Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 40964 times)

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Offline ct

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I don't own any Davis SHT31 sensors, but have a few I2C SHT3x sensors in operation in various fan aspirated shields.  I haven't done any scientific comparisons, but I am not seeing any significant difference in the humidity readings.

Yesterday the lowest humidity for the day was around 17:40.

SHT31, 22 months old, no protective cover:  41.90%
SHT35, 15 months old, Davis cap:  41.97%

3kms away I have another station:

SHT35, 5 months old, SF2 cap:  41.26%

At this time the closest meteorology bureau station was reading 41%.

Offline jgentry

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I don't own any Davis SHT31 sensors, but have a few I2C SHT3x sensors in operation in various fan aspirated shields.  I haven't done any scientific comparisons, but I am not seeing any significant difference in the humidity readings.

Yesterday the lowest humidity for the day was around 17:40.

SHT31, 22 months old, no protective cover:  41.90%
SHT35, 15 months old, Davis cap:  41.97%

3kms away I have another station:

SHT35, 5 months old, SF2 cap:  41.26%

At this time the closest meteorology bureau station was reading 41%.

So maybe the analog versions of the sensors are the bad ones?

How does the 35 & 31 compare when it comes to humidities in the upper 90 percentile? How do they compare in terms of air temp?
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline ValentineWeather

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I don't see it unless it's hot. Like tonight running lower than airport on dp.


66.0   58.2   73%    SHT31

66.2   62.6    88%  ASOS 1.3 miles away
Randy

Offline openvista

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So maybe the analog versions of the sensors are the bad ones?

I don't *believe* that the Davis sensor is analog. I've inquired about this before because I, too, assumed it must be using one of Sensirion's analog solutions and was told by someone knowledgeable that Sensibus is a digital-only platform. johnd should know.

** edited post and deleted dumb question **
« Last Edit: July 23, 2018, 11:57:48 PM by openvista »
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Offline ct

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How does the 35 & 31 compare when it comes to humidities in the upper 90 percentile? How do they compare in terms of air temp?

In my observations with a limited number of sensors, the SHT35 reaches 100% humidity more often than the SHT31.   For temperature, the SHT35 is usually about 0.3 °C lower than the SHT31.

Offline CW7491

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I have followed this forum for years, but never registered or even really considered posting. I have learned a great deal from following all of your comments and experiences over the years. So let me start by saying thank you. This particular topic, however, has brought me out of the shadows …

I have owned Davis equipment since 2005. I bought a VP2. In 2006, I moved to south Texas and upgraded to a 24hr FARS and this is where my frustration with the dew point readings of the VP2 began. My new sensor would never hit 100% humidity and the rest of the scale had a wet bias, but my VP2 from 2005 never had these issues. My 2006 VP2 with FARS had an SHT11 and my 2005 had the old "analog" sensor.

I am no expert obviously, just another guy with a borderline unhealthy obsession with this stuff, but bottom line: my opinion and experience strongly supports that despite the inherent limitation of the Sensirion sensors, there is something about Davis' manufacturing process that causes an offset toward a wet bias.

I have been obsessed with trying to fix and find the source of this bias as well as trying to find a sensor that consistently hits 100% RH. The SHT11 had these issues and disappointingly so did the SHT31. I have moved around a bunch, so I have tested these things from anywhere from Texas to Maine to DC and to the Pacific Northwest. The result has always been the same. I have spent hours over the years with my sensors within 10 feet from the HO-1088 and DTS1 of an ASOS. I have purchased a psychro-dyne for quality checking. I have had the sensors in the -20sF in Maine and Vermont and over 100F in DC and Texas. Here are the main points of what I have learned:

-Outgassing of silicon and glue offsets the Sensirion sensor. The offset seems irreversible. Silicon offsets to the high side. Some glues to the low side. (I discovered this trying to use Sensirion's own filters instead of Davis' filter.)
-The Davis filter may increase response time, but is not responsible for the wet bias.
-The Sensirion sensors do seem to have a slight wet bias as has been documented in some studies, but this wet bias is some how exacerbated by how Davis mounts the sensor.
-The wet bias for Sensirion sensors in prolonged periods near saturation as mentioned in their datasheet is a separate issue from the wet bias created by the Davis mounting.
-The SHT75 straight from the manufacturer without any filter is superior to the Davis mounted SHT31 in terms of dew point and humidity.
-The SHT75 will hit up to 99%, with random very short lived jumps to 100%.
-I miss the old analog sensor

I currently run my station with an SHT75 with no filter at all. I've been doing it for about 6mos. The first 5 of those months were in a stock Davis 24hr FARS. Now it is in the Davis 7714, which is a surprisingly good passive shield (much better than the VP2 stock passive shield) (sometimes a degree or two cooler than the FARS, sometimes a degree or two warmer, at least in the north latitudes of the Seattle area...) I am concerned with contamination and dust, but after trying to put a filter on it with silicon and other glues, I have ruined a fair number of SHT75s with seemingly irreversible offsets. So far I have not seen any evidence of poor performance for lack of a filter.

I run a calibration in Cumulus for the SHT75 (1.041x-3) and field check it with the psychro-dyne. It generally is very close. The psychro-dyne is surprisingly accurate compared side by side to ASOS. I set Cumulus to report 98% as 100% and the calibration I set ensures 99% is reported as 100% too.

This is the best I have been able to do given the limitations of these sensors. I really hope the efforts here will force Davis to look at their process as I'd be happy to have an accurate Davis stock sensor that is engineered to last.

Hope this helps...

Offline Dj1225

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I have followed this forum for years, but never registered or even really considered posting. I have learned a great deal from following all of your comments and experiences over the years. So let me start by saying thank you. This particular topic, however, has brought me out of the shadows …

I have owned Davis equipment since 2005. I bought a VP2. In 2006, I moved to south Texas and upgraded to a 24hr FARS and this is where my frustration with the dew point readings of the VP2 began. My new sensor would never hit 100% humidity and the rest of the scale had a wet bias, but my VP2 from 2005 never had these issues. My 2006 VP2 with FARS had an SHT11 and my 2005 had the old "analog" sensor.

I am no expert obviously, just another guy with a borderline unhealthy obsession with this stuff, but bottom line: my opinion and experience strongly supports that despite the inherent limitation of the Sensirion sensors, there is something about Davis' manufacturing process that causes an offset toward a wet bias.

I have been obsessed with trying to fix and find the source of this bias as well as trying to find a sensor that consistently hits 100% RH. The SHT11 had these issues and disappointingly so did the SHT31. I have moved around a bunch, so I have tested these things from anywhere from Texas to Maine to DC and to the Pacific Northwest. The result has always been the same. I have spent hours over the years with my sensors within 10 feet from the HO-1088 and DTS1 of an ASOS. I have purchased a psychro-dyne for quality checking. I have had the sensors in the -20sF in Maine and Vermont and over 100F in DC and Texas. Here are the main points of what I have learned:

-Outgassing of silicon and glue offsets the Sensirion sensor. The offset seems irreversible. Silicon offsets to the high side. Some glues to the low side. (I discovered this trying to use Sensirion's own filters instead of Davis' filter.)
-The Davis filter may increase response time, but is not responsible for the wet bias.
-The Sensirion sensors do seem to have a slight wet bias as has been documented in some studies, but this wet bias is some how exacerbated by how Davis mounts the sensor.
-The wet bias for Sensirion sensors in prolonged periods near saturation as mentioned in their datasheet is a separate issue from the wet bias created by the Davis mounting.
-The SHT75 straight from the manufacturer without any filter is superior to the Davis mounted SHT31 in terms of dew point and humidity.
-The SHT75 will hit up to 99%, with random very short lived jumps to 100%.
-I miss the old analog sensor

I currently run my station with an SHT75 with no filter at all. I've been doing it for about 6mos. The first 5 of those months were in a stock Davis 24hr FARS. Now it is in the Davis 7714, which is a surprisingly good passive shield (much better than the VP2 stock passive shield) (sometimes a degree or two cooler than the FARS, sometimes a degree or two warmer, at least in the north latitudes of the Seattle area...) I am concerned with contamination and dust, but after trying to put a filter on it with silicon and other glues, I have ruined a fair number of SHT75s with seemingly irreversible offsets. So far I have not seen any evidence of poor performance for lack of a filter.

I run a calibration in Cumulus for the SHT75 (1.041x-3) and field check it with the psychro-dyne. It generally is very close. The psychro-dyne is surprisingly accurate compared side by side to ASOS. I set Cumulus to report 98% as 100% and the calibration I set ensures 99% is reported as 100% too.

This is the best I have been able to do given the limitations of these sensors. I really hope the efforts here will force Davis to look at their process as I'd be happy to have an accurate Davis stock sensor that is engineered to last.

Hope this helps...

I agree 100% with you. I've said on this thread that I don't remember having this issue with the Weather Monitor II that I used from 1999-2002 (which is backed up by WMII users on this thread) the VP1 from 2002-2005 or even my VP2 I got in Oct. 2005 that I used until May 2016 until I upgraded and bought a new ISS with the SHT31. I'm thinking about looking for a VP1 and using it along side the VP2 with the SHT31 sensor and see what the results are. I miss the analog sensors also.

Online johnd

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I don't *believe* that the Davis sensor is analog. I've inquired about this before because I, too, assumed it must be using one of Sensirion's analog solutions and was told by someone knowledgeable that Sensibus is a digital-only platform. johnd should know.

No, I don't know with 100% certainty I'm afraid - that level of detail would only be known within Davis Engineering.

But to the absolute best of my knowledge, Davis have only ever used (what seems to be referred to as) Sensibus or Sbus - ie digital - versions of the SHT series of T/H sensors in eg the post 2006 VP2 stations. So any thought of analogue would be - I strongly suspect - completely wide of the mark.

I've never been bothered to look into it in sufficient detail, but don't the markings on the sensor reveal its type/subtype etc? They're tiny and you probably need to take a good photo and enlarge appropriately and then cross-check with the detailed Sensirion documentation.
« Last Edit: July 24, 2018, 04:47:03 AM by johnd »
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Offline dendrite

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I have followed this forum for years, but never registered or even really considered posting. I have learned a great deal from following all of your comments and experiences over the years. So let me start by saying thank you. This particular topic, however, has brought me out of the shadows …

I have owned Davis equipment since 2005. I bought a VP2. In 2006, I moved to south Texas and upgraded to a 24hr FARS and this is where my frustration with the dew point readings of the VP2 began. My new sensor would never hit 100% humidity and the rest of the scale had a wet bias, but my VP2 from 2005 never had these issues. My 2006 VP2 with FARS had an SHT11 and my 2005 had the old "analog" sensor.

I am no expert obviously, just another guy with a borderline unhealthy obsession with this stuff, but bottom line: my opinion and experience strongly supports that despite the inherent limitation of the Sensirion sensors, there is something about Davis' manufacturing process that causes an offset toward a wet bias.

I have been obsessed with trying to fix and find the source of this bias as well as trying to find a sensor that consistently hits 100% RH. The SHT11 had these issues and disappointingly so did the SHT31. I have moved around a bunch, so I have tested these things from anywhere from Texas to Maine to DC and to the Pacific Northwest. The result has always been the same. I have spent hours over the years with my sensors within 10 feet from the HO-1088 and DTS1 of an ASOS. I have purchased a psychro-dyne for quality checking. I have had the sensors in the -20sF in Maine and Vermont and over 100F in DC and Texas. Here are the main points of what I have learned:

-Outgassing of silicon and glue offsets the Sensirion sensor. The offset seems irreversible. Silicon offsets to the high side. Some glues to the low side. (I discovered this trying to use Sensirion's own filters instead of Davis' filter.)
-The Davis filter may increase response time, but is not responsible for the wet bias.
-The Sensirion sensors do seem to have a slight wet bias as has been documented in some studies, but this wet bias is some how exacerbated by how Davis mounts the sensor.
-The wet bias for Sensirion sensors in prolonged periods near saturation as mentioned in their datasheet is a separate issue from the wet bias created by the Davis mounting.
-The SHT75 straight from the manufacturer without any filter is superior to the Davis mounted SHT31 in terms of dew point and humidity.
-The SHT75 will hit up to 99%, with random very short lived jumps to 100%.
-I miss the old analog sensor

I currently run my station with an SHT75 with no filter at all. I've been doing it for about 6mos. The first 5 of those months were in a stock Davis 24hr FARS. Now it is in the Davis 7714, which is a surprisingly good passive shield (much better than the VP2 stock passive shield) (sometimes a degree or two cooler than the FARS, sometimes a degree or two warmer, at least in the north latitudes of the Seattle area...) I am concerned with contamination and dust, but after trying to put a filter on it with silicon and other glues, I have ruined a fair number of SHT75s with seemingly irreversible offsets. So far I have not seen any evidence of poor performance for lack of a filter.

I run a calibration in Cumulus for the SHT75 (1.041x-3) and field check it with the psychro-dyne. It generally is very close. The psychro-dyne is surprisingly accurate compared side by side to ASOS. I set Cumulus to report 98% as 100% and the calibration I set ensures 99% is reported as 100% too.

This is the best I have been able to do given the limitations of these sensors. I really hope the efforts here will force Davis to look at their process as I'd be happy to have an accurate Davis stock sensor that is engineered to last.

Hope this helps...
Nice first post. My observations tend to align with yours. We have fairly close CWOP IDs so we both got our VP2s when the Davis analog sensor was nearing its final days. That thing hit 100% so often that I was afraid it was hitting it too much. On a radiational cooling night I'd hit 100% around 9pm and it would flatline there until about 8-9am.

I have always ran my 75 with no filter as well and never noticed a problem, but the consensus here and from the documentation is that it will degrade performance over time so I don't want to be responsible for people ruining sensors following my filterless practices. Interesting that you noticed degraded performance with filters versus no filters though.

The SHT35 will be out in a pin form later this year. It'd be nice if someone could find a simple way to adapt it to a Davis ISS.

Offline openvista

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Nice post, CW7491! This is why the thread exists - so people can share their experiences and knowledge.

Couple questions:

1) How do you mount the 75 to the shield if you don't use glue or silicon?
2) Approximately how long did your previous 75s last before they went off the rails?
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Offline jerryg

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Well my first check with new sht15 humidity max 93, sht11 max 94 and new sht31 max96 all temps were within .1f of each other. I compared the two new 15's and they showed the same humidity and temp was within .2f of each other. Now then i have another test going starting this am and that is i put the other new 15 in a Davis dfars that i will run fan during the day and off at night. I put a high volume fan in place of the standard fan which is set up for 24 hr fars use and is a low volume fan with the shield being designed to use effectively but i figure the dfars should have a larger volume because of the open plate design. I am hoping to get decent daytime temps with the ability to shut the fan off at night and use the normal open plate air flow at night to get accurate readings without puting the sht to excessive moisture input. Of course you might know after weeks of clear skies and high heat the day i start my test the skies are cloudy lol. It is only supposed to last a day or so then back to clear and hot. :lol:


« Last Edit: July 24, 2018, 09:56:29 AM by jerryg »

Offline miraculon

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I was considering making some kind of "electronic wet/dry bulb" setup, and I found that SLOWeather already did it.

In California, dealing with the water source for the wet bulb sock would be better for more of the year than here in northern Michigan. I would have to take it down for winter. (or at least empty it)

Has anyone tried this setup?

It is an old thread but is still a valid and timely idea WRT this SHT-31 subject.

https://www.wxforum.net/index.php?topic=4844.0

Greg H.


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Offline dendrite

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Nice post, CW7491! This is why the thread exists - so people can share their experiences and knowledge.

Couple questions:

1) How do you mount the 75 to the shield if you don't use glue or silicon?
2) Approximately how long did your previous 75s last before they went off the rails?
I just wiretie the cord to the temp/hum mounting plate.

https://www.scaledinstruments.com/shop/davis-instruments/davis-7353-065-24-hour-spars-temperature-humidity-sensor-mounting-plate/


Offline jgentry

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I don't *believe* that the Davis sensor is analog. I've inquired about this before because I, too, assumed it must be using one of Sensirion's analog solutions and was told by someone knowledgeable that Sensibus is a digital-only platform. johnd should know.

No, I don't know with 100% certainty I'm afraid - that level of detail would only be known within Davis Engineering.

But to the absolute best of my knowledge, Davis have only ever used (what seems to be referred to as) Sensibus or Sbus - ie digital - versions of the SHT series of T/H sensors in eg the post 2006 VP2 stations. So any thought of analogue would be - I strongly suspect - completely wide of the mark.

I've never been bothered to look into it in sufficient detail, but don't the markings on the sensor reveal its type/subtype etc? They're tiny and you probably need to take a good photo and enlarge appropriately and then cross-check with the detailed Sensirion documentation.

I’ve heard that the version of the SHT-31 that Davis uses is the “analog.” That’s why I said it but I’m probably not correct in calling it analog.

If someone on here knows how alter the Davis transmitter so that it can take a SHT-35 would be awesome! I also might add that I would be willing to buy a SHT-35 from someone who knows how to make the SHT-35 sensor that will fit into the Davis 24hr FARS & passive shields.
« Last Edit: July 24, 2018, 10:55:43 AM by jgentry »
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline openvista

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I've been thinking about the issue of humidity reading too low on clear nights with radiational cooling resulting in dew and/or fog. Those of us with aspirated (FARS) shields tend to read warmer in those conditions because we are less subject to longwave radiation than passive shields. That's good and bad. It's good because that's technically closer to the guidelines for temperature measurement thereby achieving greater "accuracy". It's bad because the ground, the grass, and living things outdoors within a few feet of the ground ARE experiencing long wave radiation which drops the temperature until it meets the dew point. If you are outdoors during that time, your uncovered skin will feel the long-wave induced temperature, not the one reported by your weather station.

Also, there's a height difference to deal with on all shields that are at the standard 5-6ft off the ground. The further off the ground you are, the more likely there is an inversion (temperature increasing with height).

I'm not saying that the SHTxx sensors don't have a dry bias. They do. You can confirm this on any day or night with thick fog regardless of cloud cover. I'm just saying there are certain conditions that may prevent the temperature inside the shield from meeting the dew point. Yet another reason the ASOS/AWOS systems separate temperature measurement from humidity measurement and expose the humidity sensor with minimal shielding.

EDIT: I should add that even passive shielding will keep the temperature a bit higher than it would be otherwise, suppressing humidity.
« Last Edit: July 24, 2018, 11:20:52 AM by openvista »
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Offline dendrite

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I don't *believe* that the Davis sensor is analog. I've inquired about this before because I, too, assumed it must be using one of Sensirion's analog solutions and was told by someone knowledgeable that Sensibus is a digital-only platform. johnd should know.

No, I don't know with 100% certainty I'm afraid - that level of detail would only be known within Davis Engineering.

But to the absolute best of my knowledge, Davis have only ever used (what seems to be referred to as) Sensibus or Sbus - ie digital - versions of the SHT series of T/H sensors in eg the post 2006 VP2 stations. So any thought of analogue would be - I strongly suspect - completely wide of the mark.

I've never been bothered to look into it in sufficient detail, but don't the markings on the sensor reveal its type/subtype etc? They're tiny and you probably need to take a good photo and enlarge appropriately and then cross-check with the detailed Sensirion documentation.

I’ve heard that the version of the SHT-31 that Davis uses is the “analog.” That’s why I said it but I’m probably not correct in calling it analog.

If someone on here knows how alter the Davis transmitter so that it can take a SHT-35 would be awesome! I also might add that I would be willing to buy a SHT-35 from someone who knows how to make the SHT-35 sensor that will fit into the Davis 24hr FARS & passive shields.
SHT3x-LSS datasheet

https://www.glynshop.com/erp/owweb/Daten/Datenblaetter/Sensirion/SHT3x_LSS_V0.94.pdf
« Last Edit: July 24, 2018, 11:07:17 AM by dendrite »

Offline hwcorder

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So here is a rudimentary drawing of my setup. Apologies and the next chance I have I will try to take pics.  Anyway, I cut a small slot in the sensor shelf that comes with the 24hr FARS unit and inserted a 4 in connector soldering the wiring to the correct solder points.  Sorry I don't remember which color wires connect to which. I will have to take the sensor out of the shield and look at some point.  I protected and mounted the wires to the sensor shelf using electronic potting epoxy. Be careful not to use too much as to block the pins from being able to go through the 4 pin connector but enough to protect the solder joints. 
It was I little tough soldering the wires but not a huge deal. I'm okay with a soldering iron so I'm sure everybody here could do it as well.  I takes several hours to let the epoxy fully set but one thats done you are good to go. Plug in a new SHT75 and that it. I currently use the Davis filter which pop in using holes I drilled and it fits nicely around the 75 but I'm looking for a replacement that is a little more robust.

Offline Old Tele man

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I don't *believe* that the Davis sensor is analog. I've inquired about this before because I, too, assumed it must be using one of Sensirion's analog solutions and was told by someone knowledgeable that Sensibus is a digital-only platform. johnd should know.

No, I don't know with 100% certainty I'm afraid - that level of detail would only be known within Davis Engineering.

But to the absolute best of my knowledge, Davis have only ever used (what seems to be referred to as) Sensibus or Sbus - ie digital - versions of the SHT series of T/H sensors in eg the post 2006 VP2 stations. So any thought of analogue would be - I strongly suspect - completely wide of the mark.

I've never been bothered to look into it in sufficient detail, but don't the markings on the sensor reveal its type/subtype etc? They're tiny and you probably need to take a good photo and enlarge appropriately and then cross-check with the detailed Sensirion documentation.

I’ve heard that the version of the SHT-31 that Davis uses is the “analog.” That’s why I said it but I’m probably not correct in calling it analog.

If someone on here knows how alter the Davis transmitter so that it can take a SHT-35 would be awesome! I also might add that I would be willing to buy a SHT-35 from someone who knows how to make the SHT-35 sensor that will fit into the Davis 24hr FARS & passive shields.
SHT3x-LSS datasheet

https://www.glynshop.com/erp/owweb/Daten/Datenblaetter/Sensirion/SHT3x_LSS_V0.94.pdf

Quoted from page 8:

"Heater: An on chip heating element can be addressed
by writing a command into status register. The heater
may increase the temperature of the sensor by 5 – 10°C8
beyond ambient temperature. The heater draws roughly
25 mW @ 5V supply voltage.
For example the heater can be helpful for functionality
analysis: Humidity and temperature readings before and
after applying the heater are compared. Temperature
shall increase while relative humidity decreases at the
same time. Dew point shall remain the same
."


[red emphasis mine]

Anybody know if DAVIS utilize this function within the ISS?
« Last Edit: July 24, 2018, 11:38:04 AM by Old Tele man »
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Offline hwcorder

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I don't *believe* that the Davis sensor is analog. I've inquired about this before because I, too, assumed it must be using one of Sensirion's analog solutions and was told by someone knowledgeable that Sensibus is a digital-only platform. johnd should know.

No, I don't know with 100% certainty I'm afraid - that level of detail would only be known within Davis Engineering.

But to the absolute best of my knowledge, Davis have only ever used (what seems to be referred to as) Sensibus or Sbus - ie digital - versions of the SHT series of T/H sensors in eg the post 2006 VP2 stations. So any thought of analogue would be - I strongly suspect - completely wide of the mark.

I've never been bothered to look into it in sufficient detail, but don't the markings on the sensor reveal its type/subtype etc? They're tiny and you probably need to take a good photo and enlarge appropriately and then cross-check with the detailed Sensirion documentation.

I’ve heard that the version of the SHT-31 that Davis uses is the “analog.” That’s why I said it but I’m probably not correct in calling it analog.

If someone on here knows how alter the Davis transmitter so that it can take a SHT-35 would be awesome! I also might add that I would be willing to buy a SHT-35 from someone who knows how to make the SHT-35 sensor that will fit into the Davis 24hr FARS & passive shields.
SHT3x-LSS datasheet

https://www.glynshop.com/erp/owweb/Daten/Datenblaetter/Sensirion/SHT3x_LSS_V0.94.pdf

Quoted from page 8:

"Heater: An on chip heating element can be addressed
by writing a command into status register. The heater
may increase the temperature of the sensor by 5 – 10°C8
beyond ambient temperature. The heater draws roughly
25 mW @ 5V supply voltage.
For example the heater can be helpful for functionality
analysis: Humidity and temperature readings before and
after applying the heater are compared. Temperature
shall increase while relative humidity decreases at the
same time. Dew point shall remain the same
."


[red emphasis mine]

Anybody know if DAVIS utilize this function within the ISS?

No they don't. This feature is useful in high humidity situations where condensation can begin for form on the sensor.  If you can raise the temperature of the sensor far enough above the dew point you can prevent that.  However to use this functionality fully you would need a separate temperature sensor to continue to measure ambient air while the heating process in taking place.  The heated sensor of course would give an erroneous temperature reading.

Offline SLOweather

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As noted in my posts in that thread, it's certainly doable. However, as also noted, using Davis wireless stations on a VP2 console limits you to whole degrees, which I found to be too constraining.

For instance...

Dry Wet  RH at 1013 mb
70   60   55.5
71   60   52.1
70   61   59.5 
69   60   58.9

Find a way to run matched sensors with at least 0.1° resolution, and it would work.

Here's an equation for it that leaves out BP:


 
In another thread here I proposed building a chilled mirror dew point detector from an Arduino, Peltier device and a stainless steel camping mirror, but have yet to attempt it.

I was considering making some kind of "electronic wet/dry bulb" setup, and I found that SLOWeather already did it.

In California, dealing with the water source for the wet bulb sock would be better for more of the year than here in northern Michigan. I would have to take it down for winter. (or at least empty it)

Has anyone tried this setup?

It is an old thread but is still a valid and timely idea WRT this SHT-31 subject.

https://www.wxforum.net/index.php?topic=4844.0

Greg H.

Offline rdsman

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Quote

The SHT35 will be out in a pin form later this year. It'd be nice if someone could find a simple way to adapt it to a Davis ISS.


Follow this post:

http://www.wxforum.net/index.php?topic=33157.msg335987#msg335987

Ray

Offline rdsman

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Code: [Select]
Quoted from page 8:

"Heater: An on chip heating element can be addressed
by writing a command into status register. The heater
may increase the temperature of the sensor by 5 – 10°C8
beyond ambient temperature. The heater draws roughly
25 mW @ 5V supply voltage.
For example the heater can be helpful for functionality
analysis: Humidity and temperature readings before and
after applying the heater are compared. Temperature
shall increase while relative humidity decreases at the
same time. Dew point shall remain the same."

[red emphasis mine]

Anybody know if DAVIS utilize this function within the ISS?

If the VP2 is like the Vue, this wouldn't work without supplying an external voltage to the heated sensor.  In the Vue, VCC is only on during a sensor read.

« Last Edit: July 24, 2018, 12:36:43 PM by rdsman »
Ray

Offline mcrossley

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Do we know if the ISS applies the two correction values for humidity non-linearity and temperature compensation? It would require intercepting the output from the chip and comparing the raw values with those transmitted by the ISS - way beyond my capabilities!
Mark

Offline miraculon

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As noted in my posts in that thread, it's certainly doable. However, as also noted, using Davis wireless stations on a VP2 console limits you to whole degrees, which I found to be too constraining.

For instance...

Dry Wet  RH at 1013 mb
70   60   55.5
71   60   52.1
70   61   59.5 
69   60   58.9

Find a way to run matched sensors with at least 0.1° resolution, and it would work.

Here's an equation for it that leaves out BP:


 
In another thread here I proposed building a chilled mirror dew point detector from an Arduino, Peltier device and a stainless steel camping mirror, but have yet to attempt it.

I was considering making some kind of "electronic wet/dry bulb" setup, and I found that SLOWeather already did it.

In California, dealing with the water source for the wet bulb sock would be better for more of the year than here in northern Michigan. I would have to take it down for winter. (or at least empty it)

Has anyone tried this setup?

It is an old thread but is still a valid and timely idea WRT this SHT-31 subject.

https://www.wxforum.net/index.php?topic=4844.0

Greg H.

I just double checked and I am getting temperatures to 0.1°F increments using meteobridge and meteohub. This might be a workaround for the Davis console integer degrees issue.

Greg H.


Blitzortung Stations #706 and #1682
CoCoRaHS: MI-PI-1
CWOP: CW4114 and KE8DAF-13
WU: KMIROGER7
Amateur Radio Callsign: KE8DAF

Offline rdsman

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Quote

Do we know if the ISS applies the two correction values for humidity non-linearity and temperature compensation?


It does......

Ray

 

anything