Author Topic: Can a weather station actually be accurate without a fan?  (Read 15180 times)

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Offline Axel

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Can a weather station actually be accurate without a fan?
« on: October 25, 2014, 11:12:10 PM »
Several years ago I purchased an "Ambient Weather" 2080 station along with meteohub2 LINUX appliance to push the data to Wunderground. I was told the temperature sensor is inaccurate because of the radiational shield, so I also purchased a larger radiational shield. OK, it was cheap, but I figured this was a good place to start.

My station is placed with the standard 6 feet off the ground away from structures and trees so that I can get the most accurate measurements, but it's clear to me that even the bigger radiational shield doesn't really work. As soon as  the sun hits the sensor, the temp goes way up, the discrepancy is worse in comparison to nearby stations the less wind there is. Similarly, on clear nights, the lower the dew points and the less wind there is, the worse the discrepancy with nearby stations.

I grow exotic plants that are frost tender. So while I need accurate temp readings during sunny days, I need them especially on nights when radiational loss effects are the highest. In this part of California, this is when we get our freezes.

I don't particularly want to build an old-fashion 1950's style square weather box. So I am thinking of tossing out this station and replacing it with a station that has a 24h fan. But is this the only way to get accurate measurements? The question I have is, how can any of the home weather stations have accurate temperature measurements if they don't have a fan? Most of those radiational shields are tiny and look ineffective. This would also imply the data coming in through Wunderground would be relatively inaccurate given all the radiational effects.

Any suggestions?

Offline nincehelser

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Re: Can a weather station actually be accurate without a fan?
« Reply #1 on: October 26, 2014, 01:40:49 AM »
You don't need a fan if you place your sensor in a shaded area where it won't be hit by direct sunlight.

Offline Axel

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Re: Can a weather station actually be accurate without a fan?
« Reply #2 on: October 26, 2014, 01:57:21 AM »
Well, doesn't placing it in a shaded area mean your weather station is either 1) closer than four times the height of whatever structure is producing the shade, or 2) beneath vegetation like a tree that will act as an overall insulator. Both go against NWS standards when it comes to placing a weather station. You will get a temperature reading for a small spot in the garden, not anything close to what an official weather station would measure.

On a clear night with low dewpoints, radiational effects are so strong that there can be as much as a 3 to 5 degrees difference between an open area and an area under shade of a tree or structure.

Here are the NWS standards for the temperature sensor:

Temperature sensor siting:
The sensor should be mounted 5 feet +/- 1 foot above the ground.
The ground over which the [radiation] shelter is located should be typical of the surrounding area.
A level, open clearing is desirable so the thermometers are freely ventilated by air flow.
Do not install the sensor on a steep slope or in a sheltered hollow unless it is typical of the area or unless data from that type of site is desired.
The shelter should be no closer than four times the height of any obstruction (tree, fence, building, etc.)
The sensor should be at least 100 feet from any paved or concrete surface.

See http://newa.cornell.edu/index.php?page=placement-guidelines.

Offline nincehelser

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Re: Can a weather station actually be accurate without a fan?
« Reply #3 on: October 26, 2014, 02:24:40 AM »
Regardless, it is possible to put a temperature sensor in a shaded area and get accurate readings.

Having fan aspiration does widen your siting choices, though.


Offline Axel

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Re: Can a weather station actually be accurate without a fan?
« Reply #4 on: October 26, 2014, 02:37:29 AM »
OK, perhaps I'll put the old one in the forest section of my property. I played around with sensors in those different areas, and in the December 2013 freeze in Norcal, the forest section read 32F whereas the open area read 27F. Not sure what the actual temp was since the gauge isn't reading properly in the open area, but plant damage was significantly higher in the open, so it was definitely below freezing down there.

But since there is no shade in the open area, so I guess I have no choice but to get the fan aspiration shield. I will probably opt for a Davis Instrument.

Offline nincehelser

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Re: Can a weather station actually be accurate without a fan?
« Reply #5 on: October 26, 2014, 03:05:17 AM »
I guess I'm unclear on what you're trying to accomplish.

If you want to protect your plants from cold temperatures, you would want to place your sensors where your plants are.  Granted, this might not be the best data for reporting purposes, but it would best for your plants.

As for open areas registering lower temps, that is quite normal and expected.  Radiation shields and fans are to counter solar radiation (i.e. sunshine) only, not "radiation" effects from the ground.  In other words, the night temperatures in the open areas are likely going to be cooler with or without an aspiration fan. 

In any case, you can't go wrong with a fan.

« Last Edit: October 26, 2014, 03:08:33 AM by nincehelser »

Offline Cutty Sark Sailor

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Re: Can a weather station actually be accurate without a fan?
« Reply #6 on: October 26, 2014, 05:46:15 AM »
See what a Vue can Due For Yue
 


Offline johnd

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Re: Can a weather station actually be accurate without a fan?
« Reply #7 on: October 26, 2014, 06:04:51 AM »
Radiation shields and fans are to counter solar radiation (i.e. sunshine) only, not "radiation" effects from the ground.

Please, let's not perpetuate this myth. Radiation shields play an important part in measuring more accurate air temperature readings on clear nights (ie as well as during the day).

The temperature sensor is obviously trying to measure air temperature and so needs to be in equilibrium, as far as possible, with the surrounding air. But, especially on a still night, it's a relatively slow and weak interaction between the air and the sensor itself. If you compare readings from a sensor left in the open to an identical sensor in a shield on a still clear night, you'll find that the unshielded one typically can read substantially lower. The reason is that the sensor itself is losing heat (effectively by radiation into deep space) and cools more quickly than it can re-equilibrate with the surrounding air. (Still and clear are key words here - if it's cloudy then then the radiation effects are much less marked and if it's windy then this will speed up convective equilibration of sensor and sir.)

The shield prevents the sensor element from seeing deep space and so it has a much better chance of reflecting the temperature of the surrounding air.

A well-designed passive shield can do a good job of giving more accurate air temperature readings. The Davis passive shields always seem to perform well in comparative tests, but some other designs, especially on cheap stations, can be much poorer. Shield design is important. That said, any passive shield is going to be vulnerable to errors in the noonday sun, poor shields will give large errors but even good passive shields will still have some significant error, which will obviously be exaggerated on calm days compared to windy ones. If you're anxious to have noonday temperatures a accurate as possible then a FARS shield will always do better. At night I'm not sure that there's too much difference between FARS and passive, though the FARS one will obviously react more quickly when the air temperature may be changing quickly.
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Offline Cutty Sark Sailor

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Re: Can a weather station actually be accurate without a fan?
« Reply #8 on: October 26, 2014, 06:24:57 AM »
I think what John's basically saying, is that "the temperature" (and any other sensors) at the station location is "what it is",,,at that location and time...   :roll:
and attempts to obtain perfection that match some 'standard' are likely to be futile... ](*,)
again, please see attachment
« Last Edit: October 26, 2014, 06:29:06 AM by Cutty Sark Sailor »
 


Offline nincehelser

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Re: Can a weather station actually be accurate without a fan?
« Reply #9 on: October 26, 2014, 07:49:01 AM »
Radiation shields and fans are to counter solar radiation (i.e. sunshine) only, not "radiation" effects from the ground.

Please, let's not perpetuate this myth. Radiation shields play an important part in measuring more accurate air temperature readings on clear nights (ie as well as during the day).

The temperature sensor is obviously trying to measure air temperature and so needs to be in equilibrium, as far as possible, with the surrounding air. But, especially on a still night, it's a relatively slow and weak interaction between the air and the sensor itself. If you compare readings from a sensor left in the open to an identical sensor in a shield on a still clear night, you'll find that the unshielded one typically can read substantially lower. The reason is that the sensor itself is losing heat (effectively by radiation into deep space) and cools more quickly than it can re-equilibrate with the surrounding air. (Still and clear are key words here - if it's cloudy then then the radiation effects are much less marked and if it's windy then this will speed up convective equilibration of sensor and sir.)

The shield prevents the sensor element from seeing deep space and so it has a much better chance of reflecting the temperature of the surrounding air.

Reference, please?

It sounds like you're talking about a dew shield on a telescope more than a temperature sensor. 



Offline ggsteve

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Re: Can a weather station actually be accurate without a fan?
« Reply #10 on: October 26, 2014, 08:18:40 AM »
What I've learned while trying to choose sites for my stations is that every site is a compromise, and no site is likely to meet the NWS guidelines.  How am I going to get 100' from all pavement at a high school?  How am I going to get 400' from the 100' trees that surround my house? 

On the roof of my high school we were limited to sites that the maintenance crew can get to easily (fortunately the head of maintenance is a weather nut), many undesirable barriers to pure wind readings and a tar roof.  It also had to be close enough to my classroom for the signal to pass through the numerous brick walls.

At home I have tree walls close on four sides of the house.  The only passable location is the center of the roof.  Certainly not ideal for temp, but a compromise to get decent wind and precipitation readings.

It sounds like the most important measurement to you is temperature, although humidity and soil and leaf wetness could be important to your plants too.  Give yourself some peace of mind and get the Davis with the 24hr. FARS and put it in the garden.  You can add soil and leaf wetness later.  You can't really go wrong with Davis, good quality and very flexible options.
« Last Edit: October 26, 2014, 08:20:35 AM by ggsteve »

Offline johnd

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Re: Can a weather station actually be accurate without a fan?
« Reply #11 on: October 26, 2014, 10:06:30 AM »
Reference, please?

To what? To the fact that sensor elements lose heat by radiation? Just pick up any standard textbook on meteorological instrumentation. But basically it's just scientific common sense. Any object gains and loses energy by radiation (along with conduction and convection, but on a calm radiation night the radiation process will dominate). If an object is radiating energy away faster than it can re-equilibrate to by other processes then it will fall below ambient air temperature. Why can car windshields get frosted at night when the air temperature stays above freezing? That's the same process.

Quote
It sounds like you're talking about a dew shield on a telescope more than a temperature sensor.

Sorry, no idea what that is. I'm talking about meteorological temperature sensors.
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Offline nincehelser

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Re: Can a weather station actually be accurate without a fan?
« Reply #12 on: October 26, 2014, 10:48:28 AM »
To what? To the fact that sensor elements lose heat by radiation? Just pick up any standard textbook on meteorological instrumentation.

As in a citation in the literature.

Quote
But basically it's just scientific common sense. Any object gains and loses energy by radiation (along with conduction and convection, but on a calm radiation night the radiation process will dominate). If an object is radiating energy away faster than it can re-equilibrate to by other processes then it will fall below ambient air temperature. Why can car windshields get frosted at night when the air temperature stays above freezing? That's the same process.

What you're describing is true for an object with significant mass (such as a car windshield or telescope lens), but not for a temperature sensor of the kind used in personal weather stations.  They have very little mass and thus can't hold and radiate much energy to begin with. 

Quote
Sorry, no idea what that is. I'm talking about meteorological temperature sensors.

It's practically word-for-word the often-used reasoning used behind dew shields on telescopes, complete with terms like "deep space". 

If such were true, then I could turn the temperature sensor 180-degrees so it wasn't "seeing" deep space and the problem would go away?  I know that would work to inhibit dew formation on the face of a piece of glass, but I have my doubts about it changing the temperature registered by a SHT21 sensor.

Offline ValentineWeather

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Re: Can a weather station actually be accurate without a fan?
« Reply #13 on: October 26, 2014, 11:27:02 AM »
I've seen what johnd is talking about with radiational cooling. That's why we use shelters, not just for daytime but nighttime also.
If you just take a thermometer on a clear calm night (important) and set it down outside at the same level of the artificially or naturally aspirated shelter and give it time to cool, it will cool below the air temperature because of the described radiational cooling. 
Randy

Offline nincehelser

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Re: Can a weather station actually be accurate without a fan?
« Reply #14 on: October 26, 2014, 11:29:28 AM »
I've seen what johnd is talking about with radiational cooling. That's why we use shelters, not just for daytime but nighttime also.
If you just take a thermometer on a clear calm night (important) and set it down outside at the same level of the artificially or naturally aspirated shelter and give it time to cool, it will cool below the air temperature because of the described radiational cooling.

For a thermometer made of glass and mercury?  I can see that due to the mass.  I don't see it for a chip sensor.

Offline ValentineWeather

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Re: Can a weather station actually be accurate without a fan?
« Reply #15 on: October 26, 2014, 11:41:47 AM »
I'm not sure of the mass size needed to see this.
I do know the shelters prevent this for the most part. I was checking my snow heater the other day, making sure it wasn't transmitting heat down to my temperature sensor using an IR temperature gun and was surprised just how much cooler surfaces were than the actual air temperature. I was getting temperatures as low as 18 degrees off some objects when the air was 25 at the time.   
Randy

Offline AWL

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Re: Can a weather station actually be accurate without a fan?
« Reply #16 on: October 26, 2014, 11:46:35 AM »
 ](*,)
From Wikipedia:

Radiative cooling on Earth's surface at night

Radiative cooling is commonly experienced on cloudless nights, when heat is radiated into space from the surface of the Earth, or from the skin of a human observer. The effect is well-known among amateur astronomers, and can personally be felt on the skin of an observer on a cloudless night. To feel the effect, one compares the difference between looking straight up into a cloudless night sky for several seconds, to that of placing a sheet of paper between one's face and the sky. Since outer space radiates at about a temperature of 3 kelvins (-270 degrees Celsius or -450 degrees Fahrenheit), and the sheet of paper radiates at about 300 kelvins (room temperature), the sheet of paper radiates more heat to one's face than does the darkened cosmos. The effect is blunted somewhat by Earth's surrounding atmosphere which also traps heat. Note that it is not correct to say that the sheet "blocks the cold" of the night sky; instead, the sheet is literally warming your face, just like a camp fire warms your face; the only difference is that a campfire is several hundred degrees warmer than a sheet of paper, just like a sheet of paper is several hundred degrees warmer than the deep night sky.

The same radiative cooling mechanism can sometimes cause frost or black ice to form on surfaces exposed to the clear night sky, even when the ambient temperature does not fall below freezing.

-Doug

Offline nincehelser

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Re: Can a weather station actually be accurate without a fan?
« Reply #17 on: October 26, 2014, 01:59:00 PM »
Now I understand all the head banging. 

Let me rephrase this in specific practical terms.

Let's say you have two chip sensors as are commonly used it consumer weather stations (e.g. SHT2x).  One you leave exposed to open air at night.  The other you wrap up in some kind of "shield".

It sounds like some are contending there will be a substantial difference between the two.  I'm thinking there isn't as their very small mass will bring them to equilibrium with the air temperature quickly.  Note these sensors typically have a specified accuracy +/- of 1F or 2F.

Assuming there is a difference, what exactly is the "shield" shielding?  Heat energy escaping from the sensor?  Heat energy from some other source hitting the sensor?  (Just to clarify, we're talking specifically about night readings.)
« Last Edit: October 26, 2014, 02:10:23 PM by nincehelser »

Offline Axel

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Re: Can a weather station actually be accurate without a fan?
« Reply #18 on: October 26, 2014, 02:25:26 PM »
Sorry, I take it for granted that folks would know about blackbody radiation mechanisms. I should have mentioned that I was trying to minimize that effect and I need an actual air temperature reading out in the open at 6 feet above the ground. I am not so interested in the temperature of an exposed surface to the night sky, that's not the air temperature.

Air is almost a perfect insulator when no moisture is present, so any surface exposed to the night sky will be considerably lower than the surrounding air because it's radiating energy away to the night sky, and that difference is largest when dewpoints are the lowest. That's why frost forms on car windshields when the air temperature can be as high as 39F. At higher elevations, frost can form on windshields with temps as high as 41F. There's just less atmosphere for the radiation to traverse.

I believe the inexpensive weather Station I currently own is 8-12F apart during the day and 1-3F off at night.

Offline geofb13

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Re: Can a weather station actually be accurate without a fan?
« Reply #19 on: October 26, 2014, 02:53:33 PM »
Now I understand all the head banging. 

Let me rephrase this in specific practical terms.

Let's say you have two chip sensors as are commonly used it consumer weather stations (e.g. SHT2x).  One you leave exposed to open air at night.  The other you wrap up in some kind of "shield".

It sounds like some are contending there will be a substantial difference between the two.  I'm thinking there isn't as their very small mass will bring them to equilibrium with the air temperature quickly.  Note these sensors typically have a specified accuracy +/- of 1F or 2F.

Assuming there is a difference, what exactly is the "shield" shielding?  Heat energy escaping from the sensor?  Heat energy from some other source hitting the sensor?  (Just to clarify, we're talking specifically about night readings.)

In the sense of the shield at night it's shielding it from terrestrial radiation or some other source of thermal input. Yes it's a smaller change from the amount you'd see from solar radiation but there is still some. With that said I agree with your comment to Axel where you said he should put it with his plants or in them if possible. A shield (fan aspirated or not) would help give a better reading to what's happening with them or from the experience that I've seen with a shielded sensor (no fan) in a farm field.

As far as the size goes you're correct in saying that a bigger sensor would have more of an issue. In that same light it would also depend on the material the sensor is made of and for that matter if it's in a shield the design of the shield and what it's also made of. But like I said I think you presented the fact that if he's worried about the plants the sensor should be out in the open with the plants because you want the feedback from any heat that the plants are giving off. Yes it's not accurate if you're trying to report it as NWS/WMO climate data but that's not what the original post wanted to do.

Anyway if you're interested, some lit:

From the FAO on frost protection (which funnily enough has on the description of the shield for the humidity and temp sensor as protecting against shortwave radiation); http://www.fao.org/docrep/008/y7223e/y7223e0c.htm

"Limiting Solar Radiation Effects on Outdoor Air Temperature Measurement," Jeffrey K. Sonne, Robin K. Vieira and Armin F. Rudd Florida Solar Energy Center (FSEC); http://www.fsec.ucf.edu/en/publications/html/fsec-pf-240-92/

"Effect of thermometer screens on accuracy of temperature measurements," Zoltán Nagy; http://www.wmo.int/pages/prog/www/IMOP/publications/IOM-94-TECO2006/P3%2814%29_Nagy_Hungary.pdf

"Air Temperature Measurement Errors In A Naturally Ventilated Multi-Plate Radiation Shield," Reina Nakamura and L. Mahrt (From AMS Conf); https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=0CDkQFjAE&url=https%3A%2F%2Fams.confex.com%2Fams%2Fpdfpapers%2F78111.pdf&ei=tw9NVM-YNc_4yQSr54HADA&usg=AFQjCNGTctN9fsN28LbLWunWbNxlDwOw0g&sig2=N9W2hC8Vw1rdTya2tyYvbw&bvm=bv.77880786,d.aWw

and the corresponding published article; http://journals.ametsoc.org/doi/pdf/10.1175/JTECH1762.1

Sorry, I take it for granted that folks would know about blackbody radiation mechanisms. I should have mentioned that I was trying to minimize that effect and I need an actual air temperature reading out in the open at 6 feet above the ground. I am not so interested in the temperature of an exposed surface to the night sky, that's not the air temperature.
...
I believe the inexpensive weather Station I currently own is 8-12F apart during the day and 1-3F off at night.

For Axel; Are your plants at 6 feet? Also if you really need the temperature out in the open at 6 feet I'd suggest going with a fan aspirated as pretty much everyone suggested at some point or another in the thread. You are more likely to get what you're looking for on a consistent basis with one.

Offline nincehelser

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Re: Can a weather station actually be accurate without a fan?
« Reply #20 on: October 26, 2014, 03:32:32 PM »
Now I understand all the head banging. 

Let me rephrase this in specific practical terms.

Let's say you have two chip sensors as are commonly used it consumer weather stations (e.g. SHT2x).  One you leave exposed to open air at night.  The other you wrap up in some kind of "shield".

It sounds like some are contending there will be a substantial difference between the two.  I'm thinking there isn't as their very small mass will bring them to equilibrium with the air temperature quickly.  Note these sensors typically have a specified accuracy +/- of 1F or 2F.

Assuming there is a difference, what exactly is the "shield" shielding?  Heat energy escaping from the sensor?  Heat energy from some other source hitting the sensor?  (Just to clarify, we're talking specifically about night readings.)

In the sense of the shield at night it's shielding it from terrestrial radiation or some other source of thermal input. Yes it's a smaller change from the amount you'd see from solar radiation but there is still some.

This may be where some of the confusion is creeping in.

Let me ask this question to clarify.  If you use a radiation shield on a temperature sensor at night, should you expect to see higher or lower readings?

The arguments I'm hearing is that without a shield, the temperature will register too low.  Or in other words, you need a radiation shield to get a higher reading.  I don't get how that would work in a thermodynamic sense by blocking radiation.  If you block outside radiation, the sensor's reading should be lower.


Offline geofb13

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Re: Can a weather station actually be accurate without a fan?
« Reply #21 on: October 26, 2014, 04:21:41 PM »
This may be where some of the confusion is creeping in.

Let me ask this question to clarify.  If you use a radiation shield on a temperature sensor at night, should you expect to see higher or lower readings?

The arguments I'm hearing is that without a shield, the temperature will register too low.  Or in other words, you need a radiation shield to get a higher reading.  I don't get how that would work in a thermodynamic sense by blocking radiation.  If you block outside radiation, the sensor's reading should be lower.

Taking from what I know and looking back at how some of the other posts I'll try to answer it below.

On a night where there is some wind or mixing of the surface/near-surface atmosphere (this can include cloudy/partly cloudy nights) you aren't going to see much of a difference between a shielded (FARS or not) or unshielded sensor because all other things being equal the atmosphere surrounding the sensor is better mixed and is not just relying on the nearest source or sink of heat. This mixing is why the temperature on cloudy and light breeze nights stay warmer then if heat in the lower atmosphere had a direct ticket out to space on a clear night.

Now looking at it from the other side, if you have a calm clear evening you don't get any mixing which allows the heat to escape to space or the upper atmosphere. I think that calm clear part was mentioned before by johnd and Valentine which really is the sticking point. If you place an unshielded sensor out in that environment any thermodynamic heat it has acquired from solar heating it will give off quickly depending on the thermodynamic properties of the material. Obviously you could tell the difference between the air, water, and brick in the same environment. If the sensor is in a shield and it's aspirated it will work like the sensor in the mixed atmosphere example above because it's creates more of a moving equilibrium between everything whether it's the outside atmosphere, any terrestrial radiation that might enter the shield, the shield itself, and the sensor. From that the aspirated shielded sensor out in a field will read warmer then one that's allowed to radiate it's heat/energy towards space without staying closer to environment it's surrounded by. A non-aspirated shield will meet it's surrounding environments temperature but at a different rate then the aspirated sensor.

So long story short, the non-shielded temperature sensor will read lower out in the open on a clear calm night then a sensor that is shielded and has ventilation.  I apologize if my write-up is weirdly written at times, when I start writing I generally just write without overly thinking things on the first go.

Offline nincehelser

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Re: Can a weather station actually be accurate without a fan?
« Reply #22 on: October 26, 2014, 05:03:23 PM »
So long story short, the non-shielded temperature sensor will read lower out in the open on a clear calm night then a sensor that is shielded and has ventilation.  I apologize if my write-up is weirdly written at times, when I start writing I generally just write without overly thinking things on the first go.

That makes sense and is as I would expect it to be.  Having a fan certainly doesn't hurt, but I really have my doubts that it does much for temperature readings in a practical sense (humidity might be a different issue).  It would be interesting to see a controlled lab experiment test this.

Offline DoctorKnow

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Re: Can a weather station actually be accurate without a fan?
« Reply #23 on: October 26, 2014, 07:32:57 PM »
Axel,

When you say the "temp goes way up" when the sun hits your shield, how many degrees are we talking? I have seen people paint the inside of their radiation shield black, and say that it helps quite a bit. The idea is to keep as much light off the sensor as you can. You want to take the temp/humidity reading in an open area, away from any obstructions as much as practical and possible. I would focus on reducing light and not worry about a fan just yet.

Offline Axel

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Re: Can a weather station actually be accurate without a fan?
« Reply #24 on: October 27, 2014, 12:19:36 AM »
When I finally get the Davis instruments station, I'll set it up side by side, and we'll see what the difference is. Right now I am extrapolating based on what time the sun hits the shield. Temps literally go up 10 degrees at that point over a relatively short period of time.

 

anything