at the bottom of the chamber produces a small area semi-stagnate air. The sensor can be placed near this area. Above that small area is a turbulence zone ...and near the top by the fan, faster and straighter moving air. Plenty of room for all that to take place.
Also, there would be no sense in adding the SF2 Sensirion filter cap if the air is not directly hitting the sensor. According to you, the cap will block this air.
By the way, my choice of the term 'full force of air' was hyperbole. We all know there is no 'force' when air is being drawn through the chamber with the typical fan.
« Last Edit: July 27, 2018, 10:54:57 PM by Bobvelle »

Offline CW2274

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I think the fars with the big chamber it has has a lot more area to retain condensed moisture than the standard shield with the sensor located at the bottom so is more readily exposed to the drier air with all the plates being open to the air.
I don't see it. All a fan does is take ambient air and draw it across the sensor, evaporating or saturating certainly more efficient than any passive shield.
Not saying your observations are wrong, but my physics don't see your physics. :???:
but its seems to me that being in that area actually keeps the full force of the air from blowing directly on the sensor, while at the same time still 'saturated' in the air pulled in from below.
I would certainly think the small area in the chamber would negate sensor placement. The filter also keeps the "full force" of the air off the sensor.

I disagree. The baffle at the bottom of the chamber produces a small area semi-stagnate air. The sensor can be placed near this area. Above that small area is a turbulence zone ...and near the top by the fan, faster and straighter moving air. Plenty of room for all that to take place.
Also, there would be no sense in adding the SF2 Sensirion filter cap if the air is not directly hitting the sensor. According to you, the cap will block this air.
By the way, my choice of the term 'full force of air' was hyperbole. We all know there is no 'force' when air is being drawn through the chamber with the typical fan.
You put the sensor where you believe is the most beneficial. That being said, I think you're waaay over thinking this. Whether the sensor receives more or less of a laminar flow is irrelevant.

Offline dendrite

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I have the 75 almost ready to go. Here it is next to a Davis sensor for size comparison.

 [ You are not allowed to view attachments ]

Oh wow! You didnít have to solder right?  How are you going to protect the exposed wires?
Which exposed wires? I have some silicone conformal coating I may put on the pin end of the 75 PCB and up to where it meets the connector. Maybe I'll put a little on the wire end of the connector too.


And no, no soldering. It's an IDC connector so slide the wires in all the way and close it up tight. I think the Davis wires are 26 AWG and the connector accepts 26-28 AWG.

Good deal. Will probably end up doing that.




Between where you spliced the wires and the connector.
Yeah. I may add the silicone conformal coating around those holes where the wires enter to make sure no moisture finds its way in. With this connector I didn't even have to strip the wires...just a clean straight cut across. When you close the connector it slices into the wire to make the insulated connection, but better to be safe than sorry when it comes to moisture.

Offline Bobvelle

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Irrelevant in regards to the discussion of wet bias sensors? Agreed.   :-P

Offline Jim's Weather

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After a couple of PM's on this, let me address the issue of "best practice". So where does the best practice come from about not aspirating humidity sensors. It comes from us field techs. It's frustrating when I tell people in a PM what we are seeing in the field and they come back with, "but the sensor manufacturer says this" or "the manufacturers document says that". I like to use the car analogy here. Who would you trust more in getting information about minor problems being experienced with a particular type of GM car? GM or the local independent mechanic that works on their cars everyday? Us meteorological field techs are the independent mechanics.

Just like the wet bias I discovered over a year ago with the SHT-31. Did Davis know about it? Nope! A year later someone on this forum finally told them about it. My test proved that the field techs are aware of many sensor issues that the manufacturers aren't. Not everything gets back to the manufacturers and even if it does they may not think it's a big enough problem to do anything about it. From our experiences, we develop our own best practices when it comes to sensors.

So now back to the humidity sensor best practice and where it comes from. We recycle humidity sensors on average every 18 months. We lab test the sensors when they are new right before they are put in service. They are also spot checked right after being put in service and tested in more detail every 90 days thereafter. That test data gets logged by sensor serial number. That sensor is spot checked again at the 18 month recycle time before it is pulled out of service. We lab test it again right after it is pulled out of service. What we are seeing is that we have a much higher number of sensors that we pull out of FARS installations that are right on the edge of the spec or are already slightly out of spec versus those we pull out of passive shields. Unfortunately none of us field techs has taken the time to try and fully understand why this is happening. Those nearly out of spec or out of spec sensors all show a wet bias. They often fail other sensor metrics we test as well. Getting to travel all over the country to work on different installations, it's a much bigger problem in humid regions.

Again this is not happening to ALL humidity sensors in a FARS. We pull many humidity sensors that show no ill effects after 18 months of being in a FARS. However, based on what we are seeing across the board, we recommend client configurations of temperature sensor in FARS and humidity sensor in an open passive shield (non-gill plate) to maintain optimum performance and calibration. FARS is great for humidity response time and a few other positives but what good is that if we check the sensor during the 18 month in service period and it's out of spec or fails one of the other metrics we test.

Just an FYI, I don't do any work with Davis sensors. My SHT-31 tests I did were done on the side and for my own curiosity.

So then I get asked, "What should I do since I have a combined temperature and humidity sensor?" I say put it in a FARS for temperature accuracy and hope your humidity sensor holds up. If not, replace the sensor. If you don't have the means to properly test the humidity side of the sensor and you are concerned about its accuracy long term, just get on a schedule to replace it every 12 to 18 months.

Again, for the record, I'm not saying the FARS is causing the problem with the Davis sensors in any way shape or form. It's just one possibility that needs to be investigated.

« Last Edit: July 28, 2018, 01:46:48 AM by kcidwx »
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Offline CW2274

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^^^ Thank you for your expertise on this subject. This is without a doubt the most informative thread I've ever been part of on this forum.

For me, things are pretty much status quo, for others, obviously not so much.

Offline Bobvelle

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  However, based on what we are seeing across the board, we recommend client configurations of temperature sensor in FARS and humidity sensor in an open passive shield (non-gill plate) to maintain optimum performance and calibration.

What is "an open passive shield (non-gill plate)"? What would that look like? Just a Bell or Cone shaped shelter?

Offline ValentineWeather

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  However, based on what we are seeing across the board, we recommend client configurations of temperature sensor in FARS and humidity sensor in an open passive shield (non-gill plate) to maintain optimum performance and calibration.

What is "an open passive shield (non-gill plate)"? What would that look like? Just a Bell or Cone shaped shelter?

Image near top of thread Kcidwx posted of the ASOS sensor was open shield. Basically has a umbrella and fully exposed underneath.
Randy

Offline Bobvelle

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  However, based on what we are seeing across the board, we recommend client configurations of temperature sensor in FARS and humidity sensor in an open passive shield (non-gill plate) to maintain optimum performance and calibration.

What is "an open passive shield (non-gill plate)"? What would that look like? Just a Bell or Cone shaped shelter?

Image near top of thread Kcidwx posted of the ASOS sensor was open shield. Basically has a umbrella and fully exposed underneath.

Gotcha. Thanks

Offline openvista

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So then I get asked, "What should I do since I have a combined temperature and humidity sensor?" I say put it in a FARS for temperature accuracy and hope your humidity sensor holds up. If not, replace the sensor. If you don't have the means to properly test the humidity side of the sensor and you are concerned about its accuracy long term, just get on a schedule to replace it every 12 to 18 months.

OK fair enough, Ron.

But when the official parts suppliers only carry sensors with wet biases "preinstalled", what then? Davis had BETTER be working on a solution.
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Offline WheatonRon

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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 youíre like me. DPs are 3-4 degrees off being wet in bias

+4į dp when I did the afternoon check.  Thread has gone 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.

This quote is from post 475 and we thought we were done until Davis replied! Has Davis done its thing yet? I certainly havenít heard anything and I doubt anyone else has including the resellers of the world, like Johnd and Ryan Wilhour.
« Last Edit: July 28, 2018, 09:16:44 AM by WheatonRon »
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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 youíre like me. DPs are 3-4 degrees off being wet in bias

+4į dp when I did the afternoon check.  Thread has gone 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.

This quote is from post 475 and we thought we were done until Davis replied! Has Davis done its thing yet? I certainly havenít heard anything and I doubt anyone else has including the resellers of the world, like Johnd and Ryan Wilhour.

Haven't heard anything from Davis but the thread lives on.  :grin:     
Randy

Offline johnd

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This quote is from post 475 and we thought we were done until Davis replied! Has Davis done its thing yet? I certainly havenít heard anything and I doubt anyone else has including the resellers of the world, like Johnd and Ryan Wilhour.

No, for the record, not heard anything here. But then frankly I'd be surprised to have heard anything. If anyone is interested in my personal opinion:

This reported issue doesn't sound like something that can be validated, fixed and tested quickly, even assuming that any fix is possible. Add into the mix that Davis are, I suspect, very busy with other priorities right now and this issue might have to take its place in the queue (at least beyond that some bland statement that Davis are looking into the report and maybe some updated maintenance advice).

Please don't misunderstand: I'm all for the VP2 stations being as good and consistent in specification as they can be, as I'm sure that Davis are too. But there has to be some realism about what is possible at the price point. It would be good to think that a fix can be quickly identified and implemented (though how much time and effort would be involved in trying to fix a problem of excessive long-term drift - that sounds like a 2-3 year study to me?) but there's also the real possibility that the VP2 is what it is.

The reality could - but only could, I emphasise - be that users needing real accuracy and long-term stability in RH readings should consider buying eg a Vaisala station (and a suitable maintenance contract).
« Last Edit: July 28, 2018, 10:28:40 AM by johnd »
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Sorry, but I can't help with individual issues by email. Please post your issue in the relevant forum section here & I will comment there if I have anything useful to add.

Offline dendrite

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Great stuff guys.

I spent the morning refurbing my VP2...new rad plates, uv/solar covers, aerocone, etc. it almost looks like I just bought it.

Anyway, I put my old SHT75 back in the FARS for now. I put silicone conformal coating on the new one so thatíll take a little time to cure at regular ambient air temps. Maybe tomorrow morning Iíll swap it out with the new one.

C7324 is my station for those who want to follow along. I wouldnít overanalyze the readings too much until I put the new sensor in. Feel free to critique away then.