Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 45060 times)

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Offline SLOweather

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

That'll work. Good catch. As you can see from my previous calcs, the sensors need to be closely matched, either physically, or corrected in your software, to make the calculation worthwhile.

Offline mcrossley

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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......
Good, in the ISS or the console?
Mark

Offline jerryg

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Well the sun came out and the wind has been1 to 5 mph a really good test for my dfars test and i am surprised so far that the dfars has been running cooler than both 24 hr fars readings by as much as .5f to 1.0f. I am shocked that so far the high volume fan is working well and if it keeps going like this then it will be really interesting tonight when i turn off the fan an use it as a regular shield and if it works out then that might solve the humidity problem at night. Stay tuned for further updates  :grin:

Offline Old Tele man

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Well the sun came out and the wind has been1 to 5 mph a really good test for my dfars test and i am surprised so far that the dfars has been running cooler than both 24 hr fars readings by as much as .5f to 1.0f. I am shocked that so far the high volume fan is working well and if it keeps going like this then it will be really interesting tonight when i turn off the fan an use it as a regular shield and if it works out then that might solve the humidity problem at night. Stay tuned for further updates  :grin:
Get one of those "daylight/nitelight" sensors to automatically shut off fan at dusk and then turn it back on at dawn?
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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......

Good, in the ISS or the console?

In the ISS.....
« Last Edit: July 24, 2018, 03:29:15 PM by rdsman »
Ray

Offline CW7491

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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 mount it just like hwcorder showed. This is the first sensor Ive used without a filter, so its only been 6 mos so far ...

As far as the wiring to the SHT75, Ive found youll need to use the 6 wire cable even though the sensor only uses 4 wires, but Im no expert, so someone else may know better.

Referencing the data sheet, the wiring is:
1. SCL - white
2. VDD - yellow
3. VSS - green
4. SDA - blue

Red - not used
Black - not used

Offline jerryg

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I have the off/on photocell set up on my main station site but for testing l use one closer to the house, getting to old to do a lot of walking lol. So i will just up plug power tonight and see what happens if good results i will move it out to my main site and hook it up to my switchable power source.

Offline jerryg

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You can use 4 conductor wire with no problem just take the fourth wire to pin 6 leaving 4 and 5 blank.

Offline jerryg

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Quick update fars max so far 95.6   dfars 94.2 looking good.

Offline jerryg

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Final high update max fars 96.6  max dfars95.4

Offline WxLover16

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First off I wanna say how awesome it is for this topic to have ignited so much posting and enthusiasm and knowledge of everything that I love, temperature/humidity accuracy. However, so much posting of this vs. that sensor has me so confused. Some people say the 15 is better, some say the 31 is better, some say the 75 is better. I live in North GA in a generally very humid climate in summer, and my 31 has hit 100% once this summer but regularly hits the upper 90s every night, so my guess is that I have a good sensor. However, my main goal is temperature accuracy (always has been, always will be). WRT that, which is better, the 31 or 75? Also, Dendrite has possibly inspired me to remove my filter for more roller coaster readings which I love (to be able to catch all the extremes of the day, no smoothing crap!). I'm still debating it.
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Offline jerryg

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Well i have four of the 31 sensors and they all show the same temp within .1degrees, really great on temperature accuracy. The 15 and  75 are pretty good in the mid ranges but i have read not so good in the really cold climates. Here in south Texas it doesn't get cold enough for me to have tried that out.

Offline ValentineWeather

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Both the 15 and 75 are close to +/- (2F)  @ -4F when the 31 is only about +/- (.4F) @ -4.  I average around 22 days below 0F so neither is great for cold climates.

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« Last Edit: July 25, 2018, 12:00:39 AM by ValentineWeather »
Randy

Offline dendrite

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My 75 was pretty much in line with other stations during the winter I used it.

Offline CW7491

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No question the SHT31/35 are updated, improved and have better specs. If I could figure a way to use those sensors straight from the manufacturer, or without the wet bias observed from the Davis version, I would prefer it. It is purely anecdotal of course, but in terms of cold temps, I’ve tested the SHT75 as low as -8F side by side with an ASOS in New England and it was spot on.

We all have to make choices and compromises based on our unique circumstances and climate. My goal here is not to sell SHT75s (which are quickly running out of production), but to help change how Davis mounts/engineers these sensors to ultimately achieve their maximum accuracy. I invite all to try on their own, but my experience is humidity performance of the older generation SHT75, which has not been compromised by Davis mounting performed better than the SHT31 sensor with Davis mounting. I’m certain the SHT31 straight from the manufacturer would perform even better, but unfortunately that’s not an option at this point.
« Last Edit: July 25, 2018, 05:02:29 AM by CW7491 »

Offline jerryg

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Overnight report, at 6am all readings were the same with dense fog present. All three had a temp of 74.1 and humidity of 97%. When i plugged the fan in the dfars went to 98% where it is now. Just a note the fars with the 11 in it is one i made so is not contaminated like the Davis, i bought 10 sht11 sensors in factory sealed tape reel containers and mounted them on smd boards. The 11 tracks right with the new 15. If the dfars performs the same way today as it did yesterday and last night it looks like i can run the dfar and eliminate the night time fan which is great. More to come when the sun is up and the fog burned off.

Offline jerryg

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A little more info, i wish Davis would report humidity to the tenth would make it easier to compare readings. I took my modified hand held humidity meter out to the shields to see how close the readings are and the main fars was reading 96.97%, the sht11 was 97.03% and the dfars 15 was 97.98% .

Offline ValentineWeather

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Well some good news about reconditioned sensors.
I reconditioned 3 more sensors the other day 2-SHT31's and 1-SHT15 and just got around to testing.

I did the full rehydrate 20 hours unlike first time when disappointed not seeing any change in sensor performance only maxed at 96%. 
This time 2 of the 3 sensors reached 100% humidity in non polluting ESD bag (wet paper towel inside)  in about 8 minutes time very fast response.  I also went to 125c on 10 hour bake after reading how Spark FUN did all their sensors.

BAGS: https://www.amazon.com/gp/product/B07CX62R99/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1

I've been storing each sensor in these bags and WOW I've never had a sensor hit 100% until now without using software Cumulus bias. 

SHT31-100%
SHT31-98%
SHT15-100%

I'll get the 31 installed as primary sensor and see if the reconditioning helped with with high heat/hum bias.

This is the raw reading from one of the sensors s using envoy8x.  I left Davis filter on during test.
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« Last Edit: July 25, 2018, 10:00:29 AM by ValentineWeather »
Randy

Offline jerryg

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That's good news hope it proves out in the long term. =D>

Offline ValentineWeather

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That's good news hope it proves out in the long term. =D>

Yes! I'm about to install the 31 that hit 100%...Excited  \:D/
Randy

Offline ValentineWeather

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I do wonder about over saturation.  I  couldn't remember which 31 hit 100% so redid test this time it took about 15 minutes, double the first time to reach 98 and 99%. The 100% wasn't reached on second test even with new wet paper towel.  Maybe if I waited longer..
Randy

Offline DoctorKnow

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I've been thinking and have an idea, (which I know is going against proper placement of sensors), but how about raising your ISS higher away from the ground? As I recall, when I had my 5n1, (which uses sensiron) higher at like 25 feet, it read lower on humidity. Would raising the station to say 15 feet help possibly? Just a thought... ? Or try placing the sensor near pavement? (crazy I know for temp)

Offline ValentineWeather

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I've been thinking and have an idea, (which I know is going against proper placement of sensors), but how about raising your ISS higher away from the ground? As I recall, when I had my 5n1, (which uses sensiron) higher at like 25 feet, it read lower on humidity. Would raising the station to say 15 feet help possibly? Just a thought... ? Or try placing the sensor near pavement? (crazy I know for temp)

Think you are right especially those over lawns. I've had so much rain still haven't watered except early spring 1 time after weed and feed.  I do believe some moisture is always on grass because you see dew until about 1 pm many days.  Only mow late afternoons or grass gets stuck under carriage and plugs becoming pia to clean out.
Randy

Offline WxLover16

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Dendrite, please check your PMs. Thanks.
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Offline openvista

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I've been thinking and have an idea, (which I know is going against proper placement of sensors), but how about raising your ISS higher away from the ground? As I recall, when I had my 5n1, (which uses sensiron) higher at like 25 feet, it read lower on humidity. Would raising the station to say 15 feet help possibly? Just a thought... ? Or try placing the sensor near pavement? (crazy I know for temp)

Think you are right especially those over lawns. I've had so much rain still haven't watered except early spring 1 time after weed and feed.  I do believe some moisture is always on grass because you see dew until about 1 pm many days.  Only mow late afternoons or grass gets stuck under carriage and plugs becoming pia to clean out.

Raising the ISS will only make it more difficult to achieve 100% humidity on nights with radiational cooling and dew.

The issue isn't environmental. That's proven by Ron's tests in a controlled environment. Also, up my way we've gone for long stretches without rain this summer. Yet I haven't noticed any reduction in the wet bias of my defective SHT31 sensor during the dry spells.
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