Author Topic: VP2 FARS airflow  (Read 12901 times)

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Offline jerryg

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Re: VP2 FARS airflow
« Reply #25 on: June 22, 2017, 11:56:34 AM »
Just updating my reply on this old post. I had been running my shields on Davis fans using ac modules running 2.3 volts in the day and using photo/relay switching to 1 volt at night but i ran into the same problem i went to ac fans for, fan motors failing after some time. I had one fail after four months and another started the old tap to get running. I stopped using the ac fans because of high humidity down here near the cost and was getting sensor failures. Sort of a problem, fan failures or sensor failures, well i read that it was recommended to not use batteries at night with the shield to increase the life of the fans. The article said the fan was not needed at night unless you had a heat problem near by. Well i put my ac fans back in my shields except for one to compare the readings at night and after two weeks of switching the fans off at night and low to calm winds i saw no difference with the readings from the shield with the fan running. I have all my shields running 12 volt fans on 9 volts to keep the airflow lower and i switch them off at night with my photo/relay set up and everything is fine. Now i get the better fan life and air flow in the day and humidity control at night and don't have to worry about the poor fan performance of the Davis fan. My shields are located in an open field over grass so no nearby heat problems that might need the nighttime fans.

Offline jerryg

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Re: VP2 FARS airflow
« Reply #26 on: June 22, 2017, 02:18:34 PM »
As for as response time it was the same as with the fan as long as the wind was up around 3 or 4 mph and was a little slower with no wind. In the am the readings were always the same for temp and humidity as the one with the fan running.

Offline WxLover16

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Re: VP2 FARS airflow
« Reply #27 on: June 22, 2017, 04:32:14 PM »
As for as response time it was the same as with the fan as long as the wind was up around 3 or 4 mph and was a little slower with no wind. In the am the readings were always the same for temp and humidity as the one with the fan running.

That is really surprising to me, since knowing how the 24hr was designed; without any air being pulled in, the air would become more stagnant that it would be in a passive shield and it would get warmer and warmer.
Davis Wireless VP2 SHT31 24hr 24CFM FARS

Offline jerryg

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Re: VP2 FARS airflow
« Reply #28 on: June 22, 2017, 04:48:48 PM »
Well i think that it is night time so no heating being done by the sun and the bottom of the chamber is open as well as the top and you can get a slight flow of air rising up the tube. Also the shield cools down at night and transfers that into the chamber. I am no engineer when it comes to this just by observation it works lol. I like not having the fan running at night because of the 90% plus humidity we have down here near the coast every night. In the am i have had water condensed on the plates and inside that looked like it was sprayed with a hose.

Offline ct

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Re: VP2 FARS airflow
« Reply #29 on: June 22, 2017, 07:59:14 PM »
That is really surprising to me, since knowing how the 24hr was designed; without any air being pulled in, the air would become more stagnant that it would be in a passive shield and it would get warmer and warmer.

In my experience, when the fan is not working during the night response can be very slow.  I have 2 FARS out in the yard and can turn the fans on/off remotely, so comparisons can be done quite easily.

Offline WxLover16

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Re: VP2 FARS airflow
« Reply #30 on: June 22, 2017, 10:10:49 PM »
That is really surprising to me, since knowing how the 24hr was designed; without any air being pulled in, the air would become more stagnant that it would be in a passive shield and it would get warmer and warmer.

In my experience, when the fan is not working during the night response can be very slow.  I have 2 FARS out in the yard and can turn the fans on/off remotely, so comparisons can be done quite easily.

Lucky you. I'd be a kid in a candy store if I could be able to test 2 FARS or even 1 FARS 1 passive right next to each other to see the results  :lol:. I think we can pretty much all agree on the fact that without aspiration at night, the FARS will be less responsive and run warmer, whether it be as little as .1 degrees or more.
Davis Wireless VP2 SHT31 24hr 24CFM FARS

Offline ValentineWeather

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Re: VP2 FARS airflow
« Reply #31 on: June 23, 2017, 01:20:54 AM »
I've got 3 shields all VP2 design. One gutted no fan it's definitely slow responding at night vs the aspirated. 
Randy

Offline jgentry

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Re: VP2 FARS airflow
« Reply #32 on: July 04, 2017, 04:19:14 PM »
Yes i have had several failures with long term high humidity. Most of the failures happened after i increased the air flow with 12 volt fans, too much moisture inside the sensor filter. I run my fan at half voltage to cut down on the air flow. Also i am now using the sht31 sensors and they are supposed to be better at handling high humidity for long periods of time. We shall see.

Does your Dew Point readings read lower if you have the airflow coming in stronger than the Davis FARS? Does stronger inflow makes any difference in response times?
Davis Vantage Pro2 & WeatherFlow Tempest. WU: KXALJEMI2, KALJEMIS7, KFLPANAM363 & KALTHORS2. CWOP/APRS: C6353 & E6358

Offline jerryg

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Re: VP2 FARS airflow
« Reply #33 on: July 04, 2017, 04:41:04 PM »
What works for one may not work for another, if you go by all the responses about how one is better than the other. I have four fars housings and i had one running on solar, one on ac using the Davis fan at 2.4 volts during the day and 1 volt at night. I had one running at 12 volts day and night and the fourth at 7 volts during the day and off at night. The Davis shield is designed for low air flow and i saw no difference in performance between them. I don't like solar, too much depends on sunlight and battery charge at night. I run the ac fans because the Davis fan is cheap and fails too much to suit me. I have one of the ac fans that has been running along for several years with no problem. With the design of the shield for low air flow anything should be ok. I personally don't use the fan at night because of our high humidity and sensor failures due to too much moisture on the sensor chip. Like i say each to his own.








Offline jgentry

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Re: VP2 FARS airflow
« Reply #34 on: July 04, 2017, 05:20:00 PM »
What works for one may not work for another, if you go by all the responses about how one is better than the other. I have four fars housings and i had one running on solar, one on ac using the Davis fan at 2.4 volts during the day and 1 volt at night. I had one running at 12 volts day and night and the fourth at 7 volts during the day and off at night. The Davis shield is designed for low air flow and i saw no difference in performance between them. I don't like solar, too much depends on sunlight and battery charge at night. I run the ac fans because the Davis fan is cheap and fails too much to suit me. I have one of the ac fans that has been running along for several years with no problem. With the design of the shield for low air flow anything should be ok. I personally don't use the fan at night because of our high humidity and sensor failures due to too much moisture on the sensor chip. Like i say each to his own.


Aight. Thanks!  I was just curious.
Davis Vantage Pro2 & WeatherFlow Tempest. WU: KXALJEMI2, KALJEMIS7, KFLPANAM363 & KALTHORS2. CWOP/APRS: C6353 & E6358

Offline WxLover16

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Re: VP2 FARS airflow
« Reply #35 on: July 04, 2017, 08:24:54 PM »
What works for one may not work for another, if you go by all the responses about how one is better than the other. I have four fars housings and i had one running on solar, one on ac using the Davis fan at 2.4 volts during the day and 1 volt at night. I had one running at 12 volts day and night and the fourth at 7 volts during the day and off at night. The Davis shield is designed for low air flow and i saw no difference in performance between them. I don't like solar, too much depends on sunlight and battery charge at night. I run the ac fans because the Davis fan is cheap and fails too much to suit me. I have one of the ac fans that has been running along for several years with no problem. With the design of the shield for low air flow anything should be ok. I personally don't use the fan at night because of our high humidity and sensor failures due to too much moisture on the sensor chip. Like i say each to his own.

When you say the Davis shield is designed for low air flow, how low are we talking? I'm now going with a 24 cfm fan which is much more than what the stock puts out. Is that adequate for the shield?
Davis Wireless VP2 SHT31 24hr 24CFM FARS

Offline CW2274

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Re: VP2 FARS airflow
« Reply #36 on: July 04, 2017, 08:43:03 PM »
I'm now going with a 24 cfm fan
Do tell. Have you found your problem(s)?

Offline WxLover16

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Re: VP2 FARS airflow
« Reply #37 on: July 04, 2017, 08:47:21 PM »
I'm now going with a 24 cfm fan
Do tell. Have you found your problem(s)?

No, basically just "testing" fans. Maybe this one will keep working. Crossing my fingers, toes, etc.... Supposed to arrive Thursday. Can't wait to put it in. I hate how pittily the stock fan blows (I put it back in). AC fans certainly blow it out the water.
« Last Edit: July 04, 2017, 08:49:24 PM by WxLover16 »
Davis Wireless VP2 SHT31 24hr 24CFM FARS

Offline Bashy

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Re: VP2 FARS airflow
« Reply #38 on: July 05, 2017, 12:36:28 AM »
Hi jerryg

Which photo/relay switching please?

I do have one but i think its just and on/off, not v reduction
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Bashy

Offline Phil23

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Re: VP2 FARS airflow
« Reply #39 on: July 05, 2017, 06:43:54 AM »
.......the bottom of the chamber is open as well as the top and you can get a slight flow of air rising up the tube.......

Ahhh,

Now here's one of my favorite discussion points regards air flow, often gets heated discussion, and at times I get others to agree with my lecturer from the past.

Hot Air Does Not Rise!

What can happen though is colder more dense air displaces the warm lighter air upwards.

Just like dropping a brick in a bucket of water; the water doesn't really rise, it is displaced by something denser.

When you apply that philosophy to air movement it takes a different turn.

Swap hot air for just air, & cold air for water & then visualise how everything flows & it paints a bit of a different picture.


Got my flame suit on... Accustom to the Blow Torches coming out on this topic.

Phil.

Offline Bashy

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Re: VP2 FARS airflow
« Reply #40 on: July 05, 2017, 06:48:00 AM »
Something that goes up, rises, regardless how you want to put it, hot air rises
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Offline Phil23

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Re: VP2 FARS airflow
« Reply #41 on: July 05, 2017, 07:08:07 AM »
Something that goes up, rises, regardless how you want to put it, hot air rises
Cold air could be squeezing it up from different directions.

Drafts under skirting boards; high ceiling vents.

It's a favorite topic of mine.

Offline jerryg

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Re: VP2 FARS airflow
« Reply #42 on: July 05, 2017, 08:53:31 AM »
Hi Bashy; The control stuff is just homemade, a photo cell module that controls a security light at night and an ac relay i had on hand. At night the relay is pulled on and turns off my fans and during the day the relay is off and turns the fans on when the relay switches to the off state. When i was running the davis fan i used a module for the 2.4 volts during the day and at night the relay was used to switch in a few diodes to bring the voltage down to 1 volt at night. Now for the chicken or the egg thing, does hot air rise because of thermodynamics or cold air at the surface being denser causes the warmer lighter air to rise. What we need now is someone smarter than us to settle it lol. If the temp at floor level is say 100 and the temp at ceiling height is 110 degrees would the 100 degree temp at the bottom be considered cold air? lol.

Offline ValentineWeather

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Re: VP2 FARS airflow
« Reply #43 on: July 05, 2017, 09:10:36 AM »
With the design of the shield for low air flow anything should be ok. I personally don't use the fan at night because of our high humidity and sensor failures due to too much moisture on the sensor chip. Like i say each to his own.

When you say the Davis shield is designed for low air flow, how low are we talking? I'm now going with a 24 cfm fan which is much more than what the stock puts out. Is that adequate for the shield?

The issue you have WxLover16 is this discussion is about the Davis 24 hour shield not DFARS shield you mentioned in your PM. These are totally different shields. The 24 hr shield needs little airflow to be efficient. The DFARS shield can benefit from higher air flow because it's drawing air from much larger area. The 24 hr. shield is channeling air through inlet tube into triple walled core across sensor mounted inside core tube. 
Randy

Offline alexstaar

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Re: VP2 FARS airflow
« Reply #44 on: July 06, 2017, 01:08:33 AM »
Hi Bashy; The control stuff is just homemade, a photo cell module that controls a security light at night and an ac relay i had on hand. At night the relay is pulled on and turns off my fans and during the day the relay is off and turns the fans on when the relay switches to the off state. When i was running the davis fan i used a module for the 2.4 volts during the day and at night the relay was used to switch in a few diodes to bring the voltage down to 1 volt at night. Now for the chicken or the egg thing, does hot air rise because of thermodynamics or cold air at the surface being denser causes the warmer lighter air to rise. What we need now is someone smarter than us to settle it lol. If the temp at floor level is say 100 and the temp at ceiling height is 110 degrees would the 100 degree temp at the bottom be considered cold air? lol.

To comment on the thermodynamics in question here... dry air with a temperature of 100ºF is more dense than dry air with a temperature of 110ºF. Do not think of it as "hot" vs. "cold". The base equation is P=ρRT so if temperature decreases (even by a slight amount), ρ must increase by some amount in order for P to remain constant. In the case of this "unaspirated" 24-hour FARS, you could use the density acceleration equation to determine how much air would flow up through the chamber by free convection and density acceleration alone (assuming a calm ambient wind). Obviously, it won't be very fast with only very small variations in temperature, but it will certainly be a non-zero value. Hope this helps!

Sources: an Atmospheric Science degree ;)
-Alex

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Offline Bashy

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Re: VP2 FARS airflow
« Reply #45 on: July 06, 2017, 01:33:34 AM »
Hi Bashy; The control stuff is just homemade, a photo cell module that controls a security light at night and an ac relay i had on hand. At night the relay is pulled on and turns off my fans and during the day the relay is off and turns the fans on when the relay switches to the off state. When i was running the davis fan i used a module for the 2.4 volts during the day and at night the relay was used to switch in a few diodes to bring the voltage down to 1 volt at night. Now for the chicken or the egg thing, does hot air rise because of thermodynamics or cold air at the surface being denser causes the warmer lighter air to rise. What we need now is someone smarter than us to settle it lol. If the temp at floor level is say 100 and the temp at ceiling height is 110 degrees would the 100 degree temp at the bottom be considered cold air? lol.

Thanks, i have a on/off photocell relay board, but its just that, on when light, off when dark, its the small amount of on instead of
off i would like :)


As for the heat, chit chat,

Does a hot air balloon rise or is it the ground sinking?
Kind regards
Bashy

Offline alexstaar

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Re: VP2 FARS airflow
« Reply #46 on: July 06, 2017, 01:43:38 AM »
As for the heat, chit chat,

Does a hot air balloon rise or is it the ground sinking?

 :lol:

Hot air balloon rises because you create a large parcel of air with much lower density than the environmental air around it just by heating it up (or adding hot exhaust). So you create a large upward acceleration/force against gravity.
-Alex

Davis 6153 VP2 with 24-hour FARS/
Davis 6250 Vantage Vue
WU: KNJMOUNT68/KSCGREER81
CWOP: FW5135/EW6390
www.uah-mcthornmoracres-weather.com

Offline Bashy

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Re: VP2 FARS airflow
« Reply #47 on: July 06, 2017, 01:48:30 AM »
yeah but, hot air still rising, its not falling, yeah ok, bad analogy lol Thermal any better?
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Bashy

Offline alexstaar

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Re: VP2 FARS airflow
« Reply #48 on: July 06, 2017, 01:55:46 AM »
Well when you have thermals in the convective boundary layer, the mass of air in the updraft (thermal) is replaced by sinking air in a downdraft or by low level convergence of air (i.e. cold front, outflow boundary, low pressure system, etc.). You could relate that to what's going on inside an "unaspirated" 24-hour FARS. Is that what you're talking about?

Sorry if this is a bit off topic!
-Alex

Davis 6153 VP2 with 24-hour FARS/
Davis 6250 Vantage Vue
WU: KNJMOUNT68/KSCGREER81
CWOP: FW5135/EW6390
www.uah-mcthornmoracres-weather.com

Offline Bashy

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Re: VP2 FARS airflow
« Reply #49 on: July 06, 2017, 02:08:58 AM »
Pretty much, cold air sinking,  hot air rising, but you should never pay attention to most of what i say, i tend to over think things
Kind regards
Bashy