Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 5212 times)

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Offline ValentineWeather

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I would like to see the SHT31 with some side by side temperature data against vaisala temperature sensor.
The ASOS here has been running around 2° warmer than my instruments. I've taken nearby data with portable FARS and Davis console nearby and come up 1.7° lower vs what the ASOS 5 minute average shows.  I've cross checked the SHT31 with NIST certified and they are very close within .1-.2F.
Does anyone know of a study where this has been conducted?
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Offline kcidwx

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ASOS still uses the 1088 system from TSL for ambient air temperature. The chilled mirror sensor in the 1088 is still used as a backup to the DTS1 humidity sensor.

https://tslinc.com/systems/instruments-hygrothermometer/
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Offline jerryg

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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.

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 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline mcrossley

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Also read where the shield is designed for low air flow
I think the low figure for the shield (0.1µm 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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Offline Old Tele man

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Sensirion needs to develop a centripetal "filter" stack that works exactly like those used in diesel engines, where air comes in and then gets swirled around fast enough to "sling" physical particles outward away from the central flow pattern...that would mitigate the particulate clogging problem...AND...permit continuous high (AC-fan) airflow.
« Last Edit: July 12, 2018, 11:31:15 AM by Old Tele man »
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Offline 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 Old Tele man

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I'm talking FARS applications...how to mitigate the large amounts of airborne junk being continuously thrown "at" the sensor(s).
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB
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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 2° 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.
Randy

Offline Old Tele man

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'Globally Warming' by sensor bias!  :roll:
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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 Old Tele man

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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?
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

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 IDT’s HS 3001. That’s IMO.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

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 IDT’s HS 3001. That’s 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 IDT’s HS 3001. That’s IMO.

But I just saw that it can do either digital or analog output.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

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 IDT’s HS 3001. That’s 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 »
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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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Offline Old Tele man

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

If I were still active in the "industry" I would contact them for "engineering information" and possible(?) prototyping example to evaluate...but, I'm no longer in the industry.
« Last Edit: July 12, 2018, 03:55:45 PM by Old Tele man »
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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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Offline Old Tele man

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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.

You are correct, the whole SHT-3X line are I2C and/or analog. Don't know what Sensibus is.
« Last Edit: July 12, 2018, 04:36:15 PM by Old Tele man »
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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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