Author Topic: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?  (Read 25070 times)

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Offline dendrite

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1225 on: September 14, 2018, 11:29:56 AM »
Does anyone knows where you can get a crossover adapter for the RJ12?  If I can get a RJ 12 adapter, then I wonít have to worry about getting rid of the 45 plug thatís on the EH-4 cable.
The thing is you need the wires to line up (clk, vdd, gnd, data) according to the Davis ISS requirements.

Offline dendrite

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1226 on: September 14, 2018, 11:48:59 AM »
Another mostly 99% RH night with a few 100% obs thrown in. The fog started forming before sunset last night and was very thick by morning. The local ASOS at CON was reporting M1/4SM visibility at times. However, they never had an hourly METAR with 100% RH during the fog...the best they could do was a 1F dewpoint depression (DD). CON is a 1st order climate station with official obs going back to the 1860s. So there's an example of a 1st order ASOS reporting the lowest possible automated visibility in fog (M1/4SM) with an RH under 100%. So either the ASOS is wrong or we don't need to be seeing 100% RH every time we experience some fog. The funny thing is that CON reported a couple hours of 100% RH once the visibility began to rise. They even had a 100% with a visibility of 7SM (not even a BR ob). The LCI AWOS near me also reported M1/4SM, but they did have a few hourly METARs with a 0F DD. 1P1 to my north is a little different. That's an AWOS that reports to the exact 0.1C in every observation (not 1F resolution converted to degC). They had 1/4SM visibility and the RH maxed out at 97%.

So in summary...lowest vis this morning and the lowest DD during the FG (fog) period (1/2SM vis or lower)

Me 0F (100%)
CON M1/4SM 1F (97%)
LCI M1/4SM 0F (100%)
1P1 1/4SM 0.9F (97%)

Offline dendrite

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1227 on: September 15, 2018, 08:32:23 AM »
heh...right on cue...

It looks like my 75 is failing. It stayed at 89-90% RH overnight despite fog and drizzle. Everyone around me is in the 97-100% range. We’ll finally dry out later today so I’ll give it another night to see if it improves. I’ll probably get another 31 from Ryan and use that through winter. Maybe we’ll have a way to use the 35s by spring.

Offline jerryg

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1228 on: September 15, 2018, 03:23:01 PM »
Well all i have to say is these sensors really suck. I am in a high humidity area and they just plain suck. We have had humidity above 80% for the last three days, i had put out two new 75's in my two Barani shields. Well every thing was fine to start with the two reading the same most of the time but yesterday morning  when both sensors had been on 97% all night one started to go down in readings until this am when it was at 75% and the other one was at 98%, so when the rain stopped for a bit i changed it out and when i got it into the house it had returned to 98% and started dropping down to the indoor humidity just like it should. The replacement was showing 95% and holding while the other was at 98%, after a while the replacement was down to 91% and the other was still at 98%, i checked with my acurite and it showed 91%, so i changed out the 98% one for a new sensor thinking it was moisture in the shield but it too settled at 91% so it was the sensor and not the shield. These sensors are really a piece of crap above 80% but the manufacturer does tell you in the specs that it is a piece of crap in high humidity and should only be used between 20% and 80% to insure accuracy. I guess i will have to change out sensors every morning when humidity is high, wow what a great sensor Davis has. Even the new 31 has the same warning about high humidity screwing the readings up. ](*,)

Offline jerryg

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1229 on: September 15, 2018, 07:39:48 PM »
 :lol:

Offline jgentry

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1230 on: September 15, 2018, 08:24:19 PM »
Welp got a rj12 plug on using the cable from Sensirion but the transmitter does not report any data from the 75 sensor.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline jerryg

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1231 on: September 15, 2018, 09:04:08 PM »
If you are sure you got the sensor plugged in the right way then you need to ohm out the cable and make sure you got the wiring right from the jack to the rj connector and that the pins on the jack go to the correct pins on the rj plug.

Online ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1232 on: September 15, 2018, 11:37:45 PM »
Here is pin assignment for the Sensirion cable this will need to be changed to match like jerry said Ohm meter.

https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/0_Datasheets/Humidity/Sensirion_Humidity_Sensors_EK-H4_Datasheet.pdf

Here's the guide for the Davis cable:
White (clock)
Yellow (vdd)
Green (gnd)
Blue (data)

Here is cable image.
Randy

Offline jgentry

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1233 on: September 16, 2018, 03:33:03 PM »
Aight. Here is what my setup currently looks like. Keep in mind that the transmitter doesnít read anything.
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline jgentry

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1234 on: September 16, 2018, 03:39:06 PM »
Aight. Here is what my setup currently looks like. Keep in mind that the transmitter doesnít read anything.

Just taking a good look at the previous picture posted, wires might not be soldered in the right slots 🤦‍♂️
Davis Vantage Pro2. SHT 31. 24hr FARS. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

  

Offline jerryg

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1235 on: September 16, 2018, 10:10:40 PM »
The way the jack is wired now blue should go to pin 6 on rj plug yellow to 2 green to 3 and white to 1

Offline dendrite

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1236 on: September 17, 2018, 07:32:45 AM »
My RH dropped out to 0% this morning. The 31 is going back in when I get home. I wonder if water got into the connector.

Online ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1237 on: September 17, 2018, 07:42:09 AM »
My RH dropped out to 0% this morning. The 31 is going back in when I get home. I wonder if water got into the connector.
I would say sensor more likely or you would of lost everything if connector shorted.
Randy

Offline dendrite

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1238 on: September 17, 2018, 10:08:52 AM »
My RH dropped out to 0% this morning. The 31 is going back in when I get home. I wonder if water got into the connector.
I would say sensor more likely or you would of lost everything if connector shorted.
Yeah who knows. My previous 75 never had an issue and made it through an entire winter. The sensor itself should've dried out yesterday with full sun and mid 80s.

Online ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1239 on: September 17, 2018, 10:25:03 AM »
Early on Davis sensor failure was more common resulting in the water resistant version we now see.
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Offline dendrite

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1240 on: September 18, 2018, 08:34:14 PM »
The SHT31 has been in for 24hrs now. I have a new one coming in from Ryan, but I gave the old one a gentle cleaning with a wet paper towel and put it in. The temp is spot on albeit much less responsive than my 75. The RH performed well all day. Max RH this morning in the rain was 98%, but it's already 97-98% this evening with fog developing. My max RH with this sensor, when I pulled it in July, was down to 93%. So giving it a little cleaning seems to have given it new life.

Online ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1241 on: September 18, 2018, 10:08:41 PM »
Comparison of 75 and 31 in same stock Davis Fars shield at temperatures mid range this is the difference between the two sensors.
 [ You are not allowed to view attachments ]
 [ You are not allowed to view attachments ]
Randy

Offline openvista

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1242 on: September 19, 2018, 10:34:51 AM »
Comparison of 75 and 31 in same stock Davis Fars shield at temperatures mid range this is the difference between the two sensors.

Looks to me like your 75 is slowly getting wetter too. It appears to only average about 1-1.5F below the 31 in dew point during the daytime.

From my informal survey of dozens of VP2s, as well as my own experience with a wet 31, the sensors tend to run at least 2.5 to 3 degrees above the actual dew point when air temps are between 60-80F (15-25C) and humidity is between 30-70%. That translates to something like 6-9% high in humidity.

As time goes on, the SHT sensors (11, 15, 31 and I would assume the 75 since it's basically the 15) also get wetter between 70-90% and drier above 90%. It's the worst of all possible worlds. So if you see a newer, supposedly good sensor converging with your known bad sensor at night, that generally isn't good news.
« Last Edit: September 19, 2018, 10:47:23 AM by openvista »
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Online ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1243 on: September 19, 2018, 10:44:15 AM »
You may be right. I'm still at 97%-98% on all sensors 10am and may not drop out of 90's for another day or so, pushing that 60 hours of saturation before rain stops.
Randy

Offline openvista

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1244 on: September 19, 2018, 11:40:02 AM »
You may be right. I'm still at 97%-98% on all sensors 10am and may not drop out of 90's for another day or so, pushing that 60 hours of saturation before rain stops.

That being the case, then it's all about whether the 75 reads significantly lower in dew point once humidity drops below 80% for awhile. I've seen it take anywhere between 6 to 48 hours for a good sensor to automatically recalibrate after being in high humidity for more than 24 hours (Sensirion may say 60 hours, but it's more like 1-2 days). If the air continues flirting with the 80 percentiles then it can take a long time (days) for the recalibration to occur. If humidity falls into the 60s or lower, then it usually takes just a matter of hours to sort the good from the bad sensors.

My "good" 31 actually reads worse than my bad 31 before it recalibrates. I'm talking an average of 3.5 to 4F (10+%) high in dp when the humidity first starts dropping below 80%. In case anyone doubts this, I've run tests with an accurate device (factory tested to +/-2% and confirmed by testing it at my local airport).

The good news is that both your sensors are still getting up to 97-98%. That tells me they are relatively healthy. Though dendrite would have said the same about his 75 until it suddenly went bad. 

I should point out that max humidity isn't always an indication of whether a sensor is accurate throughout the entire range. My "good" 31 will now only max out at 95% when the air's been saturated for hours. 94% is more common. Generally, though, it's within 2% when humidity is consistently below 80%.
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Online ValentineWeather

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1245 on: September 19, 2018, 12:32:41 PM »
Jerry and I are testing the 100 mesh silk for filters. Jerry has reported recovery time is much faster vs the sf1 filter on the 75, so we are hoping we found the right combo. The silk has faster response times both ways and will hold droplets of water until pressed underneath so looks like it should filter dust rather well also.
Randy

Online Dj1225

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1246 on: September 19, 2018, 05:19:23 PM »
Can someone tell me why the humidity would spike almost 20% in 5 min?  [ You are not allowed to view attachments ]

Offline CW2274

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1247 on: September 19, 2018, 05:29:09 PM »
Well, because your temp went down 2.1F and your dew went up 8.4F.

Online Dj1225

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1248 on: September 19, 2018, 06:00:12 PM »
Has anyone else had spikes in temps and dew point in such a short amount of time like that?

Offline openvista

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Re: The Reliability of the SHT-31 Humidity Sensor & What Psychrometer Should I Buy?
« Reply #1249 on: September 19, 2018, 06:25:46 PM »
Over in the Atlas thread (http://www.wxforum.net/index.php?topic=35084.msg358927#msg358927), it was just revealed by one of the beta testers (nincehelser) that the temp/hum sensor is, indeed, SHT31. The Atlas was just released this week.

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