Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 30993 times)

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Offline drew1021

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Agree, but is Davis going to listen to us and begin working on the fix? If enough people complain or care enough to voice their concerns to Davis then maybe. The bottom line is if sales of the VP2 suffer then we have a better chance.
I think that the VP2 owners here on this forum are a minuscule segment of their business and not worth a "drop everything that you're doing and fix it for these whiners" type of scenario. If it was a problem that was so obvious to ALL, then it might be a different story, but I think the vast majority are oblivious, or don't care enough to voice a complaint.
I very well could be and hope I'm wrong, but doubtful because I'd think there'd be a fix already.

Couldn't agree more, I guess chalk one up to wishful thinking on my behalf. Maybe I'll look into the Ambient Weather WS-2000. I've read their specs and on paper it looks worse than the VP2, however by my observation all the local ones look to be pretty accurate. I read that the sensor they use is the sht-30. I don't really care for the all in one units, however wind is not of the highest priority for me. Rain on the other hand is. Ambients specs aren't that great, +\- 10% What to do.
VP2 with 24 hour FARS. SHT31 sensor technology

Offline CW2274

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Agree, but is Davis going to listen to us and begin working on the fix? If enough people complain or care enough to voice their concerns to Davis then maybe. The bottom line is if sales of the VP2 suffer then we have a better chance.
I think that the VP2 owners here on this forum are a minuscule segment of their business and not worth a "drop everything that you're doing and fix it for these whiners" type of scenario. If it was a problem that was so obvious to ALL, then it might be a different story, but I think the vast majority are oblivious, or don't care enough to voice a complaint.
I very well could be and hope I'm wrong, but doubtful because I'd think there'd be a fix already.
What to do.
Granted, my SHT31 is "only" set for a minus 2% correction factor, considerably less than most, but you'll not see me throwing my VP2 out the window....not even a debate.

Offline galfert

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How much traction or effect do you all think starting an online petition would do for getting Davis to take notice? I think besides the petition alone there needs to be a way to reach out to other places beyond this hobbyist forum. We would need a way to secure contact information for weather hardware industries. This is something that a hardware resellers have. So I wonder if it would be possible to enlist the support of a Scaled Instruments or Scientific Sales or Ambient (they sell Davis too). In some ways I wonder how willing they would be to be a part of a notification process that VP2s are not performing their best, as it might hurt sales. There is also the possibility that it goes against some sort of reseller contract. Another way may be to be active at yearly industry conferences where Davis shows up with a booth. Someone could pass flyers at the event. The wording needs to be delicate and not disparaging. The message should be something like Davis VP2 hardware has shown by independent tests that there is a wet bias and a ceiling of around 94% to 96% humidity ceiling and that it would be instrumental to sign the petition to ensure that Davis does something about it. That's it. No harsh language calling anything crap, or that Davis isn't worth it. Nothing like that. It needs to be supportive of Davis. There should be some positive language praising the merits of owning a Davis system. I think it is always best to critique something by also adding in a positive. If you watch a bad movie and you just complain about how terrible it was and that is all you say then you are going to turn a lot of people off or cause others to jump on the defense. But if you critique and then have a balanced view and sing some praises also about what is good then you get better reception for your input, you seem more credible, and people don't have to jump on the defense. Same works with children when you are trying to raise them to make better choices. Then it becomes a showing that you aren't really there to complain, but that you want to help and be part of the solution, and that you care.
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Offline CW2274

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Your post has merit, but make no doubt, Davis is completely aware of our 'feelings' regarding this situation. I as stated earlier today, I truly believe the 'cost to weight ratio' will leave Davis unresponsive. IMO, we'll only see a change when the next line of PWS's comes out, even if they stay in the PWS business. Who knows? I would certainly think it's lucrative enough to think they will.

Offline dalecoy

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I don't know about everyone else but I am getting impatient waiting for Davis to come up with a solution to the infamous wet bias of the 31.

I'm curious about a couple of things here:

1.  What form would that "solution" have?  Would it be hardware?  Software?  Or...

2.  If a hardware solution, then would this be "send your ISS in for replacement"?  Or optionally "do it yourself"?

3.  What production lots of VP2s would be involved?  That is, would it be available for all ages of VP2?  Cabled in addition to wireless?  Etc.

4.  What would be the estimated cost?

5.  How many units would you expect Davis to sell?

Offline SnowHiker

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I'm curious about a couple of things here:

1.  What form would that "solution" have?  Would it be hardware?  Software?  Or...

2.  If a hardware solution, then would this be "send your ISS in for replacement"?  Or optionally "do it yourself"?

3.  What production lots of VP2s would be involved?  That is, would it be available for all ages of VP2?  Cabled in addition to wireless?  Etc.

4.  What would be the estimated cost?

5.  How many units would you expect Davis to sell?
My thoughts are:

1. If it's a hardware problem, as is my understanding, then a hardware solution would be preferable.  If it's a software problem a software solution would be preferable.  If it's a hardware fix that also requires a software update, then both.

2.  Yes, sending it in or optionally fixing it yourself if feasible.  Sounds good to me.

3.  The production lots that are affected.  At the least those whose owners complain and have units that are under warranty.  More depending on how far Davis feels it is worth it to them to please their existing customers.

4.  It is Davis that needs to come up with an acceptable solution and to determine the cost of that solution, not the consumer experiencing the problem.

5.  Also not a concern for the consumer who is looking for a solution.  How many sales might Davis lose because of the problem, whether rectified or unaddressed?

I'm actually not affected by the problem, but these questions strike me as strange as the answers should be self-evident, so I assume you're trying to prove some kind of point; like it's not worth it for Davis to address and rectify the problem.  If so, maybe you're right, but that doesn't mean you should throw design or manufacturing flaws back in the face of the affected consumers.  So why not just come out and make your point?  Especially as the one you addressed your questions to had acknowledged that those who are aware of and bothered by the problem may not be enough for Davis to address (see reply # 1325).
« Last Edit: November 06, 2018, 04:00:14 AM by SnowHiker »

Offline openvista

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The problem cannot be adequately addressed through software because each sensor has varying amounts of bias, and those biases can change with time. I have 3 sensors. They all behave differently. No one solution would ensure each operates within specifications.

It's clearly a hardware problem. As has been pointed out many times already, Sensirion warns explicitly in the datasheet that the sensor isn't designed for environments with humidities above 80%. I mean how asinine is it that Davis (or anyone else) thought "screw it, we're doing it anyway?!" As Ron (kcidwx) said, this is an INDOOR sensor not intended for meteorology. He's an NWS field tech who's tested and verified the problem even with non-Davis branded SHT31s. Arguing that other manufacturers have implemented this sensor outdoors "successfully" is problematic because AFAIK, those claiming that haven't tracked non-Davis SHT31s long-term (particularly not with calibrated instrumentation). Just to make matters worse, Davis uses non-approved chemicals in mounting & packaging the sensor (material safety handling). So either way you slice it, Davis blew through vendor warnings about sensor corruption and the results are predictable.
« Last Edit: November 06, 2018, 09:03:49 AM by openvista »
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Offline EA1EF

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We thing that probably the simplest way are adopted by Belfryboy that use Sensirion original recommended proof materials based in housing with Dupont original Teflon PTFE

Probably The original Davis housing for SHT sensors aren't optimized for the sensor specs and outdoor, and users are that pay so expensive the Davis bad design because Davis caps are so far Sensitrion specs and recommendations.

Handling recommendations
https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/2_Humidity_Sensors/Sensirion_Humidity_Sensors_Handling_Instructions.pdf

https://www.sensirion.com/en/environmental-sensors/humidity-sensors/filter-cap-sf1/

https://www.sensirion.com/en/environmental-sensors/humidity-sensors/filter-cap-sf2/




« Last Edit: November 06, 2018, 09:28:19 AM by EA1EF »

Offline dendrite

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The problem cannot be adequately addressed through software because each sensor has varying amounts of bias, and those biases can change with time. I have 3 sensors. They all behave differently. No one solution would ensure each operates within specifications.

It's clearly a hardware problem. As has been pointed out many times already, Sensirion warns explicitly in the datasheet that the sensor isn't designed for environments with humidities above 80%. I mean how asinine is it that Davis (or anyone else) thought "screw it, we're doing it anyway?!" As Ron (kcidwx) said, this is an INDOOR sensor not intended for meteorology. He's an NWS field tech who's tested and verified the problem even with non-Davis branded SHT31s. Arguing that other manufacturers have implemented this sensor outdoors "successfully" is problematic because AFAIK, those claiming that haven't tracked non-Davis SHT31s long-term (particularly not with calibrated instrumentation). Just to make matters worse, Davis uses non-approved chemicals in mounting & packaging the sensor (material safety handling). So either way you slice it, Davis blew through vendor warnings about sensor corruption and the results are predictable.
Agree. It's not a meteorological sensor.

If someone has a separate temp/hum station, the probable way to go (if you're really bothered by the wet bias) is to measure the temp with your FARS/ISS and then use a passive shield with a 2nd SHTxx with some form of supplemental heat inside of the shield to keep the RH almost always < 80%. Then apply your own calculations to determine a new RH using the ISS 2m temp and the passive 2m dewpoint. I'm not sure if you could apply a resistor to the Davis temp/hum voltage line to create a little heat without affecting the sensor performance. That would be a question better answered by one of the electrical guys here.

Offline openvista

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Agree. It's not a meteorological sensor.

If someone has a separate temp/hum station, the probable way to go (if you're really bothered by the wet bias) is to measure the temp with your FARS/ISS and then use a passive shield with a 2nd SHTxx with some form of supplemental heat inside of the shield to keep the RH almost always < 80%. Then apply your own calculations to determine a new RH using the ISS 2m temp and the passive 2m dewpoint. I'm not sure if you could apply a resistor to the Davis temp/hum voltage line to create a little heat without affecting the sensor performance. That would be a question better answered by one of the electrical guys here.

Aside from the fact that what you just described, while conceptually simple, would take a variety of non-trivial hardware and software skills to accomplish, the implicit assumption is that so long as you can keep the SHT31 below 80% humidity it will report accurate readings. Otherwise, why go to all the effort? (I'm saying this for the benefit of others who may not have read the thread in its entirety.) Mine arrived with +6% bias below 80% (at temps above 55F; +4% below it). I know Randy and others have reported receiving new 31s that were wet from the start. It seems, based on past discussion, that this initially defective behavior is due to bad material handling by Davis. Reconditioning attempts have largely failed.

If the chemical contamination issues were sorted out, and the sensor were verified to be accurate before leaving Davis' warehouse, then keeping it below 80% humidity long term may be a worthwhile, if complicated, endeavor to pursue.

Speaking more generally and to no one in particular, if you think accurate humidity isn't that important, you should go ask someone who runs a decent size farm whether correct evapo-transpiration calculations matter to them. Spoiler alert: a 5+% shift in humidity during peak growing season could mean wasting thousands of gallons of water ($$$) or, worse yet, under applying irrigation to thirsty crops.  Keep in mind that agriculture is one of the primary targets for Davis products. So when the VP2 OVER reports humidity, irrigation is UNDER applied and this could cost farmers real money from lost yields (especially in hot and dry years).
« Last Edit: November 06, 2018, 01:00:20 PM by openvista »
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Offline galfert

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Speaking more generally and to no one in particular, if you think accurate humidity isn't that important, you should go ask someone who runs a decent size farm whether correct evapo-transpiration calculations matter to them. Spoiler alert: a 5+% shift in humidity during peak growing season could mean wasting thousands of gallons of water ($$$) or, worse yet, under applying irrigation to thirsty crops.  Keep in mind that agriculture is one of the primary targets for Davis products. So when the VP2 OVER reports humidity, irrigation is UNDER applied and this could cost farmers real money from lost yields (especially in hot and dry years).

Which is why I say someone needs to show up to some farm convention and find the Davis booth and start handing out informative flyers to everyone around the area. I bet Davis would then care very much and they will address the issue. If you can't get in the venue then hand out flyers at the entrance.
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Offline openvista

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Speaking more generally and to no one in particular, if you think accurate humidity isn't that important, you should go ask someone who runs a decent size farm whether correct evapo-transpiration calculations matter to them. Spoiler alert: a 5+% shift in humidity during peak growing season could mean wasting thousands of gallons of water ($$$) or, worse yet, under applying irrigation to thirsty crops.  Keep in mind that agriculture is one of the primary targets for Davis products. So when the VP2 OVER reports humidity, irrigation is UNDER applied and this could cost farmers real money from lost yields (especially in hot and dry years).

Which is why I say someone needs to show up to some farm convention and find the Davis booth and start handing out informative flyers to everyone around the area. I bet Davis would then care very much and they will address the issue. If you can't get in the venue then hand out flyers at the entrance.

I'm all for action over words (which is why I crafted my own custom solution to this problem after spending countless hours assessing and documenting it), but that sounds like a pretty good way to get sued by Davis. I suppose if you have the cash or if you are a consumer litigator, that might work. As with anything, there's always the likelihood that the truth is irrelevant to the proceedings -- in the courtroom or the court of public opinion. What's the saying? It's easier to fool people than to convince them they've been fooled?
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Offline galfert

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Speaking more generally and to no one in particular, if you think accurate humidity isn't that important, you should go ask someone who runs a decent size farm whether correct evapo-transpiration calculations matter to them. Spoiler alert: a 5+% shift in humidity during peak growing season could mean wasting thousands of gallons of water ($$$) or, worse yet, under applying irrigation to thirsty crops.  Keep in mind that agriculture is one of the primary targets for Davis products. So when the VP2 OVER reports humidity, irrigation is UNDER applied and this could cost farmers real money from lost yields (especially in hot and dry years).

Which is why I say someone needs to show up to some farm convention and find the Davis booth and start handing out informative flyers to everyone around the area. I bet Davis would then care very much and they will address the issue. If you can't get in the venue then hand out flyers at the entrance.

I'm all for action over words (which is why I crafted my own custom solution to this problem after spending countless hours assessing and documenting it), but that sounds like a pretty good way to get sued by Davis. I suppose if you have the cash or if you are a consumer litigator, that might work. As with anything, there's always the likelihood that the truth is irrelevant to the proceedings -- in the courtroom or the court of public opinion. What's the saying? It's easier to fool people than to convince them they've been fooled?

You have a very good point. So maybe we consult a lawyer or maybe we don't need to. Just use common sense. You could present the information without even using the Davis name. You could list other brand hardware that uses SHT31 and mention how they don't have these wet bias problems then post the question in big letters, "Does your weather station have wet bias?" Then close it out saying that other leading weather station hardware has been shown to have wet bias in independent tests. You see this type of advertising and information all the time.
« Last Edit: November 06, 2018, 04:16:54 PM by galfert »
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Offline CW7491

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From the testing Ive done, I think the Sensirion sensor is actually remarkably good for the price. While the sensor has its limitations as discussed here (exposure to prolonged high humidity etc) my experience is there is simply something about Davis application of the sensor that accounts for most of the poor performance.

I am in NYC for work and took a run through Central Park. To validate my obsession with this stuff, I carried my $15 Acurite SHT31 and stopped at the Central Park ASOS in time for the 351p observation. This is completely anecdotal and not exactly scientific, but its just another data point. This Acurite sensor isnt exposed to long periods of high humidity, although I have had it outside near saturation for hours at a time to see how it performs. And I dont exactly baby it. I take it on runs, throw it in my bag etc. No special handling by any means. I even took pictures for the 5 minutes, on the minute, leading up to the 351p ASOS observation since ASOS uses a running 5 min average. The entire time the Acurite SHT31 was representative of the 351p picture shown below. In the other picture you can barely see the Acurite on the corner fence post for a sense of proximity. Even without a radiation shield and considering mass of the plastic moulding, the results were spot on. Rounding 61.4 and 40.9% to 61 and 41% gives a DP of 37, which is exactly the same as ASOS at the exact same time.


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« Last Edit: November 07, 2018, 05:49:57 PM by CW7491 »

Offline CW7491

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Another at 751a ...

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« Last Edit: November 10, 2018, 11:05:59 AM by CW7491 »

Offline openvista

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We already know from previous discussion up thread that the Acurite indoor units can be expected to perform quite well for a time outdoors. But no one has exposed them long term. After all, they aren't shielded from the elements whatsoever so siting them properly year round would be ill advised.

Sensirion puts the following disclaimer in the datasheet for the sensor:

Quote
The sensor shows best performance when operated within recommended normal temperature and humidity range of 5 60 C and 20 80 %RH, respectively. Long term exposure to conditions outside normal range, especially at high humidity, may temporarily offset the RH signal (e.g. +3%RH after 60h at >80%RH). After returning into the normal temperature and humidity range the sensor will slowly come back to calibration state by itself. Prolonged exposure to extreme conditions may accelerate ageing.

"Extreme conditions" here are defined as being below 5C (41F) or above 80% humidity regularly. Notice also how they use the term "normal". Under what scenario would an environment featuring temps above 41 and humidity between 20-80% be considered "normal"? The only one I can think of would be climate controlled (home, business, lab, greenhouse, humidor, etc).
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Offline ValentineWeather

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The (Acurite SHT31)  will fit inside the Davis 7714... Yes the humidity doesn't suffer same fate as Davis. Much lower in those DP's above 55 but doesn't suffer at highest end either. Basically the Acurite 31 FOLLOWS THE DATA SHEET like we would EXPECT. Why I feel problem only exists with the LS series special order Davis SHT31.   
Randy

Offline CW7491

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We already know from previous discussion up thread that the Acurite indoor units can be expected to perform quite well for a time outdoors. But no one has exposed them long term. After all, they aren't shielded from the elements whatsoever so siting them properly year round would be ill advised.

Sensirion puts the following disclaimer in the datasheet for the sensor:

Quote
The sensor shows best performance when operated within recommended normal temperature and humidity range of 5 60 C and 20 80 %RH, respectively. Long term exposure to conditions outside normal range, especially at high humidity, may temporarily offset the RH signal (e.g. +3%RH after 60h at >80%RH). After returning into the normal temperature and humidity range the sensor will slowly come back to calibration state by itself. Prolonged exposure to extreme conditions may accelerate ageing.

"Extreme conditions" here are defined as being below 5C (41F) or above 80% humidity regularly. Notice also how they use the term "normal". Under what scenario would an environment featuring temps above 41 and humidity between 20-80% be considered "normal"? The only one I can think of would be climate controlled (home, business, lab, greenhouse, humidor, etc).

Agreed and I am not trying to suggest that Sensirion sensors are without their limitations and errors. As you point out, they are quite transparent about them. And yes, I haven't exposed the Acurite sensor to "extreme conditions" for more than a few continuous days. But I have exposed it to >80% humidity in excess of 60 continuous hours. I wanted to see if it reaches 100% humidity (it does >99.4%) and to see how the Sensirion offset, when exposed to "extreme conditions," compares to a new (<1 year old) Davis SHT31 that has never been outside (I keep it for such comparisons). Even after that exposure, the Davis SHT31 shows a wet bias in excess of the Acurite mounted SHT31. As I said initially, nothing scientific here, but just providing a data point and sharing my experience.

From what I have seen, Davis' SHT31 seems to have a wet bias before it's exposed to anything. The Davis SHT31 suffers from errors inherent to the Sensirion sensor plus whatever errors they introduce in the application of the sensor. The Acurite sensor performs extremely well (which is surprising given my experience with Davis and Acurite more generally). Here is to hoping that Davis can and will make whatever changes they need to get the maximum performance out of the Sensirion sensor even given its inherent limitations because I think it would be a great improvement.
« Last Edit: November 10, 2018, 03:07:06 PM by CW7491 »

Offline jerryg

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I discovered something in the specs for the sht1x and sht7x series sensors that i had not read before or just overlooked. I was wondering why all my old sensors and the new 75's stopped at 99% and the Sensirion specs state (Values higher than 99%RH indicate fully saturated air and
must be processed and displayed as 100%RH1) this is why they don't go over 99% because Davis doesn't show the readings over 99% as 100% unless they are 99.5% or higher. The highest reading in dense fog on mine has been 99.4% with most at 99.1 to 99.2 percent. But according to specs they should be displayed as 100% by Davis are they should show the specs as 0 to 99% instead of 100%. So again it looks like Davis doesn't follow the Sensirion instructions. The 31's don't show that in the specs and both mine hit 100% in fog with one showing 99.9% and the other 99.7% so that looks like a change made with this sensor. ](*,)

Offline CW2274

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If you've stated this before, sorry, I've forgotten, but how to you see resolution to the 0.1%? I just know that whenever I hit 99% (no idea what is is to the tenth), the temp/dew spread is always 0.4F.

Offline openvista

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I discovered something in the specs for the sht1x and sht7x series sensors that i had not read before or just overlooked. I was wondering why all my old sensors and the new 75's stopped at 99% and the Sensirion specs state (Values higher than 99%RH indicate fully saturated air and
must be processed and displayed as 100%RH1) this is why they don't go over 99% because Davis doesn't show the readings over 99% as 100% unless they are 99.5% or higher. The highest reading in dense fog on mine has been 99.4% with most at 99.1 to 99.2 percent. But according to specs they should be displayed as 100% by Davis are they should show the specs as 0 to 99% instead of 100%. So again it looks like Davis doesn't follow the Sensirion instructions. The 31's don't show that in the specs and both mine hit 100% in fog with one showing 99.9% and the other 99.7% so that looks like a change made with this sensor. ](*,)

Well, that wouldn't be a problem for any of my three SHT31 sensors. None of them have ever gotten higher than 98% including the one I just received a month or so ago. To reach 98% the newest sensor has to sit in saturated air for at least a few hours. Most of the time the max is 97%. But, according to Sensirion, as long as it reaches 96.5% it's considered within specification (max error 3.5%). 
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Offline jerryg

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I have a hand held unit that used the sht11 that i removed and fixed it up so i can plug any sht sensor that uses the Davis format and it reads the humidity to a tenth. Both of my 31's read 100% in fog. Unless you are lucky and get one of the older sht's that gets to 99.5% you won't see a 100 ever. But according the the specs 99.1 should trigger the 100% reading but it looks like Davis fixed the software to not read a 100% less than 99.5%. So i use my weather display software to adjust my readings to show 100% by adding 1% to the readings which kicks in when the reading goes above 93% and removes it when i goes back down to 93. My 31's are good on the high end but read high on the lower end so have to use the adjustments to lower the readings on the low end. I use the best sensor that reads pretty good on the low end and tops out at 99 on the high end and get pretty good results that way. I have found that when i get a decent reading on one end the other is off on the other end.

Offline CW2274

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Unless you are lucky and get one of the older sht's that gets to 99.5% you won't see a 100 ever.

I have found that when i get a decent reading on one end the other is off on the other end.
Thanks. As I stated here before, the 31 I have now (couple years old) refused to hit 100%, sat on 99 (zero correction factor), and that's after being WOXOF for hours, but all my comps were long since at 100%.

Of course, robbing Peter to pay Paul.

 :-| I guess I'm basically rehashing old news.

Offline jerryg

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Yeah it's been pretty well beaten to death  :lol:. My point is Davis should have made the software to read 100 with anything over 99 and didn't. I guess i lucked out with my two 31's reading 100 but they always do when the fog is around but when we get some really dry air after a frontal passage they read high on the low end, of course down here we only get low humidity in the winter when dry air comes in from the north. I have two of the Davis sht15's that both read 99 and do pretty well on the low end too. I use the pick of the litter when it comes to the one i use on the main station but for someone who just has the one it is pretty much luck of the draw whether you get a good one or not.

Offline Old Tele man

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SHT31's new labelling: WINTER, SPRING, SUMMER, FALL
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