Temperature inside a weather station enclosure

[GhostLoveScore]

Hello I have a digital thermometer that I put into a wooden enclosure. It has horizontal holes drilled into its sides all around to let air in. On the bottom there is an opening around 1/3 of the width, like in attached photo.

And I was thinking about certain thing. When I put a bare thermometer circuit board in the middle of the enclosure, like on the left side of the attached image, I get certain values. But sometimes sunlight can get through the holes (at sunset) so I decided to put a PVC square pipe around the sensor.
Sensor is attached inside the pipe with two plastic spacers and it's exactly in the middle of the pipe. It doesn't have top and bottom sides. It hangs from the wire in both cases (with or without PVC pipe) from the top of the wooden enclosure.

I noticed that my temperatures are about 0.3 to 0.5 degrees higher with PVC pipe around the sensor than without it. At first I thought that maybe light is getting reflected like in second attached photo,

so I taped the inside with black tape. Still same thing.

Then I though that maybe PVC pipe is warmed up with my hands and with sunlight before I put it inside, but that seemed to be wrong when I stored the PVC pipe inside the wooden weather enclosure, so it should be the same temperature as the enclosure.

So now I'm thinking that maybe ambient air is colder than either wooden enclosure or the PVC pipe, but because PVC pipe is a lot closer to the sensor than wooden enclosure, it could be heating it up those 0.3-0.5 degrees. Any ideas on what's happening here?

Maybe I'm naive, but I thought when something is in (not total) shade and you put it in additional shade it should only get colder, closer to being at ambient air temperature, not hotter.

 

[Bystander]

See "Stevenson Screen," and get back to "us." (Not a "Royal We.")

 

[GhostLoveScore]

See "Stevenson Screen," and get back to "us." (Not a "Royal We.")

This is pretty much Stevenson screen, just simlified version of it. With holes instead of angled pieces of wood. I've built real Stevenson screens before, but it's a lot of work.
But that's not important. I am wondering what's going on here, when I have an object in the shade that apparently gets warmer when I put another shade around it.

 

[Bystander]

just simlified version of it.

Welcome to the "wonderful world" of attempts at non-equilibrium temperature measurement; frosted windshields at "above freezing" ambient/air temperature are quite common, as are "opposite" effects such as you've noticed.

You might check for radiation/radiant heat sources in the immediate vicinity; anything within one to two hundred yards/meters is "integrated" into the thermal background. Station locations are "required" to be "unobstructed" to be certified by the NWS; very few actually are.

 

[Tom.G]

Hmm... Maybe that's why 'official' weather station enclosure have louvers instead of drilled holes. Put a light bulb in place of the thermometer and notice how much of the surroundings are illuminated; that's what the thermometer sees.

Cheers,
Tom

 

[GhostLoveScore]

Hmm... Maybe that's why 'official' weather station enclosure have louvers instead of drilled holes. Put a light bulb in place of the thermometer and notice how much of the surroundings are illuminated; that's what the thermometer sees.

Cheers,
Tom

yes, of course. But in that case, what is that additional pvc enclosure doing to raise the temperature? From physics point of view? Is it, in some, way higher in temperature than ambient air so that it heats up the sensor?

I would imagine that if thermometer "sees" through the holes, it would be even better now with something that blocks the way.

 

[Bystander]

https://www.physicsforums.com/threads/selective-surfaces.893834/#post-5624354

Ever burn/blister your arm on your car's chrome trim? Wait a minute...that's "highly" reflective...or, is it? Optical properties are very much functions of temperature, surface type, color, history... Find/borrow/beg/steal a copy of R&H, and remember what temperatures you are dealing with; there are hundreds of white paints out there, and they'll all reach different steady-state temperatures when exposed to "earth surface" ambient.

 

[GhostLoveScore]

https://www.physicsforums.com/threads/selective-surfaces.893834/#post-5624354

Ever burn/blister your arm on your car's chrome trim? Wait a minute...that's "highly" reflective...or, is it? Optical properties are very much functions of temperature, surface type, color, history... Find/borrow/beg/steal a copy of R&H, and remember what temperatures you are dealing with; there are hundreds of white paints out there, and they'll all reach different steady-state temperatures when exposed to "earth surface" ambient.

Yes, but the car chrome trim was exposed to the direct sunlight.

 

[Dullard]

'Digital Thermometer' covers a lot of ground. I'm still trying to figure out how you read it (inside a pipe inside a box).

 

[GhostLoveScore]

'Digital Thermometer' covers a lot of ground. I'm still trying to figure out how you read it (inside a pipe inside a box).

It's MPC9808 IC on a circuit board and I'm communicating via I2C protocol with it to a microcontroller outside the enclosure.

 

[Dullard]

I looked at the data sheet. It seems possible that you are 'auto-heating' - power dissipated by the device (And pull-ups, etc.) are warming the temp sensor/PCB when you place it in a 'confined' space.

 

[GhostLoveScore]

I looked at the data sheet. It seems possible that you are 'auto-heating' - power dissipated by the device (And pull-ups, etc.) are warming the temp sensor/PCB when you place it in a 'confined' space.

It's possible. The thermometer is placed on an "island" on the board with only thin bridges that carry power and data, so the heating should be minimal, but I guess it's possible.

 

[sophiecentaur]

I have a (non-functioning) acurite home weather station. It has a small PV panel which drives a small fan when there's sunshine. That is an easy solution to measuring the surrounding air temperature rather than the temperature of static / warmed up air inside the enclosure. Technology beyond Stevenson.

 

[GhostLoveScore]

I have a (non-functioning) acurite home weather station. It has a small PV panel which drives a small fan when there's sunshine. That is an easy solution to measuring the surrounding air temperature rather than the temperature of static / warmed up air inside the enclosure. Technology beyond Stevenson.

I might try that, thanks!

 

[sophiecentaur]

I might try that, thanks!

So easy to do with an eBay fan and a battery. T without fan and T with fan. I like that kind of experiment.

 

[GhostLoveScore]

So easy to do with an eBay fan and a battery. T without fan and T with fan. I like that kind of experiment.

Even better. I'm making 2 more sensors and I will place them inside at the same time, one bare pcb for reference and put the other one in various situations to see what affects the temperature in which way.

 

[sophiecentaur]

You are clearly doing the delux job.
Collect a brownie point!

 

[Tommy T]

How about a solar powered fan at the top of the box. Since the temp variations seem to be most severe in daylight hours, the fan should draw enough ambient air to improve accuracy. The solar powered fan would not add to the heat load inside the enclosure.

 

[sophiecentaur]

How about a solar powered fan at the top of the box. Since the temp variations seem to be most severe in daylight hours, the fan should draw enough ambient air to improve accuracy. The solar powered fan would not add to the heat load inside the enclosure.

That's exactly what I had in my previous home weather station. I would have thought that a non-mechanical compensation for insolation would be better than a fan, though. I remember, when I first bought the unit, hearing the fan working and re-read the booklet. "Ahh! that's what it is." The fan broke down but not before the rain gauge tipping bucket needed de-spidering.

 

[GhostLoveScore]

That would be a good choice, I think.
Stevenson screen is great, when it's large enough. Unfortunately, large ones are harder to make, take up space or they are expensive if you buy them.
Smaller ones are easy to carry and can be placed anywhere. A small fan at the top of the small enclosure should make measurements more accurate.

The down side is that if your weather station is solar powered like mine, there's an issue of power consumption. But i think spinning the fan for a 5-10 seconds every couple minutes should be good enough. I'll let you know the results, I'm printing out a 15cm diameter round stevenson screen and I'll compare it to my nice, large, wooden Stevenson screen.

 

[sophiecentaur]

But i think spinning the fan for a 5-10 seconds every couple minutes should be good enough.

The fan only needs to be running during full sun. Let the Sun tell it when to run by using a tiny separate cell to power the fan.

 

[sophiecentaur]

Stevenson screen is great, when it's large enough.

It all depends on what accuracy you're after. I use Wunderground which is a network of amateur weather stations and looney enthusiasts. Temperature measurements from contributors near where I live (a few miles) seem to agree with me a with each other, certainly within one C. There is a range of equipment, I think, so I can't grumble for my level of use.
Frankly, it mainly useful for rain measurements so I know whether the garden needs watering when I'm away. My neighbour will give it a splosh if I ring him in an emergency.
But, if you take it seriously, I guess you need the right gear.

 

[GhostLoveScore]

It all depends on what accuracy you're after. I use Wunderground which is a network of amateur weather stations and looney enthusiasts. Temperature measurements from contributors near where I live (a few miles) seem to agree with me a with each other, certainly within one C. There is a range of equipment, I think, so I can't grumble for my level of use.
Frankly, it mainly useful for rain measurements so I know whether the garden needs watering when I'm away. My neighbour will give it a splosh if I ring him in an emergency.
But, if you take it seriously, I guess you need the right gear.

Ideally I want to be within 0.1 degrees from real value. But way too many things affect the temperature reading so I'm not sure how realistic is that goal of being within 0.1 degrees.

 

[sophiecentaur]

Ideally I want to be within 0.1 degrees from real value.

I guess that "real value" is defined as what you'd measure in a Stevenson screen because reality implies a definition of how you measure it. But far be it from me to question the motivation of an enthusiast. I'm full of nerdiness in other directions. Plus, if we're following climate change then precision is important.

 

[GhostLoveScore]

I guess that "real value" is defined as what you'd measure in a Stevenson screen because reality implies a definition of how you measure it.

Yes, current definition of taking an air temperature is at 2 meters above grass ground, in Stevenson screen.
Like I said, I'm sure that many variables can cause different temperature readings. For example, if I moved my enclosure 5 meters to the right, the temperature would be slightly different. Or if I moved my thermometer 20cm lower or higher the temperature would also differ from the last example. So, I'm not sure how high I can go in accuracy before it starts being meaningless, but 0.1C seems to be the norm.

 

[Tom.G]

Ideally I want to be within 0.1 degrees from real value. But way too many things affect the temperature reading so I'm not sure how realistic is that goal of being within 0.1 degrees.

SInce the chip itself is specified as ±0.5°C (±0.9°F), I'm wondering how you will calibrate to ±0.1°C (±0.18°F).

https://www.google.com/url?q=https://ww1.microchip.com/downloads/en/DeviceDoc/25095A.pdf

•Accuracy:
- ±0.25 (typical) from -40°C to +125°C
- ±0.5°C (maximum) from -20°C to 100°C
- ±1°C (maximum) from -40°C to +125°C

 

[GhostLoveScore]

SInce the chip itself is specified as ±0.5°C (±0.9°F), I'm wondering how you will calibrate to ±0.1°C (±0.18°F).

https://www.google.com/url?q=https://ww1.microchip.com/downloads/en/DeviceDoc/25095A.pdf

Because since I started this thread I switched to more accurate chip, TMP117, it has an accuracy of +/-0.1C, and even better then that.