Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 44985 times)

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Offline Jim's Weather

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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.

If that's the case then you are at the mercy of the vendor. However, I would not expect a quick response from them if you even get a response at all. They will need to do a lot of troubleshooting and testing to determine the cause. That can take months, even years depending on the nature and complexity of the problem. Even then the most probable cost effective solution (if this is a widespread problem) would be for Davis to publish their own set of specs for their PCB mounted SHT-31. Not uncommon for vendors to do that. The new Davis specs might look something like this.

Temperature (-40įF to +194įF) (0 - 90 %RH)      2.0 %RH
                                           (90 - 100 %RH)   3.5 %RH
Temperature (+80įF to +120įF) (40 - 70 %RH)    4.0 %RH

Those specs are just an example for a sensor that has a wet bias in the mid humidity range during warmer temperatures.
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Offline jgentry

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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.

Thank you for your input in this discussion. Itís been very helpful and educational for me.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline openvista

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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).

Well, as long as we're discussing Davis' priorities, I'd hope they'd appreciate how inaccurate humidity readings affect their largest client base in the U.S., namely agriculture. Why do farmers buy these stations if not mainly to measure evapotranspiration? You can't measure ET accurately when humidity is well off the mark (sometimes by 10% or more) during the peak of summer when irrigation needs are most acute.


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).

So you propose, in the absence of a meaningful solution from Davis, that VP2 owners pay 5-10x more so we can have something approaching the specifications we were given when we bought the Davis station?

Here are some other requirements you failed to mention that come with owning Vaisala or equivalent stations:
  • Those stations are almost certainly incompatible with existing consumer-grade weather software requiring you to buy and learn less user-friendly software
  • Owners will need to know or learn database programming so they can import current weather stats into the new software
  • Owners also have to develop custom software to send the new station's data to CWOP, Weather Underground, etc
  • Most truly professional grade stations (not the all-in-one stations meant for marine or other applications) require separate runs of power and low voltage cable to the weather station. Sorry folks, wireless costs even more and involves satellite or cellular data contracts!
  • Part replacements are going to cost hundreds, if not thousands of dollars
This is not a realistic solution for the vast majority of members on this forum.

No, unfortunately, the "realistic" solution is only available to those comfortable with DIY electronics projects. We're knee deep in posts describing that process now and we're waiting for results. For the average person unwilling or unable to tear things apart and solve other people's problems themselves, they are very likely screwed unless Davis can miraculously come through. That's the plain truth. And Davis had better hope against hope that one of their big buyers doesn't figure out the lay of the land.


« Last Edit: July 28, 2018, 01:13:05 PM by openvista »
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Offline openvista

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If that's the case then you are at the mercy of the vendor. However, I would not expect a quick response from them if you even get a response at all. They will need to do a lot of troubleshooting and testing to determine the cause. That can take months, even years depending on the nature and complexity of the problem. Even then the most probable cost effective solution (if this is a widespread problem) would be for Davis to publish their own set of specs for their PCB mounted SHT-31. Not uncommon for vendors to do that. The new Davis specs might look something like this.

Temperature (-40įF to +194įF) (0 - 90 %RH)      2.0 %RH
                                           (90 - 100 %RH)   3.5 %RH
Temperature (+80įF to +120įF) (40 - 70 %RH)    4.0 %RH

Try 10%, not 4%. We've had reports of worse. I've been able to replicate 9% consistently, but my max testing temperature was only ~85F. As the temp goes up, it gets worse.

EDIT: I'd also add that even as low as 60F, I'm seeing 5-6% errors on both my 15 and 31 sensors.
« Last Edit: July 28, 2018, 01:16:38 PM by openvista »
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Offline Old Tele man

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It's ALL about compromise: we compromise on ACCURACY vs. PRICE, they compromise on PRICE vs. ACCURACY.

We could buy more expensive systems but balk at their prices. They could build more accurate systems but balk at their cost-to-do-so.

It's the middle, common-ground, of these compromises that the consumers tend to settle, but either ENDS of this "bath tub" analogy are still there (good vs cheap) to chose from.

« Last Edit: July 28, 2018, 08:48:21 PM by Old Tele man »
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Offline ValentineWeather

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I think a great point was made about farmers relying on this data for accuracy when humidity and evaporation rates are very important.
I'm sure a large portion of their business is agricultural with the expansion in recent years of this type of station while the weather enthusiast is probably still the largest it's important for agriculture side to trust the data.
Randy

Offline jgentry

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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.

Considering the geographical features you have and that being an old sensor, the 75 seems like itís comparing fairly well to KLCI & KCON
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline hwcorder

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So here are my stats from Gladstone. As you can see my 2 year old sht75 is running very close to local ASOS for dew point and I have increased amounts of vegetation around my property.


Site   Distance         Location   Barometer   Temperature   Dewpoint


KCLT   7.7 kilometres   East       CHARLOTTE/DOUGLAS INTERNATIONAL , NC, United States (lat 35.2225į, long -80.9543į, altitude 228 metres)   -0.7mb   -0.5įC   -0.2įC

KAKH   12.6 kilometres   Southwest       GASTONIA MUNICIPAL AIRPORT, NC, United States (lat 35.1969į, long -81.1555į, altitude 243 metres)   -0.5mb   0.5įC   0.1įC

Offline galfert

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So I thought I wanted a VP2 but now I'm not so sure. Maybe I'll wait. I'm just a hobbyists with a cheap Ambient Weather WS-2902A. I think Davis needs to fix this humidity problem and soon not months or years later. Sure things take time to fix but you all have done tremendous work in less than a month. I expect Davis to put this at the top of their priority list. Well maybe just second to releasing a decent successor, the VP3, or whatever they want to call it. The VP3 is way overdue. I implore Davis to design the VP3 with separate temperature and heated humidity sensors. That solution does not break the value / price point mark. And they need to revise their manufacturing/shipping methods to ensure better quality control. But they can't forget about their loyal VP2 customers and offer them a solution too.

I predict that if Davis doesn't fix this problem soon and also if they don't release a VP3 within a year that there will be another manufacturer that starts to take away their sales. Already the Acurite Atlas has gotten alot of excitement and the Atlas Elite sounds like it might just be a VP2 killer.

I propose  we reach out to organizations that use VP2s and let them know what you all have discovered. For example reach out to WeatherSTEM and let them know the VP2 is not all it is cracked up to be. Maybe Davis needs to see pressure from not just this forum. We need to get Davis' big customers to be informed. Anyone know any other big Davis customers?

But not just customers need to know but also resellers. Good that Scaled Instruments knows. How about letting other vendors know? Has anyone let Ed at Ambient Weather know? Or how about Scientific Sales, do they know?

If you all want this problem fixed then let's light a fire and get Davis to respond. I'm talking about getting a real response. Not some canned "we are looking at it" BS statement. They need to quickly acknowledge the issue and promise a solution. Then they need to deliver a few months later but within 6 months.

I'm not all talk. I'll be glad to reach out to whomever. Let's make a list. And let's get our messaging straight. But maybe some of you already are a better inside contact for these organizations.
« Last Edit: July 28, 2018, 05:15:02 PM by galfert »
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Offline Old Tele man

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So I thought I wanted a VP2 but now I'm not so sure. Maybe I'll wait. I'm just a hobbyists with a cheap Ambient Weather WS-2902A. I think Davis needs to fix this humidity problem and soon not months or years later. Sure things take time to fix but you all have done tremendous work in less than a month. I expect Davis to put this at the top of their priority list. Well maybe just second to releasing a decent successor, the VP3, or whatever they want to call it. The VP3 is way overdue. I implore Davis to design the VP3 with separate temperature and heated humidity sensors. That solution does not break the value / price point mark. And they need to revise their manufacturing/shipping methods to ensure better quality control. But they can't forget about their loyal VP2 customers and offer them a solution too.

I predict that if Davis doesn't fix this problem soon and also if they don't release a VP3 within a year that there will be another manufacturer that starts to take away their sales. Already the Acurite Atlas has gotten alot of excitement and the Atlas Elite sounds like it might just be a VP2 killer.

I propose  we reach out to organizations that use VP2s and let them know what you all have discovered. For example reach out to WeatherSTEM and let them know the VP2 is not all it is cracked up to be. Maybe Davis needs to see pressure from not just this forum. We need to get Davis' big customers to be informed. Anyone know any other big Davis customers?

But not just customers need to know but also resellers. Good that Scaled Instruments knows. How about letting other vendors know? Has anyone let Ed at Ambient Weather know? Or how about Scientific Sales, do they know?

If you all want this problem fixed then let's light a fire and get Davis to respond. I'm talking about getting a real response. Not some canned "we are looking at it" BS statement. They need to quickly acknowledge the issue and promise a solution. Then they need to deliver a few months later but within 6 months.

I'm not all talk. I'll be glad to reach out to whomever. Let's make a list. And let's get our messaging straight. But maybe some of you already are a better inside contact for these organizations.
Davis has been badgered by us (CWOP's) for years and seldom (but not never) responds, rather they listen and then eventually (maybe) responds. Unfortunately "silent" patience is NOT our virtue.
« Last Edit: July 28, 2018, 05:22:14 PM by Old Tele man »
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ē CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
ē WU - KAZTUCSO202, Countryside

Offline ValentineWeather

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So I thought I wanted a VP2 but now I'm not so sure. Maybe I'll wait. I'm just a hobbyists with a cheap Ambient Weather WS-2902A. I think Davis needs to fix this humidity problem and soon not months or years later. Sure things take time to fix but you all have done tremendous work in less than a month. I expect Davis to put this at the top of their priority list. Well maybe just second to releasing a decent successor, the VP3, or whatever they want to call it. The VP3 is way overdue. I implore Davis to design the VP3 with separate temperature and heated humidity sensors. That solution does not break the value / price point mark. And they need to revise their manufacturing/shipping methods to ensure better quality control. But they can't forget about their loyal VP2 customers and offer them a solution too.

I predict that if Davis doesn't fix this problem soon and also if they don't release a VP3 within a year that there will be another manufacturer that starts to take away their sales. Already the Acurite Atlas has gotten alot of excitement and the Atlas Elite sounds like it might just be a VP2 killer.

I propose  we reach out to organizations that use VP2s and let them know what you all have discovered. For example reach out to WeatherSTEM and let them know the VP2 is not all it is cracked up to be. Maybe Davis needs to see pressure from not just this forum. We need to get Davis' big customers to be informed. Anyone know any other big Davis customers?

But not just customers need to know but also resellers. Good that Scaled Instruments knows. How about letting other vendors know? Has anyone let Ed at Ambient Weather know? Or how about Scientific Sales, do they know?

If you all want this problem fixed then let's light a fire and get Davis to respond. I'm talking about getting a real response. Not some canned "we are looking at it" BS statement. They need to quickly acknowledge the issue and promise a solution. Then they need to deliver a few months later but within 6 months.

I'm not all talk. I'll be glad to reach out to whomever. Let's make a list. And let's get our messaging straight. But maybe some of you already are a better inside contact for these organizations.

This is exactly what Davis hates to hear.  If you think about it this thread will live on forever and it's not great for Davis advertising. You aren't the only person I'm sure with same thoughts about holding off purchase.
Best thing Davis can do is address and fix problem quickly or thread will be 100 pages long eventually and come up on every Google search for years.
Randy

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I certainly understand the frustration with the wet bias, but the calls for reasonability at the price point are also warranted. My primary frustration is it is quite observable that a Sensirion sensor performs better than a Davis engineered Sensirion sensor even if I have no idea why. And my experience with the Sensirion sensor is that it is quite accurate (and this is for an older generation SHT75). The VP2 is extremely well designed and reliable if maybe a bit utilitarian. My main and really only gripe is for Davis to look into and fix whatever it is about their manufacturing process that seems to offset an otherwise quite accurate sensor. If I can get a Sensirion sensor to perform better than Davis can, something is definitely wrong ...
« Last Edit: July 28, 2018, 05:55:46 PM by CW7491 »

Offline WheatonRon

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Randy of ValentineWeather and I (probably others) have contacted Brett Lane (supervisor of Davis weather support), who at least acknowledged this issue and the discussion of it in this Forum. Short of having a march to Haywood, California not sure what else can be done. As Johnd, our reseller friend from the UK, has noted the demand for PWS is not huge so we donít have a lot of pent-up demand for a better product so unless the Atlas device takes off, is reliable and well built, we donít have a lot of choices. Frustrating yes, but....
« Last Edit: July 28, 2018, 06:59:13 PM by WheatonRon »
Davis VP2 with SHT31 (3 complete VP2 systemsó2 with a daytime fan and 1 that has a 24 hour fan); CWOP--CW5020, FW3075 and FW4350; WU--KILWHEAT17, KILWHEAT36 and KILWHEAT39; WeatherCloud.net; CoCoRaHS--IL-DP-132; and Weatherlink 2.0

Offline ValentineWeather

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What if Davis offered for those unhappy an option to purchase i2c protocol transmitter that worked with the VP2 so we could pick and choose the sensor we want.
Wasn't the reason they went with LLC Legacy was the older Davis stations like VP1's so they could communicate?

Just a thought  :idea: I'm sure it would make many happy. These 31's as accurate as the temperature is the humidity wasn't meant to be used in saturated atmospheres for extended periods. It's the wrong sensor for the application.


Randy

Offline jgentry

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So I thought I wanted a VP2 but now I'm not so sure. Maybe I'll wait. I'm just a hobbyists with a cheap Ambient Weather WS-2902A. I think Davis needs to fix this humidity problem and soon not months or years later. Sure things take time to fix but you all have done tremendous work in less than a month. I expect Davis to put this at the top of their priority list. Well maybe just second to releasing a decent successor, the VP3, or whatever they want to call it. The VP3 is way overdue. I implore Davis to design the VP3 with separate temperature and heated humidity sensors. That solution does not break the value / price point mark. And they need to revise their manufacturing/shipping methods to ensure better quality control. But they can't forget about their loyal VP2 customers and offer them a solution too.

I predict that if Davis doesn't fix this problem soon and also if they don't release a VP3 within a year that there will be another manufacturer that starts to take away their sales. Already the Acurite Atlas has gotten alot of excitement and the Atlas Elite sounds like it might just be a VP2 killer.

I propose  we reach out to organizations that use VP2s and let them know what you all have discovered. For example reach out to WeatherSTEM and let them know the VP2 is not all it is cracked up to be. Maybe Davis needs to see pressure from not just this forum. We need to get Davis' big customers to be informed. Anyone know any other big Davis customers?

But not just customers need to know but also resellers. Good that Scaled Instruments knows. How about letting other vendors know? Has anyone let Ed at Ambient Weather know? Or how about Scientific Sales, do they know?

If you all want this problem fixed then let's light a fire and get Davis to respond. I'm talking about getting a real response. Not some canned "we are looking at it" BS statement. They need to quickly acknowledge the issue and promise a solution. Then they need to deliver a few months later but within 6 months.

I'm not all talk. I'll be glad to reach out to whomever. Let's make a list. And let's get our messaging straight. But maybe some of you already are a better inside contact for these organizations.

I have already made it known to some from Auburn University. Iím about to talk to a guy from Earth Networks about it.

I personally think the quick solution is to use the Sensirion mounted 75. Be on the process of redoing the transmitter board; so that it can take the new 35 (thatís replacing the 75) next year. Also, use Sensirion filter cap and not the one they use now.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline Bobvelle

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I propose  we reach out to organizations that use VP2s and let them know what you all have discovered. For example reach out to WeatherSTEM and let them know the VP2 is not all it is cracked up to be. Maybe Davis needs to see pressure from not just this forum. We need to get Davis' big customers to be informed. Anyone know any other big Davis customers?

But not just customers need to know but also resellers. Good that Scaled Instruments knows. How about letting other vendors know? Has anyone let Ed at Ambient Weather know? Or how about Scientific Sales, do they know?

If you all want this problem fixed then let's light a fire and get Davis to respond. I'm talking about getting a real response. Not some canned "we are looking at it" BS statement. They need to quickly acknowledge the issue and promise a solution. Then they need to deliver a few months later but within 6 months.

I'm not all talk. I'll be glad to reach out to whomever. Let's make a list. And let's get our messaging straight. But maybe some of you already are a better inside contact for these organizations.

Davis don't want these guys getting upset at bad data.  :lol:
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Offline ValentineWeather

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I personally think the quick solution is to use the Sensirion mounted 75. Be on the process of redoing the transmitter board; so that it can take the new 35 (thatís replacing the 75) next year. Also, use Sensirion filter cap and not the one they use now.

Has it been determined the 75 doesn't have the wet bias?
I could live with it during summer and change to the 31 for winter.
Randy

Offline jgentry

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I personally think the quick solution is to use the Sensirion mounted 75. Be on the process of redoing the transmitter board; so that it can take the new 35 (thatís replacing the 75) next year. Also, use Sensirion filter cap and not the one they use now.

Has it been determined the 75 doesn't have the wet bias?
I could live with it during summer and change to the 31 for winter.

From what Iíve read on here thatís itís better. Havenít yet heard from the two yet that are going to install the 75 soon. Think one on here is doing it tomorrow so Iím waiting on him to do it. But even though the 31 spec sheet looks good in cold temps and the 75 not as good; some have said that the 75 does perform well with temps as low as 8į (if I remember correctly).
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

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From the side by sides Iíve done, the unmodified 75 runs consistently drier than the Davis-mounted 31 in the midrange as low as humidity in the 20s. (And I stress it is unmodified). That said, itís not perfect and is still a Sensirion. It struggles to hit 100%, but gets there on occasion. It hits 98 and 99 regularly. It will still exhibit a wet bias if in high humidity for a prolonged period and Iíd say it may have a slight wet bias generally, but it compares really closely to my psycho-dyne. Iíd love to find a way to get a 31 or 35 that Davis hasnít touched to work with my VP2, but until then, my experience is having the 75 is markedly superior to the Davis 31. Temp specs are not as good obviously and not exactly a rigorous study, but Iíve tested side by side with an ASOS at -8F and it was right on. Not trying to convince anyone, but just my experience ...

Offline jgentry

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From the side by sides Iíve done, the unmodified 75 runs consistently drier than the Davis-mounted 31 in the midrange as low as humidity in the 20s. (And I stress it is unmodified). That said, itís not perfect and is still a Sensirion. It struggles to hit 100%, but gets there on occasion. It hits 98 and 99 regularly. It will still exhibit a wet bias if in high humidity for a prolonged period and Iíd say it may have a slight wet bias generally, but it compares really closely to my psycho-dyne. Iíd love to find a way to get a 31 or 35 that Davis hasnít touched to work with my VP2, but until then, my experience is having the 75 is markedly superior to the Davis 31. Temp specs are not as good obviously and not exactly a rigorous study, but Iíve tested side by side with an ASOS at -8F and it was right on. Not trying to convince anyone, but just my experience ...

How much of the difference is there between the 75 & psych-dyne?
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline WheatonRon

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While I want better and more reliable humidity readings, accurate temperature is more important to me, so whatever ďbandaidĒ or work around is developed, it canít reduce temperature accuracy. Right now the SHT31 is excellent when monitoring temperature. If need be, I will just live with my SHT31 to get the temperature and accept that its humidity readings are just plain flaky. And maybe this perspective is why Davis hasnít made humidity measurement correction a high priority.
« Last Edit: July 28, 2018, 08:56:52 PM by WheatonRon »
Davis VP2 with SHT31 (3 complete VP2 systemsó2 with a daytime fan and 1 that has a 24 hour fan); CWOP--CW5020, FW3075 and FW4350; WU--KILWHEAT17, KILWHEAT36 and KILWHEAT39; WeatherCloud.net; CoCoRaHS--IL-DP-132; and Weatherlink 2.0

Offline Old Tele man

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While I want better and more reliable humidity readings, accurate temperature is more important to me, so whatever ďbandaidĒ or work around is developed, it canít reduce temperature accuracy. Right now the SHT31 is excellent when monitoring temperature. If need be, I will just live with my SHT31 to get the temperature and accept that its humidity readings are just plain flaky. And maybe this perspective is why Davis hasnít made humidity correction a high priority.

The temp sensing 'device' in the SHt31 is an electronic PN-junction on silicon(?) wafer and is typically VERY accurate, because PN-junction voltage has a very specific volt-to-temperature ratio.
« Last Edit: July 29, 2018, 02:09:59 PM by Old Tele man »
ē SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB;
ē DBX2 & DBX1 Precision Digital Barographs
ē CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
ē WU - KAZTUCSO202, Countryside

Online CW7491

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From the side by sides Iíve done, the unmodified 75 runs consistently drier than the Davis-mounted 31 in the midrange as low as humidity in the 20s. (And I stress it is unmodified). That said, itís not perfect and is still a Sensirion. It struggles to hit 100%, but gets there on occasion. It hits 98 and 99 regularly. It will still exhibit a wet bias if in high humidity for a prolonged period and Iíd say it may have a slight wet bias generally, but it compares really closely to my psycho-dyne. Iíd love to find a way to get a 31 or 35 that Davis hasnít touched to work with my VP2, but until then, my experience is having the 75 is markedly superior to the Davis 31. Temp specs are not as good obviously and not exactly a rigorous study, but Iíve tested side by side with an ASOS at -8F and it was right on. Not trying to convince anyone, but just my experience ...

How much of the difference is there between the 75 & psych-dyne?

Usually pretty spot on. Within 1F in dew point. Sometimes 2F. The nice thing about the 75 is you can put it right in the intake of the psychro dyne to verify temp and dew point. I know some others are using the 75 so they may be able to provide more input in terms of temp/hum accuracy of the 75 across the extreme ranges
« Last Edit: July 28, 2018, 10:03:04 PM by CW7491 »

Offline ValentineWeather

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While I want better and more reliable humidity readings, accurate temperature is more important to me, so whatever ďbandaidĒ or work around is developed, it canít reduce temperature accuracy. Right now the SHT31 is excellent when monitoring temperature. If need be, I will just live with my SHT31 to get the temperature and accept that its humidity readings are just plain flaky. And maybe this perspective is why Davis hasnít made humidity correction a high priority.

The temp sensing 'device' in the SHt31 is an electronic PN-junction on silicon(?) wafer and is typically VERY accurate, because PN-junction electron-flow has a very specific rate-to-temperature ratio.



Makes sense, all the units I've tested are within .1F maybe .2F at most of each other, old units new units doesn't matter. So either they are all off or all on the same amount with temperature.
Humidity is another story.

CW7491 has my interest up on the psycho dyne. 
Randy

Offline jgentry

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From the side by sides Iíve done, the unmodified 75 runs consistently drier than the Davis-mounted 31 in the midrange as low as humidity in the 20s. (And I stress it is unmodified). That said, itís not perfect and is still a Sensirion. It struggles to hit 100%, but gets there on occasion. It hits 98 and 99 regularly. It will still exhibit a wet bias if in high humidity for a prolonged period and Iíd say it may have a slight wet bias generally, but it compares really closely to my psycho-dyne. Iíd love to find a way to get a 31 or 35 that Davis hasnít touched to work with my VP2, but until then, my experience is having the 75 is markedly superior to the Davis 31. Temp specs are not as good obviously and not exactly a rigorous study, but Iíve tested side by side with an ASOS at -8F and it was right on. Not trying to convince anyone, but just my experience ...

How much of the difference is there between the 75 & psych-dyne?

Usually pretty spot on. Within 1F in dew point. Sometimes 2F. The nice thing about the 75 is you can put it right in the intake of the psychro dyne to verify temp and dew point. I know some others are using the 75 so they may be able to provide more input in terms of temp/hum accuracy of the 75 across the extreme ranges

Thatís good!  Have you ever done a comparison between the 75 & 31 in regards to air temp?

How did your 31 compared to the psychro-dyne?
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

 

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