Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 30559 times)

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Offline jgentry

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What i have decided to try now with my test setup that has the Davis filter on it is to put a sf2 on it with the Davis shield and see what happens to the response time. Sure would make for some really good shielding, the Davis for the big stuff and the sf2 for the fine stuff. Should be interesting if nothing else. The last info i saw on the air flow was 24 cfm daytime and 11 cfm at night. Also read where the shield is designed for low air flow and in air intake is designed to increase the air flow by 2.7 times by narrowing the input.

Let us know what your test results show.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Online mcrossley

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Also read where the shield is designed for low air flow
I think the low figure for the shield (0.1Ķm particles at 0.05m/s air flow.) is related to the particles getting through the membrane at the stated flow rate *through* the membrane, not the max air flow you can subject it to.
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Online mcrossley

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But there is no air flow though (to a good approximation) through the Sensirion filter. The moisture diffuses through it. Airflow past the sensor keeps it at ambient temp, and keeps 'fresh' air ready to diffuse in/out.
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Offline jerryg

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Since we are talking about air flow i have wondered what the fan deflector does. I thought it was used to mix the air in the sensor chamber but really don't know and i can not find any info on it. :?:

Offline kcidwx

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But there is no air flow though (to a good approximation) through the Sensirion filter. The moisture diffuses through it. Airflow past the sensor keeps it at ambient temp, and keeps 'fresh' air ready to diffuse in/out.

Correct, you are not passing air through the filter like one would think like an air filtration system. However, even air moving past the sensor filter, you will still get particles that end up on the sensor. The FARS just makes the problem worse by the volume of air it moves through the radiation shield chamber. A really good sensor filter can help but the trade-off can be reduced response time.

I'd like to see an up-close photo of an SHT-31 sensor that's been in FARS service for 24 months that's had the stock filter on it to see how clean it looks. It should look clean to the untrained eye but further up-close magnified inspection might show contamination.

« Last Edit: July 12, 2018, 01:22:34 PM by kcidwx »
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Offline ValentineWeather

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I'm going to replace sensor for sure each spring and depending how humidity looks going into fall again on x2 stations so 2 to 4 new units yearly.

The passive shield idea will reduce sensor contamination 10 maybe 50x depending on aspiration rate but averaging 2į warmer over long term really changes data when it comes to averages.  In truth however the data from a good passive shield is probably closer to what the old Cotton Region (Stevenson) type shelters provided.
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Offline kcidwx

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I'm testing three of these evaluation boards right now. RH spec: 1.5%

https://www.idt.com/products/sensor-products/humidity-sensors

I'll run them against my three SHT-31 evaluation boards as well.
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Offline jgentry

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I'm testing three of these evaluation boards right now. RH spec: 1.5%

https://www.idt.com/products/sensor-products/humidity-sensors

I'll run them against my three SHT-31 evaluation boards as well.

Nice! It would be interesting to see how they compare.

Just reading the spec sheet, Davis would do well to get their transmitter to take I2c and switch over to IDTís HS 3001. Thatís IMO.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline kcidwx

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I'm testing three of these evaluation boards right now. RH spec: 1.5%

https://www.idt.com/products/sensor-products/humidity-sensors

I'll run them against my three SHT-31 evaluation boards as well.
Which kit includes the "data logging" chip & coin battery?

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Offline ValentineWeather

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I'm testing three of these evaluation boards right now. RH spec: 1.5%

https://www.idt.com/products/sensor-products/humidity-sensors

I'll run them against my three SHT-31 evaluation boards as well.
Which kit includes the "data logging" chip & coin battery?

The first kit was this one. Looks like others are now out.
https://www.sensirion.com/en/environmental-sensors/humidity-sensors/development-kit/
Randy

Online johnd

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Just reading the spec sheet, Davis would do well to get their transmitter to take I2c and switch over to IDTís HS 3001. Thatís IMO.

But aren't SHT35 and HS3001 pretty much on a par? Actually SHT35 looks better in the 90-100% region. Where would HS3001 score over SHT35?

Not saying there aren't differences - they're just not too obvious from the specs, other than details like 0.1% difference in nominal tolerance which may well get lost anyway in slightly different interpretation of the specs - eg what is the difference between tolerance and accuracy or nominal vs max - and differences in eg binning. And yes Davis would need to move away from Sensibus to modern I2C if these were to be relevant to use in VP2 units.
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Offline jgentry

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I'm testing three of these evaluation boards right now. RH spec: 1.5%

https://www.idt.com/products/sensor-products/humidity-sensors

I'll run them against my three SHT-31 evaluation boards as well.

Nice! It would be interesting to see how they compare.

Just reading the spec sheet, Davis would do well to get their transmitter to take I2c and switch over to IDTís HS 3001. Thatís IMO.

But I just saw that it can do either digital or analog output.
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline jgentry

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Just reading the spec sheet, Davis would do well to get their transmitter to take I2c and switch over to IDTís HS 3001. Thatís IMO.

But aren't SHT35 and HS3001 pretty much on a par? Actually SHT35 looks better in the 90-100% region. Where would HS3001 score over SHT35?

Not saying there aren't differences - they're just not too obvious from the specs, other than details like 0.1% difference in nominal tolerance which may well get lost anyway in slightly different interpretation of the specs - eg what is the difference between tolerance and accuracy or nominal vs max - and differences in eg binning. And yes Davis would need to move away from Sensibus to modern I2C if these were to be relevant to use in VP2 units.

True, the 35 does looks better but will it act like the 31 and not perform within specs under certain conditions? It also appears according to the specs sheet that the HS 3001 seems more durable sensor. I could be wrong however
« Last Edit: July 12, 2018, 03:07:13 PM by jgentry »
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline kcidwx

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I'd like to test the 35 but it doesn't work with my Evaluation Kit.  :sad:

https://www.sensirion.com/en/environmental-sensors/humidity-sensors/evaluation-kit-ek-h4/
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Online johnd

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To me, that (end-of-life) indicates that Sensirion is moving on to the I2C interface, necessitating a whole new "development kit."

As I read it, it's just that particular evaluation kit that's EOL, not the SHT3x series.

And isn't the SHT3x series mainly I2C already, albeit with some other options available like the legacy Sensibus variant that Davis uses? For anything that would be compatible with a VP2 you need a sensor that has a Sensibus interface rather than the mainstream I2C.


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Online johnd

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You are correct, the whole SHT-3X line are I2C and/or analog. Don't know what Sensibus is.

I'm no expert but AIUI Sensibus is a kind of older variant of I2C which is what Davis SIM boards use but which is incompatible with 'proper' I2C. Any sensor that wants to talk to a VP2 must use Sensibus and not I2C is how I understand it. If you look in detail I think you'll see that SHT31 is available with a Sensibus interface, but SHT35 isn't (was the last I heard). Here's one explanation:

https://www.i2cchip.com/humidity_sensors.html
« Last Edit: July 12, 2018, 05:02:56 PM by johnd »
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Offline openvista

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The way I understood it, Davis had to collaborate with Sensirion to devise the Sensibus solution specifically for them. I don't think anyone else uses it besides Davis. Hence, their leverage is somewhat limited when it comes to squawking about humidity performance.

I'm sure Sensirion's first defense would be: you chose to forego our OEM cap. Davis' cap reminds me of a pool filter meant to keep only the largest objects out rather than a serious attempt at protecting the sensor from corruption. The reasoning might have been airflow related but at least in FARS units those sensors are SUPER sensitive and could use some delay.

I've put in an order for 4 of the OEM caps. I'll apply 1 first to my passive shield and see how it performs. Then I'll do the active shield. That leaves 2 spares for the future (or breakage).
Davis Vantage Pro2 FARS | https://marquetteweather.com | EW7933

Offline CW2274

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2 spares for the future (or breakage).
That's why I just picked up five. #-o

Offline ValentineWeather

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The Good The Bad The Ugly.

First the good, replaced one of my sensors yesterday and now easily hits 99% this is my first sensor ever to do this. So my other sensors were probably worn out when I received them.

The bad sensor was received in same bubble wrap Davis ships to retailers because retailer happened to be out of  anti static ESD plastic bags he normally ships in so unknowing used the same bubble wrap made of gaseous polyethylene which is absolute sin and contaminates sensor. Not only that Davis is shipping sensors open air so we are receiving used sensors from the get-go. 

The ugly Davis Instruments is negligent and going against all Sensirion protocol of handling humidity sensors.
 
« Last Edit: July 13, 2018, 05:54:26 AM by ValentineWeather »
Randy

Offline jgentry

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Wow.  SMH.

Could that be a part of the reasons why Sensirion humidity sensors have issues?
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Offline ValentineWeather

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Yep and I bet Sensirion won't be all too happy either. It's their product getting a bad reputation from the mishandling.
Randy

Offline openvista

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So, here's the document with handling procedures: https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/2_Humidity_Sensors/Sensirion_Humidity_Sensors_Handling_Instructions.pdf. It gives explicit instructions about wrapping the sensor in an anti-static, sealed bag and not exposing it to volatile compounds. Bubble wrapping the sensor violates ALL these instructions. Thus, Davis has been sending everyone contaminated sensors.

Furthermore, by choosing to use their own cap which lets in nearly every particle instead of the manufacturer's cap which, supposedly, filters out 99.99% of particles, Davis dooms the sensor to inaccuracy in pretty short order.

Inexcusable doesn't even begin to describe the situation.
« Last Edit: July 13, 2018, 09:12:15 AM by openvista »
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Offline jgentry

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Iím curious if Davis used the HS3001 sensor, would their actions have the same effect on the sensor as Sensirionís?
Davis Vantage Pro2. SHT-75. WU: KXALJEMI2 & KALTHORS2. CWOP/APRS: C6353 & E6358

  

Online johnd

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Iím curious if Davis used the HS3001 sensor, would their actions have the same effect on the sensor as Sensirionís?

You're asking 'if'? But obviously they can't use it (the HS3001) without a significant redesign of the SIM board and firmware, which is I suspect pretty unlikely at this stage in its product cycle. So it's presumably just a hypothetical question that can't really be answered?
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Offline kcidwx

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Sensor mishandling according to the manufacturers handling document is not uncommon. If you inquire they will just say the handling guidelines are overkill. It's like every time I step foot into a weather office and the computer tech has computer memory modules just thrown around on his work bench. If you inquire about why don't you keep the memory in an ESD bag, use an anti-static wrist strap with an anti-static work mat, you will get told, "Been doing it this way for 20 years. Never had a problem." You'll get the same response with sensor handling. Unless they see a high failure rate, it's business as usual.
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