Topic: Davis VP2 vs Kestrel D3

[ValentineWeather]

Aspiration on temperature and no aspiration on humidity I believe is how ASOS are setup.

What do you thing about adapter between ISS and sensor.
Adapter will feed ISS with temp and HUM, but on input adapter will take other sensors than Sensirion
What do you think about adapter?

This would be nice if it could be done.

[weather34]

If the SHT31 read 99% or 100% recently then, perhaps, the 96% reading is valid. But if you notice, as I and many others have, that the ceiling for the sensor goes from, let's say, 98% upon installation but over time drops to 96%, then you can assume that the sensor isn't going to read properly during fog. I've got three 31s. The humidity maxes are: 97% (newest), 96% (2nd oldest), 95% (oldest).

The 31 I'm using now has been in service ~2.5 years and hit 100% three times in the last two weeks. Granted, if you blinked you missed it, but happened none the less. When it wasn't at 100, 98% was the norm. Once again, looks like drier climates are the 31's friend, and frankly, I think having a stronger fan helps return it to normalcy (drying it out more efficiently) more so that not having a fan. Just to add, whether it matters or not, I also use the SF2 filter cap.

vp2 console doesnt cater for the decimal places i.e 99.4%..

im not convinced on aspirated methods when dealing with humidity measurements.

No, but software does.

As far as aspiration, I realize it's not SOP for humidity, but it's damn sure better for me than no fan at all, not even close.

i understand never two locations,environments alike but i guess here where i reside the constant sea breeze works but later this year during the peak summer i might just install one out of curiosity and perhaps ill learn along the way if beneficial or not . i dont use software so guess im missing those small increments .

[ValentineWeather]

Most places don't get a constant breeze. I have 2 stations at different locations, the one in town where i live without aspiration I would average around +2 (1.1C) higher daytime and as much as +5F (2.7C) at times. The other station has a constant daytime breeze 4+ mph and I don't need outside aspiration.

[jgentry]

Here is a comparison with the humidities are in the mid ranges.

Remember I have the 75 and drying out from high humidities from the last few days.

[hwcorder]

Kestrels are extremely accurate instruments if used correctly.  We use them at our CWO for backup if ASOS goes down.

For my station I use an SHT31 in the FARS for temperature and a SHT75 in a passive shield for humidity. This is exactly like ASOS is setup except its humidity sensor can be heated when air nears saturation.  Ive had this 31 for nearly a year now without a filter and it still reaches 100% easily in fog.  The 75 tops out at 98% so I use a slight slope offset so it reports 100%.  The 75 really outperforms the 31 in the mid and lower ranges though as it is usually is between 4-8 percent lower.

[jgentry]

Kestrels are extremely accurate instruments if used correctly.  We use them at our CWO for backup if ASOS goes down.

For my station I use an SHT31 in the FARS for temperature and a SHT75 in a passive shield for humidity. This is exactly like ASOS is setup except its humidity sensor can be heated when air nears saturation.  Ive had this 31 for nearly a year now without a filter and it still reaches 100% easily in fog.  The 75 tops out at 98% so I use a slight slope offset so it reports 100%.  The 75 really outperforms the 31 in the mid and lower ranges though as it is usually is between 4-8 percent lower.

Thanks for your input! My 75 typically tops off at 98-99%. Even though it’s better in the mid-ranges than the 31, I still think it has a little wet bias at times.  Out of curiosity, how do y’all use your Kestrel meter to make measurements? I know it’s tough to get accurate readings if you don’t have constant airflow to the sensors. This is how I set up my Kestrel DROP 3 meter. Planning on using this during late freeze events for fruit crops.

[hwcorder]

Kestrels are extremely accurate instruments if used correctly.  We use them at our CWO for backup if ASOS goes down.

For my station I use an SHT31 in the FARS for temperature and a SHT75 in a passive shield for humidity. This is exactly like ASOS is setup except its humidity sensor can be heated when air nears saturation.  Ive had this 31 for nearly a year now without a filter and it still reaches 100% easily in fog.  The 75 tops out at 98% so I use a slight slope offset so it reports 100%.  The 75 really outperforms the 31 in the mid and lower ranges though as it is usually is between 4-8 percent lower.

Thanks for your input! My 75 typically tops off at 98-99%. Even though it’s better in the mid-ranges than the 31, I still think it has a little wet bias at times.  Out of curiosity, how do y’all use your Kestrel meter to make measurements? I know it’s tough to get accurate readings if you don’t have constant airflow to the sensors. This is how I set up my Kestrel DROP 3 meter. Planning on using this during late freeze events for fruit crops.

Cool little device!
My 75 seems to have a slight wet bias when compared to my Vailsala but its only about 2%, much better than the 31. 
As far as the kestrels we use at the airport, the biggest problem is finding a shady area away from the buildings and parking lots.  The airport itself is quite large and higher than surrounding topography so you can usually find a decent breeze. If not we can wave it back and fourth to create artificial aspiration lol.

[jgentry]

Kestrels are extremely accurate instruments if used correctly.  We use them at our CWO for backup if ASOS goes down.

For my station I use an SHT31 in the FARS for temperature and a SHT75 in a passive shield for humidity. This is exactly like ASOS is setup except its humidity sensor can be heated when air nears saturation.  Ive had this 31 for nearly a year now without a filter and it still reaches 100% easily in fog.  The 75 tops out at 98% so I use a slight slope offset so it reports 100%.  The 75 really outperforms the 31 in the mid and lower ranges though as it is usually is between 4-8 percent lower.

Thanks for your input! My 75 typically tops off at 98-99%. Even though it’s better in the mid-ranges than the 31, I still think it has a little wet bias at times.  Out of curiosity, how do y’all use your Kestrel meter to make measurements? I know it’s tough to get accurate readings if you don’t have constant airflow to the sensors. This is how I set up my Kestrel DROP 3 meter. Planning on using this during late freeze events for fruit crops.

Cool little device!
My 75 seems to have a slight wet bias when compared to my Vailsala but its only about 2%, much better than the 31. 
As far as the kestrels we use at the airport, the biggest problem is finding a shady area away from the buildings and parking lots.  The airport itself is quite large and higher than surrounding topography so you can usually find a decent breeze. If not we can wave it back and fourth to create artificial aspiration lol.

It would be nice if it were possible to take the sensors that Kestrel uses for temperature and humidity and somehow make it work with the Davis unit.

[jcookjr82]

My Vue hit 100% humidity last January without offsets/adjustments.

[jgentry]

My Vue hit 100% humidity last January without offsets/adjustments.

How old is your Vue?

[CW7491]

Kestrels are extremely accurate instruments if used correctly.  We use them at our CWO for backup if ASOS goes down.

For my station I use an SHT31 in the FARS for temperature and a SHT75 in a passive shield for humidity. This is exactly like ASOS is setup except its humidity sensor can be heated when air nears saturation.  Ive had this 31 for nearly a year now without a filter and it still reaches 100% easily in fog.  The 75 tops out at 98% so I use a slight slope offset so it reports 100%.  The 75 really outperforms the 31 in the mid and lower ranges though as it is usually is between 4-8 percent lower.

My experience with the SHT75 and the Davis SHT31 is exactly the same, except for the easily hitting 100%. The SHT75 is demonstrably more accurate in the mid and low ranges of humidity, even if it has its own wet bias. What I find frustrating and what I have been trying to understand is why this is the case. I have been recently following data from a few new Acurite Atlas weather stations, and despite the issue related to that weather station, I have seen from their forum posts (http://www.wxforum.net/index.php?topic=35220.0) that it also uses the SHT31, but using the I2C protocol. There is an Atlas in Eastern Washington state (https://www.wunderground.com/personal-weather-station/dashboard?ID=KWAANATO5&cm_ven=localwx_pwsdash) that has reported sustained 100% humidity at 20F, which is almost unimaginable on a VP2, aside from the occasional 100% spike. I've also seen an Acurite SHT31 perform better at the midranges as I've posted before in this forum.

I know absolutely nothing about I2C and Sbus or the 'analog' version of the SHT31 that I have learned from this forum that Davis uses. What I have noticed looking at the Sensirion datasheets between their I2C and 'analog' versions of the SHT31 is that the analog version requires a conversion of signal output to achieve a temperature and humidity readout and Sensirion publishes on their datasheets the equations to do so. The I2C version does not require such a conversion. Looking back to the SHT11, which Davis originally used, these sensors also required conversions of the signal output and those equations are also published. I presume the VP2 must make these conversions. Is that correct? And if it does, I presume it does these conversions at the ISS, so that a firmware update to the console would not be helpful in updating the new conversion equations for the SHT31. Where I am going with all this and what I'd be interested to hear about from people with bigger brains that understand this stuff is this:

Is the VP2 SHT31 poor humidity performance possibly due to the ISS using a signal output conversion for the old SHT11 and not the appropriate equations for the new SHT31 because there is no way for Davis to retroactively update them in the ISS? Is the use of these old conversions also possibly why they direct putting in a -0.9F temperature correction in an "old" transmitter to approximate the difference in conversion of signal output between the SHT11 and SHT31? Is it also a possible explanation as to why the humidity performance of the SHT75 is superior because it is using a conversion more accurate for the SHT75's signal output?

Just putting it out there because I'd love to get something that at least approximates the stated accuracy specifications of the SHT31. Assuming I'm understanding this correctly (someone who gets this stuff please correct me if I am wrong), I really wish Davis would move to I2C with their Sensirion sensors.

[johnd]

I know absolutely nothing about I2C and Sbus or the 'analog' version of the SHT31 that I have learned from this forum that Davis uses.

To the best of my knowledge the VP2 does not use the analog SHT31. It is a digital sensor with the SBus interface (which doesn't seem to appear in the public listing of SHT31 data sheets - it's presumably an OEM-only part) using a digital data format similar to I2C. So any allusion to analog is probably a red herring.

[CW7491]

I know absolutely nothing about I2C and Sbus or the 'analog' version of the SHT31 that I have learned from this forum that Davis uses.

To the best of my knowledge the VP2 does not use the analog SHT31. It is a digital sensor with the SBus interface (which doesn't seem to appear in the public listing of SHT31 data sheets - it's presumably an OEM-only part) using a digital data format similar to I2C. So any allusion to analog is probably a red herring.

johnd, as always, thanks for your knowledgeable input ...

Forgive my ignorance, but why would the SHT11 and 75 (both to my understanding digital sbus sensors) require conversions of the their signal output as outlined in their datasheet? Would the sbus SHT31 require a similar conversion? And why would you have to apply an offset to a digital readout to get an accurate temperature measurement? What am I misunderstanding?

[johnd]

Sorry I don't have any more detailed info, beyond being pretty certain that that the VP2 SHT31 is not analog. I guess it hinges around what 'conversion' means - I'd assume that it was just some algorithm for processing the raw sensor readings which are maybe not in final real-world units, rather than any implication that it relates to any A/D conversion. And personally I don't read too much into the slightly different calibration needed for 3x vs 1x - I've just assumed that it's exactly that - ie a very slight difference in calibration.

[jerryg]

I have blown a few brain cells over this stuff, just keep in mind that all of the sensors used by Davis uses the SBus format and the reason for the offset is because the new 31 uses a different formula for calculating the temp and the old iss needs the offset to get the correct temp. All the calculations are done on board in the sensor and sent digitally to the iss. Just ignore the i2c format, it is just another way of communicating. If the i2c would work on the iss there would be a lot more sensors we could try to use with the iss. I read somewhere that guys using hobby boards with the i2c format tried using the SBus sensor and some got results by rewriting some sections of the format to make it work. That is way more than i can even consider doing and until Davis changes over to i2c or someone comes up with a plug and play converter it is what it is.

[CW7491]

Yes, that’s right. The data sheet states it’s to correct for non-linearity of the raw output and for temperature compensation. I just can’t seem to understand why these Davis mounted sensors don’t seem to perform as well as the same sensors mounted by other manufacturers or even as well as older generation sensors, with poorer specs, not mounted by Davis. The errors that I see are systemic and predictable (ie not random). They always read too wet at the mid and low humidity range. I suppose it’s something I’ll just need to accept (but obviously am having trouble doing!)

[CW7491]

I have blown a few brain cells over this stuff, just keep in mind that all of the sensors used by Davis uses the SBus format and the reason for the offset is because the new 31 uses a different formula for calculating the temp and the old iss needs the offset to get the correct temp. All the calculations are done on board in the sensor and sent digitally to the iss. Just ignore the i2c format, it is just another way of communicating. If the i2c would work on the iss there would be a lot more sensors we could try to use with the iss. I read somewhere that guys using hobby boards with the i2c format tried using the SBus sensor and some got results by rewriting some sections of the format to make it work. That is way more than i can even consider doing and until Davis changes over to i2c or someone comes up with a plug and play converter it is what it is.

Thanks jerryg

[jgentry]

Yes, that’s right. The data sheet states it’s to correct for non-linearity of the raw output and for temperature compensation. I just can’t seem to understand why these Davis mounted sensors don’t seem to perform as well as the same sensors mounted by other manufacturers or even as well as older generation sensors, with poorer specs, not mounted by Davis. The errors that I see are systemic and predictable (ie not random). They always read too wet at the mid and low humidity range. I suppose it’s something I’ll just need to accept (but obviously am having trouble doing!)

Same here!

[mcrossley]

Yes, that’s right. The data sheet states it’s to correct for non-linearity of the raw output and for temperature compensation.

Yep, I've raised this before on the forum, the correction factors for the SHT15 are different from the SHT31.
The firmware in the "old" ISS's hasn't changed, so maybe they read the SHT31 slightly incorrectly (and need the additional 0.5 correction).
The "new" ISS's don't need the 0.5 correction, so hopefully have the correction factors for the SHT31 applied - and presumably would be wrong if an SHT15 range sensor were used?

[jerryg]

Yes it would, you would need to add +.9 correction factor in the case of new iss and old sensor.

[mcrossley]

I meant more that the correction factors would be wrong rather than the temperature offset.

[CW7491]

If it’s not the conversions, is it the potting material/conformal coating they use to prevent the corrosion issues they had a few years ago with the SHT11 that pollutes the sensor? How can my homebrew SHT75 with my miserable soldering skills give more accurate results than a Davis manufactured and tested SHT31? It makes no sense but the results are plainly evident

[jerryg]

If you look at the specs on the sensors the 75 is just about as good as the 35 and 31 and as long as i have been testing with the 75 i haven't noticed any wet bias at all and it has been pretty linear from 18%, the lowest rh so far to 99% and i got one that will hit 100% in dense fog, so as long as the 75 is still available i will stay with it. The 75 was used in humidity probes by major makers of weather gear and only dropped it because of the phasing out of the sensor. I had one stretch of humidity that was above 98% for 2 days and when it finally cleared out the readings went down just like they should with no wet bias at all. I am surprised Davis didn't go to the 75 as an upgrade, could have made it with a socket so sensors could easily be changed out. Now that would have been a great sensor.

[jgentry]

If you look at the specs on the sensors the 75 is just about as good as the 35 and 31 and as long as i have been testing with the 75 i haven't noticed any wet bias at all and it has been pretty linear from 18%, the lowest rh so far to 99% and i got one that will hit 100% in dense fog, so as long as the 75 is still available i will stay with it. The 75 was used in humidity probes by major makers of weather gear and only dropped it because of the phasing out of the sensor. I had one stretch of humidity that was above 98% for 2 days and when it finally cleared out the readings went down just like they should with no wet bias at all. I am surprised Davis didn't go to the 75 as an upgrade, could have made it with a socket so sensors could easily be changed out. Now that would have been a great sensor.

I still noticed a wet bias at times with the 75 but at other times, it does well. But I’ll rather deal with the 75 errors than the 31.

[jgentry]

But like CW7491, just do not understand why Davis has the humidity problems it has. I have a Acurite temp/hum/lightning sensor and it doesn’t have the wet bias issues. But I will say it does have a dry bias above 70% (IMO). Even the Ambient Weather WS-2902 humidity sensor doesn’t have the issues like Davis has.