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Offlinepsyeye
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Fan inside FC Question
    #14481633 - 05/19/11 10:48 PM (12 years, 8 months ago)

I was pondering my first grow (like a true noob) and wondered... If evaporation is the main pinning trigger (worked instantly here) and lack of FAE or RH are the most common probs during fruiting, what about putting a small solar powered fan oscillating in a PMP, SGFC or Monotub?

A fan on the inside (not blowing air from outside) would move the air around inside providing evap, should help with FAE and wont affect humidity too much.

Is this an elegant addition :einstein:  or is there some simple fact my noobness is missing? :noob:


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OfflineBillyMaysHere
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Re: Fan inside FC Question [Re: psyeye]
    #14481709 - 05/19/11 11:03 PM (12 years, 8 months ago)

IME a fan directly in the chamber tends to dry things out too much.  I think if you could find a timer that would allow you to turn the fan on for 1 minute every 15 minutes or so, that would do a good job of cycling fresh air without drying everything out, but I've never been able to locate a timer with such small increments.

A properly built SGFC will work wonders though.


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OfflineMycjunky
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Re: Fan inside FC Question [Re: psyeye]
    #14481712 - 05/19/11 11:03 PM (12 years, 8 months ago)

It would most likely lower the humidity and cause to much evaporation not a good idea. You want FAE but that would be overkill.


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OfflineLord_McLovin
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Re: Fan inside FC Question [Re: Mycjunky]
    #14483493 - 05/20/11 09:35 AM (12 years, 8 months ago)

I'm always wondering why you guys use fans for FAE. Physically, there is not much point to it. Yes, you then have a directed air flow, yet the gas molekules are extremely fast anyway and you increase evaporation (i.e. loss of humidity). I mean, as a good approximation:

U = f/2 k T
E = 1/2 mv²
E=U
=> v = sqrt (fkT/m)

k= 1.3807×10^-23 J/K
T= 75 °F = 297 K
for o2:
f=5 m= 5.31352×10^-26 kg
for CO2
f=5 m= 7.30795×10^-26 kg

=>
v(O2)= 326 m/s = 729.7 mph
v(CO2) = 278.1 m/s = 622.2 mph

Didn't you learn that in school?


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Offlineesquaredx
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Re: Fan inside FC Question [Re: Lord_McLovin]
    #14483557 - 05/20/11 09:58 AM (12 years, 8 months ago)

If you build a proper sgfc you wont ever need a fan.
Could always have a fan in the room indirectly blowing air around.
Build your shits right, and no worrys.


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OfflineLord_McLovin
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Re: Fan inside FC Question [Re: esquaredx]
    #14483587 - 05/20/11 10:05 AM (12 years, 8 months ago)

Quote:

esquaredx said:
If you build a proper sgfc you wont ever need a fan.




True that.

Quote:

Could always have a fan in the room indirectly blowing air around.




As I said, I do not see the point. This only increases evaporation, loss of humidity and the chance of contamination.


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OfflineMycjunky
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Re: Fan inside FC Question [Re: Lord_McLovin]
    #14485573 - 05/20/11 05:29 PM (12 years, 8 months ago)

It is beneficial to have fans blowing air around in the same room just not inside the fruiting chamber. You don't want totally stale air and if you don't have any fans in say a basement for example it can definitely make it easier for mold to pop up.

Just gotta make sure the fans not blowing directly at it. In commercial mushroom operations they usually have fans for exhaust so you want some air circulation in the same room.


Edited by Mycjunky (05/20/11 05:31 PM)


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OfflineLord_McLovin
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Re: Fan inside FC Question [Re: Mycjunky]
    #14489349 - 05/21/11 11:57 AM (12 years, 8 months ago)

Quote:

Mycjunky said:
You don't want totally stale air




That was exactly my point. Stale air does not exist - just have a look at my calculations a few posts above and you will see the average velocity of an oxygen and a carbondioxide molecule respectively for 75°F. That is why we have air pressure. By fanning you do actually not increase the average speed of your gas molecules much - you only lead them into one direction.
Thus as long as you have one large or several small holes in your FC, you will be fine with FAE. The amount of air exchanged per unit time depends mainly on the size of the area fresh air can enter in.
If you use fans you only move larger particles around like mold spores - your mycelium does not actually get much more oxygen to breathe.

Quote:

and if you don't have any fans in say a basement for example it can definitely make it easier for mold to pop up.




Why? Concerning mold spores there is one major difference between using a fan and not using one. In the first case you move them around your FC - in the second most of them just drop to the ground.
Why does the latter increase your contamination rate?
Of course you should make sure fresh air can enter your FC from the outside.

Quote:

Just gotta make sure the fans not blowing directly at it. In commercial mushroom operations they usually have fans for exhaust so you want some air circulation in the same room.




In commercial mushroom operations they filter the air to the FC, however. That makes a huge difference since you thus decrease the area fresh air can enter through. In order to cope with that you need to create a positive or negative pressure environment in your FC in order to force osmosis through your HEPA filter.


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OfflineMycjunky
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Re: Fan inside FC Question [Re: Lord_McLovin]
    #14492168 - 05/22/11 12:31 AM (12 years, 8 months ago)

I've seen commercial grows that don't filter the air. You can talk all day long about your fancy calculations but i don't care how many contams are in the area that doesn't mean they will grow and speaking from experience I've seen a reduced rate of contams in a room with a fan blowing air around than with not.

Just my 2 cents from an experienced person.


Edited by Mycjunky (05/22/11 12:32 AM)


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OfflineLord_McLovin
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Re: Fan inside FC Question [Re: Mycjunky]
    #14492629 - 05/22/11 03:56 AM (12 years, 8 months ago)

Quote:

Mycjunky said:
I've seen commercial grows that don't filter the air. You can talk all day long about your fancy calculations but i don't care how many contams are in the area that doesn't mean they will grow and speaking from experience I've seen a reduced rate of contams in a room with a fan blowing air around than with not.

Just my 2 cents from an experienced person.




I was by no means trying to insult you or question your experience. I'm sorry if you got that impression.
I'm just trying to figure out what the reason might be for this reduced contam rate using a fan.


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OfflineTerry M
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Re: Fan inside FC Question [Re: Lord_McLovin]
    #14492632 - 05/22/11 03:57 AM (12 years, 8 months ago)

Quote:

Lord_McLovin said:
I'm always wondering why you guys use fans for FAE. Physically, there is not much point to it. Yes, you then have a directed air flow, yet the gas molekules are extremely fast anyway and you increase evaporation (i.e. loss of humidity). I mean, as a good approximation:

U = f/2 k T
E = 1/2 mv²
E=U
=> v = sqrt (fkT/m)

k= 1.3807×10^-23 J/K
T= 75 °F = 297 K
for o2:
f=5 m= 5.31352×10^-26 kg
for CO2
f=5 m= 7.30795×10^-26 kg

=>
v(O2)= 326 m/s = 729.7 mph
v(CO2) = 278.1 m/s = 622.2 mph

Didn't you learn that in school?




Assuming your calculations are correct, this is simply the average velocity of gas molecules from their thermodynamic kinetic energy. You get such velocities over very small distances. No way does this extend to the macroscopic scale. Thermodynamic speeds are sufficient to mix gases, but not turn over stale air for fresh at any significant rate.


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OfflineLord_McLovin
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Re: Fan inside FC Question [Re: Terry M]
    #14492660 - 05/22/11 04:25 AM (12 years, 8 months ago)

Quote:

Terry M said:
Quote:

Lord_McLovin said:
I'm always wondering why you guys use fans for FAE. Physically, there is not much point to it. Yes, you then have a directed air flow, yet the gas molekules are extremely fast anyway and you increase evaporation (i.e. loss of humidity). I mean, as a good approximation:

U = f/2 k T
E = 1/2 mv²
E=U
=> v = sqrt (fkT/m)

k= 1.3807×10^-23 J/K
T= 75 °F = 297 K
for o2:
f=5 m= 5.31352×10^-26 kg
for CO2
f=5 m= 7.30795×10^-26 kg

=>
v(O2)= 326 m/s = 729.7 mph
v(CO2) = 278.1 m/s = 622.2 mph

Didn't you learn that in school?




Assuming your calculations are correct, this is simply the average velocity of gas molecules from their thermodynamic kinetic energy.




Absolutely true.

Quote:

You get such velocities over very small distances. No way does this extend to the macroscopic scale.




Well, you have to keep in mind that the motion is non-directed (Brownian) and that this is indeed, physicallly speaking, the average absolute value of the instantaneous velocity of our gas molecules. They can go anywhere.

Quote:

Thermodynamic speeds are sufficient to mix gases, but not turn over stale air for fresh at any significant rate.




Turning stale air over for fresh is in fact nothing else but mixing gases or better to say a balancing of concentrations (diffusion). Thus ,if you want the process to be faster, just increase the area where diffusion can take place.


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OfflineTerry M
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Re: Fan inside FC Question [Re: Lord_McLovin]
    #14492765 - 05/22/11 05:28 AM (12 years, 8 months ago)

Lord_McLovin, you are forgetting that the distance over which gases mix requires time. In fact, a given distance for mixing goes as the square root of the time.

I'm not much of a chemist, so bear with me.

The diffusion coefficient D for the mixing of oxygen and CO2 is 0.153 cm2/s at 20 degrees C. From Fick's second law, we can approximate the distance over which molecules mix with time by:

diffusion length = 2sqrt(Dt)

Plugging in the numbers:
Over a period of one minute, the two gases mix to a distance of 3 cm.
Over a period of one hour, the two gases mix to a distance of 23 cm.

In an hour, the gases will not exchange enough for anything close to one complete change of air volume inside a grow chamber. Mushrooms need a rate of fresh air exchanges in the range of 2 to 8 per hour, depending on variety. So purely thermodynamic gas exchange is completely insufficient.


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OfflineRogerRabbitM
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Re: Fan inside FC Question [Re: Lord_McLovin]
    #14493012 - 05/22/11 08:07 AM (12 years, 8 months ago)

Quote:

Lord_McLovin said:
Quote:

Mycjunky said:
You don't want totally stale air




That was exactly my point. Stale air does not exist - just have a look at my calculations a few posts above and you will see the average velocity of an oxygen and a carbondioxide molecule respectively for 75°F. That is why we have air pressure. By fanning you do actually not increase the average speed of your gas molecules much - you only lead them into one direction.
Thus as long as you have one large or several small holes in your FC, you will be fine with FAE. The amount of air exchanged per unit time depends mainly on the size of the area fresh air can enter in.
If you use fans you only move larger particles around like mold spores - your mycelium does not actually get much more oxygen to breathe.

Quote:

and if you don't have any fans in say a basement for example it can definitely make it easier for mold to pop up.




Why? Concerning mold spores there is one major difference between using a fan and not using one. In the first case you move them around your FC - in the second most of them just drop to the ground.
Why does the latter increase your contamination rate?
Of course you should make sure fresh air can enter your FC from the outside.

Quote:

Just gotta make sure the fans not blowing directly at it. In commercial mushroom operations they usually have fans for exhaust so you want some air circulation in the same room.




In commercial mushroom operations they filter the air to the FC, however. That makes a huge difference since you thus decrease the area fresh air can enter through. In order to cope with that you need to create a positive or negative pressure environment in your FC in order to force osmosis through your HEPA filter.




I can't begin to address all the mistakes you've made in that post.  There is stale air in non-properly built fruiting areas.  Circulation helps prevent contaminant spores from germinating because molds prefer still, stale air.  One or two holes in a terrarium isn't going to cut it. Osmosis and HEPA filters have nothing to do with each other, and in my commercial mushroom farm, there is no filter on the intake and less than one substrate in 1000 ever gets mold.  Mold spores don't drop to the ground, they float on the natural air currents, since air is rarely completely still.  Air pressure is not caused by velocity, but by rather by weight.

I could go on and on, but please learn a bit about mushrooms and science before giving bad advice to others and stating it as fact.
RR


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OfflineLord_McLovin
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Re: Fan inside FC Question [Re: Terry M]
    #14494449 - 05/22/11 03:03 PM (12 years, 8 months ago)

Quote:

Terry M said:
Lord_McLovin, you are forgetting that the distance over which gases mix requires time. In fact, a given distance for mixing goes as the square root of the time.

I'm not much of a chemist, so bear with me.

The diffusion coefficient D for the mixing of oxygen and CO2 is 0.153 cm2/s at 20 degrees C. From Fick's second law, we can approximate the distance over which molecules mix with time by:

diffusion length = 2sqrt(Dt)

Plugging in the numbers:
Over a period of one minute, the two gases mix to a distance of 3 cm.
Over a period of one hour, the two gases mix to a distance of 23 cm.

In an hour, the gases will not exchange enough for anything close to one complete change of air volume inside a grow chamber. Mushrooms need a rate of fresh air exchanges in the range of 2 to 8 per hour, depending on variety. So purely thermodynamic gas exchange is completely insufficient.




Actually the relation is x= sqrt(2Dt) and I think you did some error plugging in the numbers. Using the D you gave one gets:
sqrt(0.153*3600*2) cm= 33.2 cm
Even though this relation is derived under the assumption that at t=0 all the particles are concentrated at one point in (one-dimensional) space and is thus not really applicable, one gets at least an impression of the order of magnitude we are talking about here.
I think I really underestimated the effect of Brownian motion. Thank you for your constructive criticism.
Unfortunately this also implies that if the FC is rather large, one does really need to direct the air flow using a fan, since as you said, thermodynamic gas exchange will probably not be sufficient. Too bad.


Quote:

I can't begin to address all the mistakes you've made in that post.  There is stale air in non-properly built fruiting areas.




I think this is a language issue on my side. By stale I meant not moving. Air that is not moving (i.e. molecules) cannot exist. Air that is CO2 laden certainly does exist. Excuse me, if I misunderstood something.

Quote:

Osmosis and HEPA filters have nothing to do with each other




Air moves through you HEPA filter, being a semipermeable membrane. How on earth can that not be osmosis?

Quote:

, and in my commercial mushroom farm, there is no filter on the intake and less than one substrate in 1000 ever gets mold.


 

I would never even dare to question your expertise concerning mycology, yet I do question it regarding physics.

Quote:

Mold spores don't drop to the ground, they float on the natural air currents, since air is rarely completely still.


 

In fact it never is. I think there's a minor misconception. Even though your mold spores might not be moving, the gas particles air consists of always move. Due to the large difference in mass and the extremely unordered motion of your gas molecules, the momentum transfer is, however, not apparent.

Quote:


Air pressure is not caused by velocity, but by rather by weight.




That is nonsense. Classically air pressure is caused due to the large number of collisions of the gas molecules with the container wall. Just imagine a completely closed box in free space with some gas in it. It will have no weight (lack of gravity), yet there will still be pressure due to the particles hitting the container wall.
Our planet is like such a container - just that there is no real wall keeping the air on the planet - it is gravitational attraction. Of course, if you switch off gravity :laugh: our nice atmosphere will be gone and there is no more pressure. But if you somewhat manage to keep your atmosphere in place by other means and then switch off gravity - you will still have pressure.


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