Topic: SHT31 temperature spikes with solar fluctuations

[wxnut99]

Greetings..

I have a 24hr FARS DAVIS VP2 with recently installed SHT31 T/H sensor and new fan.  Even with the new hardware, I'm noticing rapid temperature spikes of 0.5-1C (1-2F) depending on solar radiation. This is most noticeable when sunshine is fluctuating in and out of cloud cover (as opposed to straight sunny skies)  I realize the air warms up when the sun comes out, but I thought the FARS would dampen out these temperature spikes a bit more. Rather, it seems like the SHT31 sensor has a more rapid response to fluctuating solar input. I have an older VP1 beside this unit (non aspirated), and the temperature signal with that unit is less spiky (smoother)   

Just curious to get some feedback on experience of others regarding "spikiness" of temperature readings with the SHT31 sensor, and/or FARS vs non FARS units.

[CW2274]

I thought the FARS would dampen out these temperature spikes a bit more.

Nope, what you're seeing is exactly as should be. The air sample in the sensor chamber is much more reflective of ambient with the fan than not, whereas the air can stagnate and change more slowly. The 31's pretty quick too. Actually, mine spikes more than yours does, but the solar insolation is through the roof where I live and I use a much stronger a/c fan to boot.

[ValentineWeather]

Yes, agree with CW, naturally aspirated always takes longer to respond to changes but works both ways when the temperature rises and falls. 
You are getting the most accuracy as long as the shield has full sun exposure. Shields with artificial shading ( trees, or other shaded areas) are reading lower than normal day time. 
The aspirated shields will prevent heat build (as much as 5°F, 2°C ) on low wind speed days and has more accurate nighttime temperatures by reducing radiational cooling by keeping air moving. 

[CW2274]

The aspirated shields will prevent heat build (as much as 5°F, 2°C ) on low wind speed days

I'd actually add the word "good" before aspirated....some even with fans ain't worth a damn.

[wxnut99]

Thanks for the replies and feedback. The more rapid response of the SHT31 and FARS is a bit new for me, as I've always had the slower dampened response of an older non-FARS unit. One thing I'm wondering about is if the heightened sensitivity of the SHT31 can actually pick up micro-thermals that are not really representative of the general airmass, which would go unrecorded by slower response units. For example, today my station was recording temperatures very consistently under bright sunshine with afternoon temperatures around 16.5-17.0C. By 4 pm, some cumulus cloud developed and a cloud over my station dropped the temperature down to 15.5C at 4:00 pm. A couple minutes later, the sun reappeared and my station temperature spiked up to a maximum of 17.8C at 4:08 pm, about 1C above the general airmass temperature during the afternoon (as noted by most stations around me as well). A few minutes later we were in a sun/cloud mix and my station temperature stabilized in the 16.5-17.0C range once again (*see attached graph)  So for a brief moment, my temperature spiked to 17.8C which was a bit elevated compared to nearby stations today (including my local airport site 10 km away). This makes me wonder if the faster response of the SHT31 and FARS can sometimes lead to overshooting max temps during variable cloud cover or times of strong solar heating due to sampling of micro-thermals that would otherwise go undetected by slower response units.

[ValentineWeather]

Overshoot is one way of looking at it, but it is the true temperature. 
Down south NWS ASOS works like this.

Quote: At RDU, the weather station is of a type known as "ASOS," for Automated Surface Observing System. The system has to be able to operate continuously with relatively little maintenance and to provide reliable and useful data any time of day. In the case of its temperature sensor, the designers chose to use the following protocol: an instantaneous temperature sample is read from the resistance thermometer once every 10 seconds, then at the end of one minute the six resulting values are combined into a one-minute average reading. At that same time, an average of the most recent 5 one minute readings is calculated to produce a 5-minute average value, which is rounded to the nearest degree F and also converted to the nearest tenth of a degree C. That 5-minute, reported each minute, becomes the basis of all further processing and reporting by the system. Near the top of each hour, the 5-minute average that exists at that time is reported as the "hourly" value. In addition, the system checks each 5-minute average to keep track of the highest one each hour, and eventually to report (just before midnight) the highest one that occurs all day, and similarly track the lowest temperatures. If you think about the variable trace in the image above, a 5-minute average would have a lower value than one of the peaks of those fluctuations, but would also be more representative of what most of us would experience over the course of that 5-minute period. On Sunday, it is quite possible that some of the 10-second readings, and even a few of the one-minute averages, may have reached 106 or even a bit higher, but since those were fluctuations that did not last very long, the 5-minute average never rose higher than 105, which eventually was reported as the high for the day.

Source: https://www.wral.com/weather/blogpost/11302787/?comment_order=forward

[CW2274]

One thing I'm wondering about is if the heightened sensitivity of the SHT31 can actually pick up micro-thermals that are not really representative of the general airmass,

Ha, you outta try it in a bone dry air-mass, heat undulations across the desert floor, much stronger than stock fan, and especially in the summer, my temp looks like a yo-yo.

[havtrail]

Overshoot is one way of looking at it, but it is the true temperature. 
Down south NWS ASOS works like this.

At RDU, the weather station is of a type known as "ASOS," for Automated Surface Observing System. The system has to be able to operate continuously with relatively little maintenance and to provide reliable and useful data any time of day. In the case of its temperature sensor, the designers chose to use the following protocol: an instantaneous temperature sample is read from the resistance thermometer once every 10 seconds, then at the end of one minute the six resulting values are combined into a one-minute average reading. At that same time, an average of the most recent 5 one minute readings is calculated to produce a 5-minute average value, which is rounded to the nearest degree F and also converted to the nearest tenth of a degree C. That 5-minute, reported each minute, becomes the basis of all further processing and reporting by the system. Near the top of each hour, the 5-minute average that exists at that time is reported as the "hourly" value. In addition, the system checks each 5-minute average to keep track of the highest one each hour, and eventually to report (just before midnight) the highest one that occurs all day, and similarly track the lowest temperatures. If you think about the variable trace in the image above, a 5-minute average would have a lower value than one of the peaks of those fluctuations, but would also be more representative of what most of us would experience over the course of that 5-minute period. On Sunday, it is quite possible that some of the 10-second readings, and even a few of the one-minute averages, may have reached 106 or even a bit higher, but since those were fluctuations that did not last very long, the 5-minute average never rose higher than 105, which eventually was reported as the high for the day.

That's a very interesting description, offering lots of insight.

A jittery temperature chart line does not necessarily mean that a sensor is more accurate or responsive. The rapidly-changing SHT31 readings may not be "true" temperatures at all, but simply the result of an underdamped electrical circuit, one that bounces back and forth instead of settling on the "true" temperature somewhere in the middle. The ASOS procedure of multiple samples and averaging is a arithmetic way of dealing with this issue by smoothing out the readings.

Randy's studies of readings during longer periods, like climbing or declining temperatures, with and without sun and wind, are more meaningful in looking at accuracy of a sensor than thinking that a jittery chart line is somehow more accurate.

Rich K.

[ValentineWeather]

Sorry forgot to add source, I thought it was a good explanation of how the ASOS averages the temperature.

[CW2274]

Overshoot is one way of looking at it, but it is the true temperature. 
Down south NWS ASOS works like this.

At RDU, the weather station is of a type known as "ASOS," for Automated Surface Observing System. The system has to be able to operate continuously with relatively little maintenance and to provide reliable and useful data any time of day. In the case of its temperature sensor, the designers chose to use the following protocol: an instantaneous temperature sample is read from the resistance thermometer once every 10 seconds, then at the end of one minute the six resulting values are combined into a one-minute average reading. At that same time, an average of the most recent 5 one minute readings is calculated to produce a 5-minute average value, which is rounded to the nearest degree F and also converted to the nearest tenth of a degree C. That 5-minute, reported each minute, becomes the basis of all further processing and reporting by the system. Near the top of each hour, the 5-minute average that exists at that time is reported as the "hourly" value. In addition, the system checks each 5-minute average to keep track of the highest one each hour, and eventually to report (just before midnight) the highest one that occurs all day, and similarly track the lowest temperatures. If you think about the variable trace in the image above, a 5-minute average would have a lower value than one of the peaks of those fluctuations, but would also be more representative of what most of us would experience over the course of that 5-minute period. On Sunday, it is quite possible that some of the 10-second readings, and even a few of the one-minute averages, may have reached 106 or even a bit higher, but since those were fluctuations that did not last very long, the 5-minute average never rose higher than 105, which eventually was reported as the high for the day.

A jittery temperature chart line does not necessarily mean that a sensor is more accurate or responsive. The rapidly-changing SHT31 readings may not be "true" temperatures at all, but simply the result of an underdamped electrical circuit, one that bounces back and forth instead of settling on the "true" temperature somewhere in the middle. The ASOS procedure of multiple samples and averaging is a arithmetic way of dealing with this issue by smoothing out the readings.

You're the first I've heard of this, and not doubting it as I have no clue, but I would certainly think sensor designers would be fully aware of this potential condition (i.e. hysteresis) and incorporate it into their "stats". You can "what if" just about anything and I think this is where simple common sense plays into the picture of whether your data is corrupt or not.

[CW2274]

I thought it was a good explanation of how the ASOS averages the temperature.

Certainly the most comprehensive I've read, and I've read a lot.

[hwcorder]

The 5minute running average was done in part to preserve some continuity in the climate record. Obviously the thermometer in asos is much more sensitive than the old glass/Mercury hi, lo thermometers of the past. A five minute avg more closely represents the measurements we would get if they were still in place. If you took this away you would probably see a huge increase in record highs and lows.
Also since a lot asos are installed at major airport it's plausible that jet exaust could cause short fluctuations in temp so that would dampen that effect as well.