Author Topic: VP2 FARS airflow  (Read 5091 times)

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Offline CW2274

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Re: VP2 FARS airflow
« Reply #50 on: July 06, 2017, 02:11:45 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 100F is more dense than dry air with a temperature of 110F.
And just to further expand on a common misconception, wet air is less dense than dry air. ;)
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Offline alexstaar

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Re: VP2 FARS airflow
« Reply #51 on: July 07, 2017, 01:34:09 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 100F is more dense than dry air with a temperature of 110F.
And just to further expand on a common misconception, wet air is less dense than dry air. ;)

Yes, because water vapor is less dense than dry air. So when you add water vapor to air, it becomes less dense.
-Alex

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Offline dupreezd

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Re: VP2 FARS airflow
« Reply #52 on: July 11, 2017, 09:52:53 AM »
Since this came up, I had this setup in my back yard. A piece of plexi glass with a fan (120cfm) blowing on it. Last night we had perfect conditions. Lots of rain yesterday, high humidity and no wind.

 [ You are not allowed to view attachments ]


This morning this is the result. A bit difficult to see the condensation on the pictures but I think you will get the idea even in the picture above. Where there were good airflow, no to minimal condensation. Also discussed in another thread.

More pictures
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« Last Edit: July 11, 2017, 03:12:53 PM by dupreezd »
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Offline jgentry

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Re: VP2 FARS airflow
« Reply #53 on: August 08, 2017, 08:08:54 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.

This is off topic but since you live in a very humid environment, does your SHT-31 sensor reports 100% or does it usually tops off at 98-99%? I'm thinking the reason why the Sensirion SHT-31 struggles to reach 100% is due to the sensor chip being really small. I might be wrong with my thinking.
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Offline CW2274

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Re: VP2 FARS airflow
« Reply #54 on: August 08, 2017, 08:24:43 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.
I'm thinking the reason why the Sensirion SHT-31 struggles to reach 100% is due to the sensor chip being really small. I might be wrong with my thinking.
As I've stated here many times before, some sensors can easily reach a 100% humidity reading whereas others do not. I'm not just talking PWS's, I'm talking the big boys too. It's a very finicky thing when the air becomes close to saturation.
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Offline jgentry

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Re: VP2 FARS airflow
« Reply #55 on: August 08, 2017, 08:48:26 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.
I'm thinking the reason why the Sensirion SHT-31 struggles to reach 100% is due to the sensor chip being really small. I might be wrong with my thinking.
As I've stated here many times before, some sensors can easily reach a 100% humidity reading whereas others do not. I'm not just talking PWS's, I'm talking the big boys too. It's a very finicky thing when the air becomes close to saturation.

I'm just trying to make sense of it. But then again, I guess I also need to keep in mind that all humidity sensors have wet/dry biases.
Davis Vantage Pro2. SHT 31. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

Offline jerryg

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Re: VP2 FARS airflow
« Reply #56 on: August 08, 2017, 09:09:02 PM »
Generally mine show 100% mostly in the fall and winter when we get long periods of high humidity/fog. In the spring and summer mostly shows in the 96 to 98 range. I have noticed the sensor used works pretty well until it gets to the 94/95 range then the response slows way down. That is why it hits a 100 only once in a while. The humidity has to be high for most of the night and morning for it to finally reach the 100 reading. I quit worrying about the 100 readings awhile back knowing  that 97 and above is within specs of the sensor.

Offline ValentineWeather

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Re: VP2 FARS airflow
« Reply #57 on: August 08, 2017, 09:20:06 PM »
I've posted about the airport ASOS here never exceeding 94% even in near zero visibility.  Hard to believe and same time I'm hitting 98-99% in those conditions with 3 different SHT31's.
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Offline jgentry

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Re: VP2 FARS airflow
« Reply #58 on: August 08, 2017, 09:41:12 PM »
Generally mine show 100% mostly in the fall and winter when we get long periods of high humidity/fog. In the spring and summer mostly shows in the 96 to 98 range. I have noticed the sensor used works pretty well until it gets to the 94/95 range then the response slows way down. That is why it hits a 100 only once in a while. The humidity has to be high for most of the night and morning for it to finally reach the 100 reading. I quit worrying about the 100 readings awhile back knowing  that 97 and above is within specs of the sensor.

Thanks for your input! My sensor typically tops off at 98-99%. I'm curious to why the response time goes downhill when it reaches 95%? I guess it's just a weakness of the Sensirion sensors. But I have noticed that RainWise humidity sensor (MK III) doesn't do well when it hits in the 90s. It typically has a drier bias. But having said that, today it was reporting 100% and the nearby Airport station in Tuscaloosa had 100%. The only difference was that the RH dropped down to 94% at the Airport whereas the RainWise station in T-town was stuck on 100%.
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Offline jgentry

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Re: VP2 FARS airflow
« Reply #59 on: August 08, 2017, 09:42:23 PM »
I've posted about the airport ASOS here never exceeding 94% even in near zero visibility.  Hard to believe and same time I'm hitting 98-99% in those conditions with 3 different SHT31's.

Nice! The ASOS & AWOS here in Alabama have no problem reaching 100% humidity
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Offline CW2274

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Re: VP2 FARS airflow
« Reply #60 on: August 08, 2017, 09:50:47 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.
I'm thinking the reason why the Sensirion SHT-31 struggles to reach 100% is due to the sensor chip being really small. I might be wrong with my thinking.
As I've stated here many times before, some sensors can easily reach a 100% humidity reading whereas others do not. I'm not just talking PWS's, I'm talking the big boys too. It's a very finicky thing when the air becomes close to saturation.

I'm just trying to make sense of it. But then again, I guess I also need to keep in mind that all humidity sensors have wet/dry biases.
When it comes to humidity there is nothing linear about it's relation to temp and dew point. The closer you get to 100% the less the spread is. For instance, to get from 99 to 100% humidity may be a mere increase of only 0.4F dew point with the temp constant. At 50% the spread could be 3, 4, 5F, whatever, of dew just to reach 51%. Crunching at the end of the spectrum is much more problematic.
Davis Wireless VP2 SHT31 24hr 67CFM FARS
RW Tipper w/ CoCoRaHS

Offline jgentry

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Re: VP2 FARS airflow
« Reply #61 on: August 08, 2017, 09:52:55 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.
I'm thinking the reason why the Sensirion SHT-31 struggles to reach 100% is due to the sensor chip being really small. I might be wrong with my thinking.
As I've stated here many times before, some sensors can easily reach a 100% humidity reading whereas others do not. I'm not just talking PWS's, I'm talking the big boys too. It's a very finicky thing when the air becomes close to saturation.

I'm just trying to make sense of it. But then again, I guess I also need to keep in mind that all humidity sensors have wet/dry biases.
When it comes to humidity there is nothing linear about it's relation to temp and dew point. The closer you get to 100% the less the spread is. For instance, to get from 99 to 100% humidity may be a mere increase of only 0.4F dew point with the temp constant. At 50% the spread could be 3, 4, 5F, whatever, of dew just to reach 51%. Crunching at the end of the spectrum is much more problematic.

Gotcha. Thanks!
Davis Vantage Pro2. SHT 31. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358

Offline jerryg

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Re: VP2 FARS airflow
« Reply #62 on: August 08, 2017, 10:11:41 PM »
I have been reading about humidity and the warmer the temperature the lower the humidity reads. Warm air holds less water than cooler air. So that is why down here in the hot summer months the humidity does not get to 100 because the low temp only gets down to upper 70's to low 80's at night, not enough cooling to increase the moisture in the air. The norm is around mid 90's. So when the temp drops down into the low 70's here the humidity does read higher. Wow, could you imagine the dew point readings if the temps were high as well as the humidity.

Offline jgentry

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Re: VP2 FARS airflow
« Reply #63 on: August 08, 2017, 10:54:07 PM »
I have been reading about humidity and the warmer the temperature the lower the humidity reads. Warm air holds less water than cooler air. So that is why down here in the hot summer months the humidity does not get to 100 because the low temp only gets down to upper 70's to low 80's at night, not enough cooling to increase the moisture in the air. The norm is around mid 90's. So when the temp drops down into the low 70's here the humidity does read higher. Wow, could you imagine the dew point readings if the temps were high as well as the humidity.

That would be suffocating!
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Offline alexstaar

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Re: VP2 FARS airflow
« Reply #64 on: August 08, 2017, 11:12:20 PM »
I have been reading about humidity and the warmer the temperature the lower the humidity reads. Warm air holds less water than cooler air. So that is why down here in the hot summer months the humidity does not get to 100 because the low temp only gets down to upper 70's to low 80's at night, not enough cooling to increase the moisture in the air. The norm is around mid 90's. So when the temp drops down into the low 70's here the humidity does read higher. Wow, could you imagine the dew point readings if the temps were high as well as the humidity.

Let's be clear here... relative humidity is NOT a measure of the absolute water vapor content of air. This is better determined using dewpoint. If the dewpoint remains constant throughout a day, for example, the absolute water vapor content (vapor pressure) of the air remains exactly the same. Relative humidity decreases during the day because warmer air can hold higher concentrations of water vapor than cooler air. Therefore, relative humidity decreases because, relative to the air temperature, the air can hold more water vapor than it is currently holding at warmer temperatures than cooler temperatures for air that has not changed it absolute water vapor content. If dewpoint begins to increase or decrease, some sort of addition or subtraction of water vapor content in the air must be occurring (usually due to evaporation (transpiration from plants/water), mixing in the atmosphere, or advection). This is not necessarily the case when relative humidity increases or decreases.
-Alex

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Offline jerryg

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Re: VP2 FARS airflow
« Reply #65 on: August 09, 2017, 08:49:42 AM »
Yep you are right, just reread some more stuff about water vapor and it can get confusing real fast.  It looks like dp is the best way to get an accurate reading.

Offline Old Tele man

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Re: VP2 FARS airflow
« Reply #66 on: August 09, 2017, 02:13:38 PM »
When it comes to humidity there is nothing linear about it's relation to temp and dew point. The closer you get to 100% the less the spread is. For instance, to get from 99 to 100% humidity may be a mere increase of only 0.4F dew point with the temp constant. At 50% the spread could be 3, 4, 5F, whatever, of dew just to reach 51%. Crunching at the end of the spectrum is much more problematic.

You can use this NOAA(*) empirical formula to get some idea of that non-linearity:

RH% = 100%*[(173 - 0.1*Ta + Tdp)/(173 + 0.9*Ta)]^8 ...an '8th-power' is a pretty steep rising curvature.

where:
RH% = relative humidity, percentage
Tdp = dew point temperature, F
Ta = ambient air temperature, F

(*) developed by Julius F. Bosen, Office of Climatology, US Weather Bureau, 17-Nov-1958
reference: "An Approximation Formula to Compute Relative Humidity from Dry Bulb and Dew Point Temperatures," MONTHLY WEATHER REVIEW, Dec.1958, page 486.
« Last Edit: August 09, 2017, 06:22:49 PM by Old Tele man »
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Offline CW2274

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Re: VP2 FARS airflow
« Reply #67 on: August 09, 2017, 03:37:33 PM »
It looks like dp is the best way to get an accurate reading.
The only time I give humidity any real value is inside my home, but only because on the relative constant temp. Outdoors, humidity is pretty much useless, but it's what the masses "understand".
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Offline ValentineWeather

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Re: VP2 FARS airflow
« Reply #68 on: August 09, 2017, 04:39:31 PM »
This is getting way off topic but one of my rules on dewpoint is anything below 60 feels the same or cooler and above warmer.  In Arizona they used the 55 dewpoint standard as when evaporative cooling stopped working and AC was needed but I found the 60 DP rule is cut off for evaporative cooling also.   
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Offline Old Tele man

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Re: VP2 FARS airflow
« Reply #69 on: August 09, 2017, 04:51:48 PM »
This is getting way off topic but one of my rules on dewpoint is anything below 60 feels the same or cooler and above warmer.  In Arizona they used the 55 dewpoint standard as when evaporative cooling stopped working and AC was needed but I found the 60 DP rule is cut off for evaporative cooling also.
Actually, Tucson used "3-day" 54F dewpoint average (≈100 grains H20/lb of dry air) as the 'threshold' for start of the monsoon; Phoenix used "3-day" 55F dewpoint average. I can't remember what Yuma, AZ, used.
« Last Edit: August 09, 2017, 04:58:17 PM by Old Tele man »
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Offline CW2274

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Re: VP2 FARS airflow
« Reply #70 on: August 09, 2017, 05:36:13 PM »
This is getting way off topic but one of my rules on dewpoint is anything below 60 feels the same or cooler and above warmer.  In Arizona they used the 55 dewpoint standard as when evaporative cooling stopped working and AC was needed but I found the 60 DP rule is cut off for evaporative cooling also.
Actually, Tucson used "3-day" 54F dewpoint average (≈100 grains H20/lb of dry air) as the 'threshold' for start of the monsoon; Phoenix used "3-day" 55F dewpoint average. I can't remember what Yuma, AZ, used.
Key being past tense. Once again another benchmark eliminated so as to dumb it down for the public. :roll:
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Offline ValentineWeather

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Re: VP2 FARS airflow
« Reply #71 on: August 09, 2017, 05:39:11 PM »
This is getting way off topic but one of my rules on dewpoint is anything below 60 feels the same or cooler and above warmer.  In Arizona they used the 55 dewpoint standard as when evaporative cooling stopped working and AC was needed but I found the 60 DP rule is cut off for evaporative cooling also.
Actually, Tucson used "3-day" 54F dewpoint average (≈100 grains H20/lb of dry air) as the 'threshold' for start of the monsoon; Phoenix used "3-day" 55F dewpoint average. I can't remember what Yuma, AZ, used.

I remember that but with all their wisdom now go by the calendar.  :lol:  Its like hey look at us we can One-up-Ourselves on stupidity. Like it or not start date now June 15 when its still bone dry.  ](*,)
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Offline ValentineWeather

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Re: VP2 FARS airflow
« Reply #72 on: August 09, 2017, 06:06:27 PM »
Once again another benchmark eliminated so as to dumb it down for the public. :roll:

I can tell who works for the govs and who doesn't  :lol:
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Offline Old Tele man

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Re: VP2 FARS airflow
« Reply #73 on: August 09, 2017, 06:19:04 PM »
Once again another benchmark eliminated so as to dumb it down for the public. :roll:

I can tell who works for the govs and who doesn't  :lol:
Yes (past-tense), but NOT for the Weather-guessing branch of our Gobberment.
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Offline jgentry

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Re: VP2 FARS airflow
« Reply #74 on: August 13, 2017, 09:10:37 AM »
Read 100% this morning. Still have the passive shield installed for now. Even though all humidity sensors have their strengths/weaknesses, I still think the reason why the SHT-31 struggles/lags getting to 100% is due to the sensor chip's size. If it was bigger, I think we would see more times of the SHT 31 hitting 100%
Davis Vantage Pro2. SHT 31. WU: KXALJEMI2 & KALTHORS2 CWOP: C6353 & C6358