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

[Bobvelle]

Oh and 1oz. of Epo-Tek® H20E  Adhesive is on sale at Amazon for $277 + shipping... But only if you hurry 

[CW2274]

I was noticing when putting on the filter that the rubbery substance around the sensor allows for what I think would be a good seal if the filter is smoothed off and the wire torqued down enough. At least you would avoid a sealant if it works.

Agreed.
 But I noticed 2 things while reading that link you provided.
1. touching the membrane can contaminate with oils and reduce the amount of humidity that can transfer to the sensor.  Yeah.. I touched it... like  a lot. I thought it was just a screen.

Yeah, I manhandled it too, just to see how it would go. Now that I have, I'll be using latex gloves, then washing my hands with them on before handling one of the other filters I bought (5). 

[WxLover16]

I was noticing when putting on the filter that the rubbery substance around the sensor allows for what I think would be a good seal if the filter is smoothed off and the wire torqued down enough. At least you would avoid a sealant if it works.

Agreed.
 But I noticed 2 things while reading that link you provided.
1. touching the membrane can contaminate with oils and reduce the amount of humidity that can transfer to the sensor.  Yeah.. I touched it... like  a lot. I thought it was just a screen.

Yeah, I manhandled it too, just to see how it would go. Now that I have, I'll be using latex gloves, then washing my hands with them on before handling one of the other filters I bought (5).

So, I take it this sensirion SF2 filter is now all the craze? I might just have to get me one, or two.

[CW2274]

I was noticing when putting on the filter that the rubbery substance around the sensor allows for what I think would be a good seal if the filter is smoothed off and the wire torqued down enough. At least you would avoid a sealant if it works.

Agreed.
 But I noticed 2 things while reading that link you provided.
1. touching the membrane can contaminate with oils and reduce the amount of humidity that can transfer to the sensor.  Yeah.. I touched it... like  a lot. I thought it was just a screen.

Yeah, I manhandled it too, just to see how it would go. Now that I have, I'll be using latex gloves, then washing my hands with them on before handling one of the other filters I bought (5).

So, I take it this sensirion SF2 filter is now all the craze? I might just have to get me one, or two.

Well, it's certainly more protective than the stock filter, but certainly more difficult to mount properly. 

[Bobvelle]

I was noticing when putting on the filter that the rubbery substance around the sensor allows for what I think would be a good seal if the filter is smoothed off and the wire torqued down enough. At least you would avoid a sealant if it works.

Agreed.
 But I noticed 2 things while reading that link you provided.
1. touching the membrane can contaminate with oils and reduce the amount of humidity that can transfer to the sensor.  Yeah.. I touched it... like  a lot. I thought it was just a screen.

Yeah, I manhandled it too, just to see how it would go. Now that I have, I'll be using latex gloves, then washing my hands with them on before handling one of the other filters I bought (5).

So, I take it this sensirion SF2 filter is now all the craze? I might just have to get me one, or two.

Well, it's certainly more protective than the stock filter, but certainly more difficult to mount properly.

Ain't that the truth.
I'm wondering if i need to re-mount a new one now.... and then I wonder if I should just put the stock cover back on it and be done with it, because it's not worth trouble.

[Bobvelle]

I was noticing when putting on the filter that the rubbery substance around the sensor allows for what I think would be a good seal if the filter is smoothed off and the wire torqued down enough. At least you would avoid a sealant if it works.

Agreed.
 But I noticed 2 things while reading that link you provided.
1. touching the membrane can contaminate with oils and reduce the amount of humidity that can transfer to the sensor.  Yeah.. I touched it... like  a lot. I thought it was just a screen.

Yeah, I manhandled it too, just to see how it would go. Now that I have, I'll be using latex gloves, then washing my hands with them on before handling one of the other filters I bought (5).

So, I take it this sensirion SF2 filter is now all the craze? I might just have to get me one, or two.

All the cool kids are doing it now. 
C'mon man, Put one on, then share your results er frustrations.

[jerryg]

Well for those of you who are interested in the 75 performance i have had it running as my main sensor for a few days not i i am well pleased with the overly readings i have gotten from it. Humidity wise it out does my 11 or 15 in response to rapid changes in temp and humidity with the humidity readings below 50% pretty much on where the 11 and 15 always ran 2 to 3% high. On the high end pretty even on readings but haven't had any heavy fog to see how high it will go but this am it was bouncing up to 98 and the 15 was a solid 97. The main thing is when the humidity went down the 75 dropped faster than the 15. The 15 does catch up to the 75 after some time. Now compared to the new 31 same results as the 15 on the top end and the 31 was about the same on the lower end. Had a good test of the sensors this pm when a thunderstorm came through and dumped some much needed rain. The temp was at 96 and humidity at 49 and the temp dropped to 72 and the humidity went up to 94% in about a half hour. The 75 humidity was running about 7% higher than the 15 and 31 with the temp running about 2 degrees higher. It took about 10 extra minutes for the 15 and 31 to get up to the 75 readings. I really like the 75 and it will be my main sensor for now.

[BCJKiwi]

Mounting an SF1 or SF2

An Option would be to cut the legs off and sit them on top of the ~2mm thick protective coating on the Davis board.
Then place a coil spring on top of the SF1/SF2 and then fit the standard davis filter on top.

The Sensirion Filters have a circular protrusion on top and the underside of the top of the inner Davis filter carrier has rounded corners with a flat sentral portion so a coil spring should be reasonably centrally located.

The SF1 protrusion is 6.4mm dia and the SF2 is 5.05mm dia (according to the spec sheets).
So different inside diameters would be required depending on the filter in use. Suggest around 0.5mmm dia wire and a soft spring (a lot of turns) about 25 to 30mm long with ID to suit the filter. Note that on the SF2 there is very little flat area around the circular prootrusion do the Spring ID will need to be close to the 5.05 dia and preferably ground flat on the end. Have not tried this yet as am waiting on parts (SF2) and SHT15 - see below.

I also note there is very little difference in specification between the SHT15 and SHT7x. My VP2 came with an SHT11 so I have ordered a couple of SHT15s and will swap out the SHT11.
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[dendrite]

Hit 97% on the 75 this morning despite BKN/OVC skies and no fog (just some light dew). I see jerryg is up to 99% RH this morning...maybe he snuck a 100% in there between Mesowest obs.

[jgentry]

Hit 97% on the 75 this morning despite BKN/OVC skies and no fog (just some light dew). I see jerryg is up to 99% RH this morning...maybe he snuck a 100% in there between Mesowest obs.

According to his website, it says the peak RH is 99%. The RW station (has SHT-15) hit 100%. Now I will say that give the 75 a few more high humidity events, it will probably trigger 100% more often.

[jerryg]

Well things got cleared up a little better this am, finally got some decent humidity lol. Now all three sensors were at 99%, the new 31 went first then the 75 went about 50 minutes later and the 2 yr old 31 that i put in place of the 15 went to 99 about an hour after the 75. The new 31 bounced to 100 a couple times. Now i still got the same results when the humidity started to drop, the 75 went first and the two 31's were about a half hour later before they dropped. It's been 50 minutes now and the 75 is still dropping at about 3% lower than the 31's.

[jerryg]

Update, one hour since 75 dropped and now the new 31 is 2% higher but the old 31 is 4% higher, wet bias showing up?

[jerryg]

All three are now in the same range of humidity, almost two hours to get there.

[Bobvelle]

I also note there is very little difference in specification between the SHT15 and SHT7x. My VP2 came with an SHT11 so I have ordered a couple of SHT15s and will swap out the SHT11.

And I just realized 2 days ago that I've never owned a SHT15. I went fro the 11 to 31. I'd at least like to try the 15 at some point.

As far attaching the SFx filter cap.... Hot Glue! Albeit, a very small amount on each side. Because there is relatively a "country mile" of room around the perimeter of the sensor, you wouldn't even need to put any under the cap. Just a drop on each side should lock it down.

[ValentineWeather]

All three are now in the same range of humidity, almost two hours to get there.

For me this is a red flag because my 31's always run 5%+ above actual humidity (moist warm environment I assume you are in on the Gulf coast at 87°) even my new units do. And if they all are the same tells me the 75 isn't much better and running high. 

[dendrite]

All three are now in the same range of humidity, almost two hours to get there.

For me this is a red flag because my 31's always run 5%+ above actual humidity (moist warm environment I assume you are in on the Gulf coast at 87°) even my new units do. And if they all are the same tells me the 75 isn't much better and running high.

Didn't one of those 31s hit 100% this morning? Maybe they haven't experienced much drift yet?

Maybe he can run the data through a spreadsheet and find the average afternoon dewpoint (or RH) for each sensor. One observational point in time doesn't necessarily tell the story. A psychrometer reading would help too.

[jerryg]

Well need to keep in mind that the 75 and 31 are new and the two year old 31 has been cooked and hydrated so first time i have tried it out to see how it works. This is the first time since i have had the new sensors that we had a long period of high humidity and i don't know if just one time is enough to really get a feel for things. Right now i just like the faster response time of the 75 compared to the 31's. Heck maybe i lucked out and got a good 31 lol.

[jgentry]

All three are now in the same range of humidity, almost two hours to get there.

For me this is a red flag because my 31's always run 5%+ above actual humidity (moist warm environment I assume you are in on the Gulf coast at 87°) even my new units do. And if they all are the same tells me the 75 isn't much better and running high.

Jerryg DP is matching up fairly well with Port Lavaca Airport. Don’t know how far he is from it though. But his max DP is one degree higher than Port Lavaca.

[ValentineWeather]

Yes, lets see if it stays near the 31 after saturation all day that would be my main concern and tells me it's just a senserion issue with all their sensors, some worse than others.   

[jgentry]

Yes, lets see if it stays near the 31 after saturation all day that would be my main concern and tells me it's just a senserion issue with all their sensors, some worse than others.

What’s amazes me is how the RW stays very close to the airport (KVCT).

[jerryg]

Just to inform you i am 10 miles se of kvct and 12 miles nw of port lavaca metar. Port lavaca station is located a few miles from lavaca bay and my readings very seldom agree because i am inland from the bay and on the other side i generally run wetter than kvct being in the country over grass and not over gravel. The rw station being on the roof of a building matches the runway and gravel of kvct lol. So i am on an island by myself with the closest pws about 7 miles away, i have to run my own comparisons lol. Another thing kvct is at 115 feet and port lavaca is at 27 feet and i am at 59 feet. I have always found it strange that the metar near the bay always has lower humidity readings than most, strange being near that much water. Of course the temps are higher due to the fact they get the warm air currents off of the water and keeps their temps up at night and cooler in the daytime. I don't get any cooling from the bay until the sea breeze kicks in and then the temp drops in the after noon and the humidity goes up, makes it real hard to get a handle on things. Sometimes the sea breeze doesn't make it to kvct and i have a big temp and humidity difference.

[jgentry]

Just to inform you i am 10 miles se of kvct and 12 miles nw of port lavaca metar. Port lavaca station is located a few miles from lavaca bay and my readings very seldom agree because i am inland from the bay and on the other side i generally run wetter than kvct being in the country over grass and not over gravel. The rw station being on the roof of a building matches the runway and gravel of kvct lol. So i am on an island by myself with the closest pws about 7 miles away, i have to run my own comparisons lol. Another thing kvct is at 115 feet and port lavaca is at 27 feet and i am at 59 feet. I have always found it strange that the metar near the bay always has lower humidity readings than most, strange being near that much water. Of course the temps are higher due to the fact they get the warm air currents off of the water and keeps their temps up at night and cooler in the daytime. I don't get any cooling from the bay until the sea breeze kicks in and then the temp drops in the after noon and the humidity goes up, makes it real hard to get a handle on things. Sometimes the sea breeze doesn't make it to kvct and i have a big temp and humidity difference.

That makes good sense. Elevation can also play a role too when it comes to DP.

[openvista]

If I were jerryg and trying to compare to a good station, I might pick the RAWS right next to KVCT. RAWS stations have their temp/humidity sensor replaced annually. I checked the overdue maintenance report for RAWS stations and it wasn't on it. So it should be accurate (+/- 2% humidity), assuming good siting.

On a sunny hot day with not much wind, the dew point is going to bounce around. That's especially true in a FARS and even truer if there's a sensor with a small thermal mass like the SHT75 placed within that FARS. So it's best to take an average of several samples and compare that to the reference weather station from the same time. Averaging is less important when its cloudy and windy.

I looked around 11AM (CDT) and took a 5 minute average of Jerry's DPs. They ranged from 73 to 77. Avg was 74.8. Then I looked at the 11:04AM report from VCRT2 (https://www.wrh.noaa.gov/mesowest/timeseries.php?sid=VCRT2&num=48&banner=gmap&raw=0&w=325) and it was reporting 72. That's about a 3 degree difference. The KVCT airport was also reporting a DP of 72 as of 10:51AM.

Those readings are a bit concerning, even if they are about 9 miles away. Something to keep an eye on.

[openvista]

Just to inform you i am 10 miles se of kvct and 12 miles nw of port lavaca metar. Port lavaca station is located a few miles from lavaca bay and my readings very seldom agree because i am inland from the bay and on the other side i generally run wetter than kvct being in the country over grass and not over gravel. The rw station being on the roof of a building matches the runway and gravel of kvct lol. So i am on an island by myself with the closest pws about 7 miles away, i have to run my own comparisons lol. Another thing kvct is at 115 feet and port lavaca is at 27 feet and i am at 59 feet. I have always found it strange that the metar near the bay always has lower humidity readings than most, strange being near that much water. Of course the temps are higher due to the fact they get the warm air currents off of the water and keeps their temps up at night and cooler in the daytime. I don't get any cooling from the bay until the sea breeze kicks in and then the temp drops in the after noon and the humidity goes up, makes it real hard to get a handle on things. Sometimes the sea breeze doesn't make it to kvct and i have a big temp and humidity difference.

That makes good sense. Elevation can also play a role too when it comes to DP.

Dew point changes with elevation at a rate of about 1F per 1000ft, generally.

[jerryg]

here is a sample of reports from victoria and port lavaca metars and mine at 1 pm victoria temp 93 dp 71 h 48  port lavaca temp 92 dp 65 h 41 and mine temp 91 dp 72 h 54 That is a lot of variations to try to compare to my dp was pretty close to victoria but way off from port lavaca just too many variations to try and compare being i am half way in between the two. I just have to trust that my 29 dollar sensor is doing as good as it can without some type of calibrated instruments to compare to on site. Heck for my site i am right and they are wrong