Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 59830 times)

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Offline hwcorder

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Randy...

I brought it up before, but have you ever tried an SHT75? If you're really suspicious about Davis' handling of the sensors it may be worth giving a sensor soldered at the Sensirion factory a try. I'm sure you have an old Davis SHTxx that you can sacrifice to solder directly to the 75 pins or insert into an acceptable connector.

So remove sht15 chip and solder SHT75 in its place? How is the 75 with humidity? Does it have the same high bias even after reconditioning the 31 nothing really changed.




Yes
Randy...

I brought it up before, but have you ever tried an SHT75? If you're really suspicious about Davis' handling of the sensors it may be worth giving a sensor soldered at the Sensirion factory a try. I'm sure you have an old Davis SHTxx that you can sacrifice to solder directly to the 75 pins or insert into an acceptable connector.

Correct me if I'm wrong, Randy, but didn't you say that your 15s were fried too when it came to humidity?

Yes it's not much better.
The 75 is basically a 15 with slightly better RH specs between 10-90% (1.8% versus 2.0%). The difference is that it is presoldered by Sensirion. Solder the correct wires to an old 6-conductor Davis temp/hum and it becomes plug and play. With your FARS the temp response will be through the roof because of the low thermal mass...especially if you run it with the filter off.

http://www.wxforum.net/index.php?topic=14175.msg189484#msg189484

http://www.mouser.com/ds/2/682/Sensirion_Humidity_SHT7x_Datasheet_V5-469726.pdf

I havent done any actual testing on humidity drift but just from what I can remember the sht75s I order directly from Newark14 have less drift than the two or three sht31s I bought.
 I recently bought another sht31 and used the sf2 filter like jerryg had done and when I compared the results it immediately ran 2-3% higher than the sht75 that is almost two years old.  SHT75 was reading right along with the ASOS 5 miles away.  Got my Vaisala HMP46 out to spot check and it was reading 2% lower than the sht75 and about 4-5% lower than the brand new 31. 
I can switch out my 75s directly since I soldered a connector making them plug and play.  Really think it may be something happening in the manufacturing process possibly making these sensor defective or perhaps its a flaw in the design of the sht31 itself, who knows. All I know is I have no plans of switching to anything else anytime soon until something changes, protocol, or Davis manufacturing process.  Thinking about making a diy temp/hum probe using some old magic marker housing and some epoxy then topping it off with a better filter from Vaisala or Cambell Scientific.
« Last Edit: July 22, 2018, 10:50:54 PM by hwcorder »

Offline dendrite

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I have a few of these and a new 75 coming in tomorrow from Newark. They should work. We'll find out tomorrow. No soldering required.

http://www.newark.com/harting/14310410301000/receptacle-4way-1row-1-27mm-har/dp/53X2803

Offline jgentry

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I have a few of these and a new 75 coming in tomorrow from Newark. They should work. We'll find out tomorrow. No soldering required.

http://www.newark.com/harting/14310410301000/receptacle-4way-1row-1-27mm-har/dp/53X2803

Iím not very knowledgeable with this stuff so here is my question.

1. If I order a SHT-75, what all exactly I need to do to put it together so that I can use it in my FARS?

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

  

Offline jgentry

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Will note that Sensirion says that the SHT-75 is designed for weather stations
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline ValentineWeather

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I know this may be getting off subject a little but for those interested in switching software from WL or Cumulus over to Weather Display.... WD has a nice slope adjustment that would work. I just downloaded and impressed, my biggest problem I have 4 years of data on Cumulus and website data all using Cumulus lot's of work to change over.

Anyway the slope adjustment at -3% looks like it would work, here's the PDF. I tried it out and works.
There is a setting for decreasing slope as humidity drops lower end of scale also which I have checked.  It doesn't show on PDF however.



Randy

Offline jgentry

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I know this may be getting off subject a little but for those interested in switching software from WL or Cumulus over to Weather Display.... WD has a nice slope adjustment that would work. I just downloaded and impressed, my biggest problem I have 4 years of data on Cumulus and website data all using Cumulus lot's of work to change over.

Anyway the slope adjustment at -3% looks like it would work, here's the PDF. I tried it out and works.
There is a setting for decreasing slope as humidity drops lower end of scale also which I have checked.  It doesn't show on PDF however.

Nice! Personally donít care for WD. I personally donít want to keep doing calibration offsets etc. Iím thinking about giving the 75 a try if I can set everything up without soldering. Iím hoping someone on here can tell me how to install the SHT-75. And Iím also curious too if the 75 needs a filter cap?
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline dendrite

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I know this may be getting off subject a little but for those interested in switching software from WL or Cumulus over to Weather Display.... WD has a nice slope adjustment that would work. I just downloaded and impressed, my biggest problem I have 4 years of data on Cumulus and website data all using Cumulus lot's of work to change over.

Anyway the slope adjustment at -3% looks like it would work, here's the PDF. I tried it out and works.
There is a setting for decreasing slope as humidity drops lower end of scale also which I have checked.  It doesn't show on PDF however.

Nice! Personally donít care for WD. I personally donít want to keep doing calibration offsets etc. Iím thinking about giving the 75 a try if I can set everything up without soldering. Iím hoping someone on here can tell me how to install the SHT-75. And Iím also curious too if the 75 needs a filter cap?
Read the first few pages of this thread...specifically this post.

http://www.wxforum.net/index.php?topic=14175.msg138241#msg138241

Sensirion makes a filter for it, but you need a way to make it fit and stay secure. I have run mine no filter, but that will obviously degrade performance with time. I don't have much time this morning...maybe more later.

Offline jgentry

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I know this may be getting off subject a little but for those interested in switching software from WL or Cumulus over to Weather Display.... WD has a nice slope adjustment that would work. I just downloaded and impressed, my biggest problem I have 4 years of data on Cumulus and website data all using Cumulus lot's of work to change over.

Anyway the slope adjustment at -3% looks like it would work, here's the PDF. I tried it out and works.
There is a setting for decreasing slope as humidity drops lower end of scale also which I have checked.  It doesn't show on PDF however.

Nice! Personally donít care for WD. I personally donít want to keep doing calibration offsets etc. Iím thinking about giving the 75 a try if I can set everything up without soldering. Iím hoping someone on here can tell me how to install the SHT-75. And Iím also curious too if the 75 needs a filter cap?
Read the first few pages of this thread...specifically this post.

http://www.wxforum.net/index.php?topic=14175.msg138241#msg138241

Sensirion makes a filter for it, but you need a way to make it fit and stay secure. I have run mine no filter, but that will obviously degrade performance with time. I don't have much time this morning...maybe more later.

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

  

Offline jerryg

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Ok got to try make this short lol. After 12 hrs in the bag the acurite, as expected, showed 71% which is 4 low. I calibrated to 75 and put it outside in the light fog at the time. My main station was showing 98% the acurite went up slowly to 98 and stayed there for some time before going to 99. Looks pretty good. Now i put the acurite in unused room and let it settle in to 62% and steady and put the 3 old 31 and the new 31 and let them settle in. One read 1% low, two of them read 3% high and one was right on. So i took the one that was showing 1 low and bagged it for a while and it showed to be 73.7% which is right at 1 low and right now is showing 61% next to acurite which still shows 62%. I bagged the one right on an it showed 75.3% and it shows the same as the acurite. I have not check the two that have 3% high but probably will check one though it sure looks like they will be high on the bag test. 3% high is not within specs.

Offline jerryg

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Well the acurite went up to 64 and the one went to 64 and the other to 63 so tracking as tested. Now the sad news i bagged one of the two that was running 3 high and it is the new one i just got and sure enough it showed 78.1, out of spec. I guess i could send it back to Ryan but not sure if it's worth the effort. I am waiting for the new 15's to get here in the mail this am so i can do some comparing while it is fresh on my feeble brain lol.

Offline openvista

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Ok got to try make this short lol. After 12 hrs in the bag the acurite, as expected, showed 71% which is 4 low. I calibrated to 75 and put it outside in the light fog at the time. My main station was showing 98% the acurite went up slowly to 98 and stayed there for some time before going to 99. Looks pretty good. Now i put the acurite in unused room and let it settle in to 62% and steady and put the 3 old 31 and the new 31 and let them settle in. One read 1% low, two of them read 3% high and one was right on. So i took the one that was showing 1 low and bagged it for a while and it showed to be 73.7% which is right at 1 low and right now is showing 61% next to acurite which still shows 62%. I bagged the one right on an it showed 75.3% and it shows the same as the acurite. I have not check the two that have 3% high but probably will check one though it sure looks like they will be high on the bag test. 3% high is not within specs.

The problem with testing at 75% humidity on the 31s is that's where they are the MOST accurate, even the ones that are out of spec. 75% is the crossover point from wet to slight dry bias (assuming that particular sensor contains either bias). So if you're looking for drift, you need to test below 70%. I know they make 62% humidity kits, for instance.
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline jerryg

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Right now i am looking at comparing readings at a given calibration point. I have a 33% packet for testing and i also have potassium sulfate for a 97% reference. My real pain is that the brand new 31 is 3% high to start.

Offline openvista

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Right now i am looking at comparing readings at a given calibration point. I have a 33% packet for testing and i also have potassium sulfate for a 97% reference. My real pain is that the brand new 31 is 3% high to start.

33% might be a better test, I don't know. I've seen notable bias around 35%. But I haven't been able to replicate bias consistently at 75%. Of course, the other problem is that those kits need to be run at room temperature, no? The highest biases start north of 25C (77F).

Also, weren't you one of the original proponents of the SHT75? Could you test one (or more) that's been in use for awhile and see if it's truly more accurate with humidity than the 31s?

Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline jerryg

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Well heck, i tried a couple 75 but alas they both fell victims of too much humidity and not enough knowledge of how humidity kills the sensor in long periods of high humidity with a fan running. I have one on order and am going to do some comparisons when i get it in.

Offline dendrite

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Ok. Thanks!
Just remember that you don't need the capacitor for the SHT75 since it's preinstalled. That post I referenced was for the SHT15. You just need to worry about the proper wiring.

I don't have a huge gripe with the RH values of any of the SHT sensors so I really couldn't tell anyone, in any scientific way, how it compares to the 31 or 15. I do know I was regularly hitting 98-99% with it before I took it out. I think the temp geeks with the AC FARS are going to really like the rapid response though.

Offline dendrite

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And one more thing...

If you go from the 31 to the 75 make sure you adjust your temp cal settings. I have an older ISS so I had to go back to a 0 cal, but you guys with the newer ones will need to adjust the 0.5C.

Offline JudinNorman

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I dug my old Weather Monitor II station out few days ago to hook up the temp/hum sensor to compare to my sht31 vp2. 
Even after all these years the dewpoints last few days were constantly close to those of nearby weather service station than the vp2.

Offline jgentry

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For those who are getting the SHT-75 and installing it soon, please report back your findings. Iím curious if the DP readings are better. Iím also curious to how the temps compare with the 31. Thanks!

Also, if you have experience using the SHT-75 when temps got below 32į?  How did the sensor perform under those conditions? Spec sheet suggests that the temp will start drifting under 32į
« Last Edit: July 23, 2018, 06:50:11 PM by jgentry »
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline jgentry

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I dug my old Weather Monitor II station out few days ago to hook up the temp/hum sensor to compare to my sht31 vp2. 
Even after all these years the dewpoints last few days were constantly close to those of nearby weather service station than the vp2.

And thatís not a good thing for the VP2...
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline openvista

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Finally today the humidity fell below 70% and I was able to test the midrange on the SHT15. Not good. I'm seeing an average dew point error of about 2-3 degrees F (54F on my good 31 vs 57F on my bad 15). That's a 5-7% humidity difference. So, it appears my 15 has succumbed to the same problem as other Davis sensors. It reads wetter with time (it's about 3 years old now). My guess, based on past experience with this problem, is that as humidity descends toward 50% and/or the air temp goes up the errors will worsen.
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline jgentry

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Finally today the humidity fell below 70% and I was able to test the midrange on the SHT15. Not good. I'm seeing an average dew point error of about 2-3 degrees F (54F on my good 31 vs 57F on my bad 15). That's a 5-7% humidity difference. So, it appears my 15 has succumbed to the same problem as other Davis sensors. It reads wetter with time (it's about 3 years old now). My guess, based on past experience with this problem, is that as humidity descends toward 50% and/or the air temp goes up the errors will worsen.

Iím now curious how a new SHT-75 will perform overall. Are you thinking about getting the 75?
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline ValentineWeather

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I gave up on WD was having so many weird issues, spent all day on it. Back to old reliable. Wish Cumulus had the humidity slope it's an easy fix.
Randy

Offline openvista

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Finally today the humidity fell below 70% and I was able to test the midrange on the SHT15. Not good. I'm seeing an average dew point error of about 2-3 degrees F (54F on my good 31 vs 57F on my bad 15). That's a 5-7% humidity difference. So, it appears my 15 has succumbed to the same problem as other Davis sensors. It reads wetter with time (it's about 3 years old now). My guess, based on past experience with this problem, is that as humidity descends toward 50% and/or the air temp goes up the errors will worsen.

Iím now curious how a new SHT-75 will perform overall. Are you thinking about getting the 75?

No. I don't really have any more confidence in that sensor than a 15 at this point. I realize it's cheaper. But it has the same issues, ultimately, as the 15. It's obsolete, no longer manufactured and, most likely it will eventually develop a wet bias. jerryg had two 75s and both had issues with humidity.

Hoping against hope that Davis can come up with a solution. I'm lucky because I have 1 sensor (out of 4) that's still within spec for humidity.
Davis Vantage Pro2 AC FARS | https://marquetteweather.com | EW7933

Offline WxLover16

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I know this may be getting off subject a little but for those interested in switching software from WL or Cumulus over to Weather Display.... WD has a nice slope adjustment that would work. I just downloaded and impressed, my biggest problem I have 4 years of data on Cumulus and website data all using Cumulus lot's of work to change over.

Anyway the slope adjustment at -3% looks like it would work, here's the PDF. I tried it out and works.
There is a setting for decreasing slope as humidity drops lower end of scale also which I have checked.  It doesn't show on PDF however.

Nice! Personally donít care for WD. I personally donít want to keep doing calibration offsets etc. Iím thinking about giving the 75 a try if I can set everything up without soldering. Iím hoping someone on here can tell me how to install the SHT-75. And Iím also curious too if the 75 needs a filter cap?
Read the first few pages of this thread...specifically this post.

http://www.wxforum.net/index.php?topic=14175.msg138241#msg138241

Sensirion makes a filter for it, but you need a way to make it fit and stay secure. I have run mine no filter, but that will obviously degrade performance with time. I don't have much time this morning...maybe more later.
     

Just wondering cause I'm all about accuracy, how would you really know the sensor is degrading, slower response times? Also, I know you run a 40 cfm fan, but with no filter that has really got to be doing a huge number on your sensor without the filter on. You probably have to change out your sensor about every month, no?
Davis Wireless VP2 SHT31 24hr 24CFM FARS

Offline dendrite

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I know this may be getting off subject a little but for those interested in switching software from WL or Cumulus over to Weather Display.... WD has a nice slope adjustment that would work. I just downloaded and impressed, my biggest problem I have 4 years of data on Cumulus and website data all using Cumulus lot's of work to change over.

Anyway the slope adjustment at -3% looks like it would work, here's the PDF. I tried it out and works.
There is a setting for decreasing slope as humidity drops lower end of scale also which I have checked.  It doesn't show on PDF however.

Nice! Personally donít care for WD. I personally donít want to keep doing calibration offsets etc. Iím thinking about giving the 75 a try if I can set everything up without soldering. Iím hoping someone on here can tell me how to install the SHT-75. And Iím also curious too if the 75 needs a filter cap?
Read the first few pages of this thread...specifically this post.

http://www.wxforum.net/index.php?topic=14175.msg138241#msg138241

Sensirion makes a filter for it, but you need a way to make it fit and stay secure. I have run mine no filter, but that will obviously degrade performance with time. I don't have much time this morning...maybe more later.
     

Just wondering cause I'm all about accuracy, how would you really know the sensor is degrading, slower response times? Also, I know you run a 40 cfm fan, but with no filter that has really got to be doing a huge number on your sensor without the filter on. You probably have to change out your sensor about every month, no?
I'm still doing fine. Just throwing the disclaimer out there.

https://weather.gladstonefamily.net/site/C7324

 

anything