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  1. #1
    Seedling rangergord's Avatar

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    Cool Hydro: One year In

    Allright, first off the number of sections here in the TY forums, considering the number of members is INSANE! So this thread will be mostly about my new hydro setup but if I veer off course a little bit, I am not gonna start a whole new thread somewhere else!

    Got a few pictures to share as well. I am just starting my second indoor season (I shut down for summer and use the great outdoors) with a new hydroponic growroom (closet).

    For many years I have been growing in soiless mix, and using either salt based or organic fertilizers. Now soil has its advantages, its accessible, affordable and has a simplicity that is easily embraced. So why did I switch to hydro? Well it took me a while to figure hydro out in terms of the fundamental concepts. I rarely went near a hydro store so I just slowly picked things up as I read online.

    Then I began to figure out what the disadvantages of soil were. I mixed my own soil from peat moss, perlite and a bit of lime. I sometimes added a bit of vermiculite. I learned to mix soil that had lots of ability to absorb water but also had porosity to hold air for good root growth.

    Then when I watered it would be too wet at first, then as it dried out it would be just right and then it would be too dry and need watering again. This may seem elementary but in containers of soil, conditions are really only ideal for root growth part of the time. The rest of the time they are either too wet or too dry.

    Next there is water quality. That became evident once I obtained a TDS meter and a pH meter. At that point I realized that my well water was a big problem. It tested out at up to 500ppm of dissolved minerals. So my fertilizer solutions were way too strong even mixed at low levels and the pH was way up there.

    So I needed better water. An RO unit is often recommended but I had no room for one and they are best installed in tandem with a water softener in water as hard as mine. That put the price up way beyond what I could justify. So I realized that naturally distilled water was all around me. Its called rain, and around here it piles up and hangs around for almost 6 months as snow! I started off by collecting and melting snow in 5 gallon pails. This is what I use now. Click image for larger version. 

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    I collect rainwater and in the winter bring a barrel inside where I can dump pails of snow into it to replenish the supply. I have at least 25 gallons of naturally distilled water on hand at all times.

  2. #2
    Seedling rangergord's Avatar

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    Once I started paying attention to what I was putting in my plants I started measuring what was coming out. Now I could track the pH and the TDS of the run off when watering and fertilizing. I saw it start out at a few hundred ppm and about 6-6.5 then it would start rising until it was a pH of over 7 and ppm of over 1000. Then all of a sudden I would get leaf tip burning and yellowing and most of all stunted growth after about 6-8 weeks into growth. One mistake I was making, was I let the pots sit in the run off like a houseplant in a saucer. Big mistake! I realized they needed to be able to drain freely. I put a fridge shelf grate under them and that helped a bit.

    Things went a bit better but still not what I wanted to see in terms of growth. It still was difficult to stop the salt buildup, pH change and nutrient burning and lockout. The soil itself was part of the problem because every time I fertilized the plants the soil would start out wet and then as it dryed the concentration of fertilizer that may have originally been at a safe level would then go up dramatically as the water evaporated.

    It was at this point that I began to see that hydroponics could offer a solution to the problem. If I grew in water, instead of soil the concentration of nutrients and the pH would probably be more stable. If things got out of control, I could just refill the reservoir. Now I could just flush my pots of soil with lots of clean water as well but it is not an ideal solution because the soil is far too wet for several days and growth comes to a halt until it dries out. When there is too much water in the soil there is not enough air for good root growth. So then I thought, What is the difference between over watering a plant in soil and hydroponics? The answer is that the water in hydroponics has an abundant supply of dissolved oxygen and an over watered plant does not.

    The whole principal behind hydroponics is working water or water that moves with the assistance of a water or air pump. In this way the water comes into contact with lots of oxygen that is dissolved into solution. Thats why your plants don't drown in hydroponics but they can in over watered soil.

    Again this might seem elementary to some but when you discover its truth for yourself in an experiential manner it makes all the difference.

    Next I decided to make a reservoir of water and put an air pump and stone on it. Then I would hand water with nice oxygen filled water instead of the dead water I was using before. This actually worked quite well. I saw the plants perk up considerably.

    I started using more water than ever and as the excess was draining away it became a problem. So I needed something to catch all the run off an allow it to be disposed of. I saw a nice looking flood table that looked like it would do the job so I bought it. Then I put the potted plants inside and watered away while the excess drained neatly into a pail. Naturally that lead to the idea of putting a pump in my reservoir and watering automatically. I figured though that using soil was not a good idea at that point because it would just end up soggy and overwatered most of time. That was when I made the decision to go to a full blown flood and drain system with expanded clay and rockwool.

    The point of all this is to illustrate some important principles and show how a natural progression can lead to adopting hydroponic methods.

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    Seedling rangergord's Avatar

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    So here is my new reservoir. It holds 60-80 liters. I have a 250 GPH water pump and a 150 watt aquarium heater and air pump with air stone. The pump feeds water up into the table with 1/2" hose and drains back into the rez with 3/4" flood and drain fittings.

    Over top you can see the 6" deep, 2' x 4' grotek high tide flood and drain table.
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    Seedling rangergord's Avatar

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    I keep the aquarium heater set to 68 deg F. Enough to keep it warm during the cold weather but low enough that the water holds as much oxygen as possible.

    Next there is my cables and controls. Timers for the growlamp and water pump. Thermostat and speed control for the fan. The E ballast for the Lumatek 400. I love being able to control every part of the system. The ballast can be dimmed to 250 watts and I was amazed to discover that my veg cycles grew much more quickly dimmed down than with my previous 400 mag ballast lamp. Temperatures were much more conducive to vegetative growth and I save lots of power. Currently I am running 12+1 hours of light in veg and notice no difference in growth rates over 18-24 hours of light.
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    Vegetative Member schmade420's Avatar

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    I'm glad you've found an ideal way for you to grow mate. Nothing wrong with doing what works for you. I myself grow organically the True Living Organics method. I've done the hydro thing and switched back to soil. I just believe my meds have a richer bolder smell and taste this way. Happy growing!

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    Seedling rangergord's Avatar

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    I used to use a 265 CFM active air squirrel cage blower but it would not handle a carbon filter and did not respond well to a solid state speed controller without overheating.

    So I upgraded to a CanFan Max 6. This is an awesome fan. It is a mixed flow fan that excels at moving air more easily through ducts and handles back pressure well. It has built in speed control and is quiet while using very little power. The price was right and I am very pleased with it. It works so well in my space that my fan only runs part time and keeps things at about 75 F with ease.

    Then I added this carbon filter. It is a Canlite 6" carbon filter. It was a much better fit for my setup than the regular filters. Much lighter and smaller while still having all the capacity I need and a long lifespan as well. It is an awesome feeling not having to worry about smells during flowering.
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  7. #7
    Seedling rangergord's Avatar

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    Here you can see my table with what I am currently growing. The plants are started in 1" rockwool cubes that I have never really liked that much because they remind me of fiberglass insulation. But on the plus side I find that I have higher germination rates and no damping off. Once I have them germinated and growing I transplant them into the 6" net pots with hydroton.

    The first crop I grew, I used the table without a cover, just placing the pots in the tray and leaving it open. I found that sometimes the roots were a bit dry and over exposed to the light. So I added a coroplast cover and now the roots have a nice sheltered humid space to grow in while the light bounces back onto the plants rather than falling on the roots. I can also transform this table into a drip or aeroponic system if I wanted to.
    I really do like the flood and drain system at this point however. My plants grow constantly and don't lock up and shut down prematurely like they did in soil. I run things pretty lean as far as fertilizer goes. I start off at 600ppm and build up to about 800ppm in flowering. Have gone as high as about 900 but at that level everything is dark dark green and they stop feeding and just want water. Even at that when it comes time to flush the last couple of weeks I just let the TDS fall to 300 or less and it finishes just right in terms of taste and flavour.

    Growth rates are much higher than in soil. Yeilds are up big time and I discovered there really is a big difference in the density and flavour of hydro compared to soil grown buds. My indoor used to look and taste like my outdoor grown herb. Now the indoor tastes like hydrobud and is distinctly different from the outdoor.
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    Seedling rangergord's Avatar

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    The first picture is a shot of 11 plants that I started. The second pic shows two afghani #1 plants. The others in the third pic are a new hybrid I created from a feminized white widow crossed with Leda Uno. Will post more pics as they progress.

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    Seedling rangergord's Avatar

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    Took some more pics today. The plants have now just been sexed. From 11 plants I have culled 3 males and kept a 4th afghani #1 male for creating progeny.


    Here is an overall shot of the 8 plants remaining. I male 7 females
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    Seedling rangergord's Avatar

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    Day 9 of flowering after 3-4 weeks of veg.

    Here is Click image for larger version. 

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    And sulking in the back is the Afghani#1 male. I'll be keeping a close eye on him. I want 6 sensi females and 1 moderately seeded Afghani female!

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    Seedling rangergord's Avatar

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    Lastly a couple female White widow x Leda Uno's

    Not sure what these will turn out like. They have a strong smell. I really was impressed with the white widow and its spicy resinous buds with the most amazing thin sativa leaves. The leaves on these plants are just like Leda Uno.
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  12. #12
    Flowering Member oddjobs's Avatar

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    Good choices ranger . I,ve been growing in flood and drain for many years due to ease of growing ,I can't carry the dirt it takes to grow that way properly. Question ? How many time during a grow cycle are you changing you complete resivour. I have been using general hydroponics nutes with good success . Real easy to do with flood and drain just add as you need . I try to do complete res. change at least every couple weeks .some one told me once after I add as much as my res holds it's time to do a res change . Like your grow nice an clean , I also find I don't have as much trouble with plant pests as I do in soil. Good luck to you ranger keep up the good work . Oj

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    Seedling rangergord's Avatar

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    Quote Originally Posted by oddjobs View Post
    Good choices ranger . I,ve been growing in flood and drain for many years due to ease of growing ,I can't carry the dirt it takes to grow that way properly. Question ? How many time during a grow cycle are you changing you complete resivour. I have been using general hydroponics nutes with good success . Real easy to do with flood and drain just add as you need . I try to do complete res. change at least every couple weeks .some one told me once after I add as much as my res holds it's time to do a res change . Like your grow nice an clean , I also find I don't have as much trouble with plant pests as I do in soil. Good luck to you ranger keep up the good work . Oj
    Hey Oddjobs Thanks. Reservoir changes on the basis you describe are certainly the standard best advice I have seen. My res is filled with Advanced Nutrients 3 part which of course is a similar thing to GH. I am using up a whole bunch of AN supplements I am doing res changes but not every week like AN prescribes in their schedules.

    My first fill lasts me about 6 weeks. The first 2-3 weeks there is not much being taken up by the plants. Mostly evaporation. I just run at about 600ppm of base nutes for the first 4 weeks then add bud blood at sexing time. Then I refill with base and big bud for two weeks up to 800ppm- drain refill with base and overdrive for two weeks then drain and run fresh or just add water to the last rez until it falls to less than 300ppm. So thats 3-4 complete res changes. I see no need to adhere to a strict 1 or 2 week change schedule. If your water is pure like mine, pH will tend to be more stable and as long as the plants get enough to eat but not too much everything works out fine.

    I can't see going much less than that personally but I am moving towards GH ferts for simplicity. Got a bunch of maxibloom that I am going to use as lucas formula. Should be able to ditch the armload of supplements.

    It may not be standard procedure but it works well for me, so until otherwise I will keep at it!
    Last edited by rangergord; 11-09-2013 at 07:51 PM.

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    Seedling rangergord's Avatar

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    Here is an update. Next time I am going to have to put in a MH bulb for this. I can't remove the plants for pics. The first pic is a female afgani #1. I had a male but it seemed to be sterile because I did not find any stray pollen as the buds opened and fell off. Thats weird.
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    The next three pics are all White widow x Leda Uno. The topmost leaves are all beginning to look more like WW than LU. The smell is starting to come on now as well and it is more like WW too. Just what I was hoping for.
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  15. #15
    Seedling rangergord's Avatar

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    Here we have the inside of the flood and drain tray showing the 1/2" inlet with debris guard and the 3/4" overflow.
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    And a peek at the roots showing them expanding nicely. This is what I like about the coroplast tray cover. The roots are free to grow in the humidity without being dried out in the light or being littered by falling debris such as dead leaves.
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    Data Nazi Lightly_Toasted's Avatar

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    Very nice!

    Yes we do have many different sections, but most was here when I signed up! Eventually, some will be condensed if needed. But organization is key.

    LT

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    Seedling rangergord's Avatar

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    Thought I should update this thread a bit. I have a preharvest pic or two of the LU x WW around somewhere. I'll have to dig for it. I named it Sativa Domina and it turned into the best bud I have ever grown because I learned to control my humidity during drying and curing and also did a full two week flush in pure water.

    Right now though I think I'll show you the Humidifogg 420 atmospheric control system I just built! Click image for larger version. 

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    You can see the state of art 5 gallon pail outfitted with a computer fan and foghorn in the lid. Inside is a 27 watt ultrasonic fogger designed for hydroponics. It is all controlled by a CAP HUM-1 humidity controller. This baby puts out lots of fog and runs intermittently. Now my closet is running between 50- 70% RH.

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    Seedling rangergord's Avatar

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    I am on a bit of a humidity kick right now. I live in a dry climate and heat with wood so my indoor humidity is very low, sometimes less than 20% and ranging up to 25-30%. For years I have thought it to be no real problem, at least I had no mold right? Then circumstance set out to prove me wrong.

    When I grew in soil, no matter how little fertilizer I used or how soft my water, I ended up with plants suffering from tip and leaf burn, ph changes, and nutrient lockout.

    When I grew autoflowers indoors I would end up with lot of stunted plants. When I grew the same plants outdoors under a crop cover, they exploded in growth and delivered bumper crops.

    Then when I harvested and cured my herb it was good and potent but never seemed to have great taste and you had to squish it a bit to get it to smell.

    So I got to reading about advanced curing methods using an accurate hygrometer to measure the amount of moisture inside my curing containers. I dried my last crop inside my grow closet and monitored the humidity raising it to 60-70%. Normally it would be low at 40% or less. I used wet towels and circulation fan and managed to slow drying down enough that it was a full 7 days to semi-cracking stem stage. Then I transferred it to a rubbermaid tub and burped it until I had a steady humidity of 65%. After another week of curing I went to the jars.

    Well the difference in my harvest was like night and day! All of sudden it had flavour, taste and smell. It was dank! It was clearly the difference in humidity that was at work.

    So I started exploring how to create a controlled atmospheric humidity in my closet. Then I discovered the term Vapour Pressure Deficit. The definition is a bit complex but basically it is about how low and high relative humidity determine the transpiration rate of your plants. At high humidity plants transpire less water through though their leaves and at low humidity plants transpire much more water. They may transpire so much that the plant has to close its pores to survive and growth stops. As it transpires it draws in water and nutrients, too much water and nutrients, leading to nutrient toxicity and lockouts! Slow growth and stunted plants are common. Now many people grow in climates with more moderate and higher humidity and never experience any problems with low humidity. They may need an air conditioner or dehumidfier to avoid mold. Not me! All I have to do to lower humidity is turn on the fan. I also noticed along the way that my plants grew better with less light. This too was related to humidity. Less light= less heat and less ventilation= higher humidity.

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    Seedling rangergord's Avatar

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    humidity.pdf

    Here is a great read about humidity and flower growing in greenhouses. The principles hold true for indoor growing as well.

    Humidity can be the most difficult environmental factor to control in
    greenhouses. Maintaining setpoints and correcting for too little or too
    much humidity can be a challenge for even the most sophisticated
    monitoring and control equipment. Humidity levels fluctuate with changes
    in air temperature, and plants are constantly adding water to the air
    through transpiration. Although automated controls have added a higher
    level of precision to the art of sensing and correcting humidity levels, it is
    still important to have a good understanding of the dynamics of
    atmospheric water vapour. There is a natural tendency with sophisticated
    equipment to just ‘set it and forget it’. However, lost yields, plant stress,
    disease outbreaks, and wasted energy are still as possible as ever unless we
    realize the limitations of our equipment and the implications of
    environmental control decisions.
    What is Humidity?
    Humidity is an expression of the amount of water vapour in air. It is an
    invisible gas that varies between 1 - 4% of our atmosphere by volume (see
    Figure 1). Fogs, mists, and other tiny water droplets are not water vapour.
    Figure 1. Composition of the Atmosphere
    The maximum amount of water vapour in any given air sample is
    dependent on the temperature and to a lesser extent the air pressure (See
    Figure #2). The actual amount of water vapour present is also determined
    by the availability of free water to evaporate. Water vapour will always
    move from an area of high concentration (such as inside the leaf cavities)
    to an area of lower concentration (the greenhouse air). This is the principle
    behind evaporative transpiration.
    Understanding Humidity
    Control in Greenhouses
    * % in dry air
    C02 &
    Other Gases*
    1%
    Water Vapour
    1 - 4%
    Oxygen*
    21%
    Nitrogen*
    78%
    File No. 400-5
    Floriculture
    FACTSHEET
    June, 1994
    Ministry of Agriculture,
    Fisheries and Food
    Abbotsford Agriculture Centre
    1767 Angus Campbell Road
    Abbotsford, BC V3G 2M3
    Phone: (604) 556-3001
    Fax: (604) 556-3030
    2
    Figure 3. Vapour Pressure Deficit
    Tables 1 & 2 (See Page 8) outline vapour
    pressure deficits (the difference between
    saturated air and air at various relative
    humidities). Although different crops vary in
    their response to humidity levels, a VPD range
    of 8 - 10 mb has been suggested as an optimum
    range. VPD can be used for both dehumidifying
    and humidifying, but it is particularly useful for
    humidifying.
    Role of Humidity
    The main plant mechanism for coping with
    humidity is the adjustment of the leaf stomata.
    Stomata open and close in response to vapour
    pressure deficit, opening wider as humidity
    increases. When humidity levels drop to about 8
    grams/m3 (12 mb VPD) the stomata apertures
    on most plants close to about 50% to help guard
    against wilting. This also reduces the exchange
    of C02, thereby affecting photosynthesis.
    Figure 4.
    We usually talk of air moisture in terms of
    relative humidity. Because the absolute amount
    of water that can be held by air is constantly
    fluctuating with temperature, relative humidity is
    a handy way of describing the ratio of water
    vapour compared to the total amount of water
    that could be held in the air at saturation.
    Therefore, 50% relative humidity indicates that
    the air has half the water vapour that it could
    hold if it were completely saturated. As the air
    temperature rises, more water vapour can be
    held in a given amount of air. And as the air
    becomes warmer, more moisture must be added
    to the air to maintain the same relative humidity.
    What is vapour pressure deficit?
    Relative humidity is still the most commonly
    used measurement for greenhouse control, even
    though it is not a perfect indication of what the
    plants ‘feel’. Plants respond to the difference
    between humidity levels at the the leaf stomata
    and the humidity levels of the surrounding air. At
    the same relative humidity levels, but at different
    temperatures, the transpiration demand for water
    from the leaves may be double (See Figure 3.)
    Therefore, another kind of measurement, called
    the Vapour Pressure Deficit is often used to
    measure plant/air moisture relationships. Some
    environmental control companies now offer VPD
    measurements as a part of their humidity
    management programs.
    Figure 2. The maximum moisture content of air
    increases as temperatures increases.
    Vapour Pressure mb
    0 Temperature 0C
    5
    10
    15
    20
    25
    30
    0 5 10 15 20 25 30
    Moisture
    Saturation
    Point
    Temperature (o Celcius)
    Water saturated air exits
    via the leaf stomata
    The difference in the amount of water vapour in the leaf
    (always assumed to be saturated or 100% RH) and the
    outside air is the VPD (Vapour Pressure Deficit). The
    higher the VPD, the greater the evaporation rate.
    Vapour Pressure
    Deficit increases with
    temperature, even
    when the relative
    humidity remains
    constant.
    3
    • Transpiration - Plants can control their rate
    of water loss. Because the leaf stomata have an
    ability to limit transpiration rates, a doubling of
    the moisture deficit may result in only a 15%
    increase in the transpiration rate. However,
    when humidity levels are very high, the total
    uptake of minerals is reduced since plants are
    unable to evaporate enough water.
    ••• • Photosynthesis - Humidity levels indirectly
    affect the rate of photosynthesis because C02 is
    absorbed through the stomatal openings. At
    higher daytime humidity levels, the stomata are
    fully opened allowing more C02 to be absorbed
    for photosynthesis. Photosynthetic levels can
    vary by about 5% between VPD’s of 2-10 mb.
    ••• • Growth and Quality - Most greenhouse
    plants tend to grow better at higher relative
    humidities. However, mineral deficiencies,
    disease outbreaks, smaller root systems, and
    softer growth are possible consequences of
    excess humidity. There is no one level of
    humidity that is good for all crops.
    Quality Problems Due to Humidity
    Too Low Too High
    Dry Tip Burn Oedema
    Wilting Edge Burn (Guttation)
    Small Leaves Soft Growth
    Stunted Plants Mineral Deficiencies
    Spider Mites Disease Outbreaks
    Leaf Curl
    Dehumidification
    In greenhouses, we usually try to avoid humidity
    levels near the dewpoint* since free water
    condensing onto plant surfaces can promote the
    growth of disease organisms. Under saturated
    humidity conditions plants cannot evaporate
    water from their leaves so the uptake of nutrients
    such as calcium and boron may be limited. It is
    important to remember that when the relative
    humidity reaches 90%, it takes only a slight drop
    in temperature to reach the dewpoint. The
    problem is compounded by the fact that not all
    surfaces in the greenhouse are necessarily at the
    same temperature as the air. Any surfaces that
    are cooler than the air at high relative humidities
    will condense water vapour. That is why
    dripping can be such a problem with glazing
    materials during the heating season.
    Monitoring and controlling the relative humidity
    of the greenhouse air is not always a guarantee
    that the dew point will be avoided. Local
    condensation problems can still occur due to
    uneven heat distribution and the thermal mass of
    plant materials, particularly on plants with fruits
    and other large waterfilled parts. This causes
    their surface temperatures to lag behind when
    sudden changes in air temperature occur. It’s the
    same reason a glass of ice water sweats even
    when the relative humidity of the room air is well
    below the dewpoint. Cold surfaces within the
    greenhouse cool the air immediately surrounding
    them. If the cooling reaches the dewpoint
    temperature, water condensation occurs.
    Excess humidity is usually more problematic in
    the spring and fall seasons when the weather is
    cool and moist. (See Figure 5.) High humidities
    are not likely to occur during freezing weather,
    since the relative humidity of the outside air is
    very low. A combination strategy of venting to
    exchange moist air with drier outside air, and
    heating to reduce the relative humidity levels,
    raise the temperature of plant surfaces, and
    warm the incoming air is usually employed.
    Glass panes and other cold surfaces in the
    greenhouse serve as natural dehumidifiers when
    the outside air is colder, but this, of course, can
    cause problems with dripping.
    * See page 6.
    4
    0
    2
    4
    6
    8
    10
    12
    J F M A M J J A S O N D
    3 Start dehumidifying at or about 85% RH.
    Relative humidities above this level are not
    easily managed without an increased risk of
    condensation problems and nutrient uptake
    interference due to inactive plants (lack of
    transpiration).
    Humidification
    Although dehumidification is sometimes
    expensive, it is usually easier to reduce humidity
    levels than to increase them. Raising humidity
    levels without creating excessive free water
    requires some sort of evaporative device such
    as misters, fog units, or roof sprinklers, all of
    which add water vapour to the air, or screens
    that help hold in the water that is being
    evaporated from the plant canopy.
    Evaporative devices accomplish 3 things: first,
    they cool the air, raising the humidity and
    relieving stress on the crop. Second, they add
    water vapour to the air, further increasing the
    relative humidity. And third, they reduce the
    vapour pressure deficit which is the force that
    evaporates water from the leaves. Screens may
    also reduce leaf temperatures and help to trap
    the large amount of water that the plants are
    evaporating. Evaporative cooling and screening
    are often used together. When humidifying
    under sunny conditions, some venting is
    necessary since the greenhouse would soon
    become a steam bath without the introduction
    of fresh dry air to evaporate more water, and to
    cool, humidify, and displace hot greenhouse air.
    Anyone who has stood in an empty greenhouse
    on a hot summer’s day, knows that plants, by
    themselves, can do an excellent job of cooling
    and humidifying a greenhouse. Evaporative
    cooling equipment works with the plants,
    helping relieve some of the transpirational stress
    and allowing them to grow at optimum rates.
    The benefits of maintaining a humidification set
    point include: better plant quality, faster
    cropping, and lower disease and insect
    problems.
    * See Page 6.
    H20 Grams/m3
    Figure 5.
    Average Monthly Water Content of Outdoor Air
    There are several steps you can take to
    help avoid crop condensation problems:
    3 Make sure your temperature and humidity
    sensors are accurate, and located in the crop
    canopy. Test your temperature sensors
    regularly against an accurate thermometer.
    Bring various humidity sensors to one spot to
    see that they are the same. Relative humidity
    can be checked with a sling psychrometer*.
    3 Use thermal screens at night to prevent
    radiative heat loss from plant surfaces.
    3 Avoid sudden temperature elevations at sunup
    by programming a gradual pre-dawn
    temperature rise and dehumidification period.
    (Sudden temperature drops can cause
    condensation problems as well, particularly on
    cold glazing materials as the capacity of the air
    to hold water decreases. However, in this case,
    thermal lag should prevent condensation on
    plant surfaces, at least temporarily.)
    3 Place radiant heat sources near the crop to keep
    plant surfaces as close as possible to or slightly
    warmer than air temperatures.
    3 Use horizontal air flow fans or poly tubes to
    maintain even temperatures throughout the
    crop.
    3 Use a combination of venting and heating to
    reduce excessive humidities.
    5
    Using Screens
    Impermeable moisture screens maintain higher
    humidities and can reduce night time
    transpiration rates by about 20% under low
    overall humidities and by about 60% when
    humidities are already high. They are normally
    employed in the winter months when crops are
    young, and humidity levels are very low.
    Permeable screens block heat transport but allow
    moisture to escape. Plants under sun screens
    (shade cloth) tend to have lower transpiration
    needs due to less radiation heating. However,
    humidity levels are not significantly affected,
    since there is a corresponding reduction in
    transpiration rate. It is important to remember
    that crops vary in their ability to benefit from sun
    screens.
    Closing vents during full sun to
    increase humidity
    It has been suggested that limiting the ventilation
    rate under full sun conditions may actually reduce
    plant stress by raising humidity levels. Although
    at first, transpiration rates are reduced, the rapid
    increase in air and leaf temperatures causes an
    increase in the VPD and the transpiration rates
    climb again. In this case, the only alternative is to
    increase the ventilation rate and provide
    additional cooling/humidification by fogging,
    roof sprinkling, etc.
    Fogs, Mists, Roof Sprinklers, and
    Pan & Fan Systems
    Many evaporative cooling and humidifying
    systems are available. They add water vapour to
    the air, and may subsequently reduce the amount
    of water that the plants need to transpire.
    Systems should be sized to permit a vapour
    pressure deficit of no greater than 7 grams/m3
    (11 millibars) when operated in conjunction with
    a transpiring crop.
    Roof sprinklers add water vapour and cool the
    incoming air. On large ranges, it is possible to
    decrease the temperature by 3 - 5oC and increase
    the humidity 5-10%. Pad and fan systems consist
    of porous wet pads at the inlet end of a fan
    ventilated greenhouse. As the exhaust fans draw
    air through the wet pads, water evaporates,
    cooling and humidifying the air. Temperatures
    tend to be coolest nearer the fans and hottest at
    the exhaust when using these systems. Mist and
    fog systems produce tiny water droplets that
    evaporate, thereby cooling and humidifying the
    greenhouse air.
    Points to remember about
    humidification
    3 Plants are the primary humidifiers/coolers of
    greenhouse air. Ensure adequate irrigation for
    evapotranspiration needs on hot days.
    3 Greenhouses with sealed floors tend to be
    drier, since evaporation from the soil is
    prevented.
    3 Heat and humidity levels are easier to manage
    in taller greenhouses.
    3 If wetting of floors or foliage occurs, stop
    humidifying in the late afternoon or early
    evening to allow enough time for drying.
    3 Evaporative cooling depends upon the total
    amount of water that can be evaporated.
    Evaporative cooling systems must therefore be
    engineering with water output needs in mind.
    3 Evaporative cooling devices require good
    ventilation rates. It is the evaporative process
    that does the cooling. Fresh air must be
    continually introduced and warm, humidified
    air exhausted.
    3 To measure leaf vapour pressure deficit,
    accurate sensors for leaf and air temperature,
    as well as an accurate relative humidity sensor,
    are required.
    6
    Water Vapour Basics
    There are a bewildering number of terms and measurement units used in the discussion of humidity. Here are
    a few you may encounter:
    Relative Humidity
    The maximum amount of water that can be present in air as a vapour depends on the temperature of the air
    (assuming constant pressure). That is why we use the term relative humidity - it’s a measure of water vapour
    capacity relative to the air temperature. As the air temperature rises, its capacity to hold water also increases.
    Therefore, warm air can hold a lot more water than cold air. Relative humidity is a measure, in percent, of
    the vapour in the air compared to the total amount of vapour that could be held in the air at a given
    temperature.
    Dew-Point Temperature
    At any given temperature and pressure, there is a maximum amount of water that can be held in the air. This
    is known as the saturation point. At any time when the air is nearly saturated with water vapour, all it takes is
    a slight drop in temperature to reach the dewpoint. At this point, liquid water begins to condense on surfaces.
    This is also the phenomenon responsible for rainfall. The higher the moisture content of the air, the higher
    the dewpoint temperature.
    Absolute Humidity
    Absolute humidity is a ratio of the weight of water vapour contained in a given weight of dry air. It is used in
    the calculation of relative humidity.
    Dry Bulb Temperature
    When you measure the air temperature using an ordinary thermometer, this is called dry bulb temperature. It
    is independant of the amount of moisture contained in the air.
    Wet Bulb Temperature
    A wet bulb temperature reading is obtained by measuring the degree of cooling effect, if any, on a
    thermometer wrapped in a wet cloth. Whenever the relative humidity is below 100% a certain amount of
    evaporative cooling will occur on a wet bulb producing a lower temperature than the dry bulb reading. At
    100% relative humidity, the wet and dry bulb temperatures are equal, because no further evaporation is
    possible. The lower the relative humidity, the greater the temperature drop on the wet bulb. So by knowing
    both the wet and dry bulb temperatures, it is possible to determine the relative humidity of an air-vapour
    mixture. (See Figure 6, Page 7)
    Enthalpy
    Though not as important in humidity determinations, enthalpy describes the amount of heat contained in an
    air vapour mixture. It is used in heating and cooling calculations, since it takes more energy to raise or lower
    the temperature of moist air than dry air. Also, when moist air condenses onto greenhouse surfaces, it gives
    up a considerable amount of heat due to the latent heat of vapourization. Similarly, when water is evaporated
    from surfaces, it has a cooling effect on the air as well as adding to the humidity. Greenhouse evaporative
    cooling devices operate on this principle.
    Sling Psychrometer (or Whirling Hygrometer)
    A sling psychrometer is a wet bulb/dry bulb combination that is whirled about vigorously in the air. The
    resulting wet bulb temperature depression is then compared to a table of relative humidity values.
    7
    Vapour Pressure
    All gasses in the air exert a pressure. The combination of theses gasses including water vapour produce a
    pressure at sea level of 1013 millibars. Water vapour pressure accounts for about 2 mb of pressure under
    extremely dry and cold conditions to about 42 mb of pressure at 30oC and 100% RH.
    Vapour Pressure Deficit
    A measure of the atmospheric demand for water. At any time the humidity is below 100%, liquid water will
    evaporate. The lower the relative humidity, the greater the demand or rate of evaporation. Vapour pressure
    deficit measures the difference between the amount of water vapour that can be held in saturated air at a given
    temperature, and the actual amount of water that is held in a sample of air that is not saturated. VPD units are
    sometimes expressed in pressure units (millibars, kilo pascals, or pounds per square inch) or mass deficit
    concentration units (grams of water per cubic meter of dry air, or grams of water per kilogram of dry air). Mass
    deficit is sometimes called humidity deficit. The air/water vapour mixture leaving the leaf stomata is always
    assumed to be at saturation. Consequently, it is the VPD of the surrounding greenhouse air compared to the leaf
    surface that causes water to evaporate. The greater the VPD, the greater the evaporative demand. VPD’s for
    growing crops can only be calculated accurately when the surface temperatures of the leaves is known.
    The temperature
    difference
    between
    matched wet and
    dry bulb
    thermometers is
    used to calculate
    relative
    humidity.
    Units and Equivalents used
    in Air and Humidity
    Measurement
    1 atmosphere = 14.696 pounds
    per square inch (psi)
    at sea level
    = 1.033 kilograms per square
    centimeter (kg/m2)
    = 1013 millibars (Mb)
    = 101.3 kilopascals (kpa)
    = 760 millimeters of
    mercury (mm Hg)
    = 29.92 inches of mercury
    (in. Hg)
    = the standard value for
    air pressure at sea level
    1 millibar = .1 kilopascals
    = 100 pascals
    1 kilopascal = 10 millibars
    = 1000 newtons/square meter
    = 1000 pascals
    1 psi = 68.95 mb
    1 torr = 1 mm Hg
    1 pascal = 1 newton/square meter
    mass deficit (MD) g/kg = VPD (kpa) x 6.3
    Dry Bulb Wet Bulb
    Figure 6.
    Wick and
    water reservoir
    8
    Temp C 100% 95% 90% 85% 80% 75% 70% 65% 60% 55% 50%
    15 0.0 0.8 1.7 2.5 3.4 4.2 5.1 5.9 6.8 7.6 8.5
    16 0.0 0.9 1.8 2.8 3.7 4.6 5.5 6.4 7.3 8.2 9.1
    17 0.0 1.0 2.0 2.9 3.9 4.9 5.8 6.8 7.8 8.8 9.7
    18 0.0 1.0 2.0 3.1 4.1 5.1 6.2 7.2 8.2 9.3 10.3
    19 0.0 1.1 2.2 3.3 4.4 5.5 6.6 7.7 8.8 9.9 11.0
    20 0.0 1.2 2.4 3.5 4.7 5.9 7.0 8.2 9.4 10.6 11.7
    21 0.0 1.2 2.4 3.7 4.9 6.2 7.4 8.6 9.9 11.1 12.4
    22 0.0 1.3 2.6 3.9 5.3 6.6 7.9 9.2 10.5 11.9 13.2
    23 0.0 1.4 2.8 4.2 5.6 7.0 8.5 9.9 11.3 12.7 14.1
    24 0.0 1.5 3.0 4.5 5.9 7.4 8.9 10.4 11.9 13.4 14.9
    25 0.0 1.6 3.2 4.8 6.4 8.0 9.5 11.1 12.7 14.3 15.9
    26 0.0 1.7 3.4 5.1 6.7 8.4 10.1 11.8 13.4 15.1 16.8
    27 0.0 1.8 3.5 5.3 7.1 8.9 10.7 12.4 14.2 16.0 17.8
    28 0.0 1.9 3.8 5.7 7.6 9.5 11.4 13.3 15.1 17.0 18.9
    29 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0
    30 0.0 2.1 4.2 6.4 8.5 10.6 12.7 14.8 17.0 19.1 21.2
    Table 1. Vapour Pressure of Water in Millibars at Various Temperatures and Relative Humidities
    To find the Leaf Vapour Pressure Deficit:
    1. Measure the leaf temperature and look up the vapour pressure at 100% RH on the above table.
    2. Measure the air temperature and relative humidity and look up the nearest vapour pressure figure.
    3. Subtract the air vapour pressure from the leaf vapour pressure
    Example: Leaf Temperature = 25oC (100% RH) Leaf VP: 31.7
    Air Temperature = 24oC @ 65% RH Air VP: 19.4
    VPD 12.3
    Table 2 depicts a humidity control strategy based on vapour pressure deficit. A VPD between 8 and 10
    millibars (the middle shaded area) has been chosen as ideal. About 4.5 mb and below (shaded area at
    the left) is the setpoint for active dehumidification. VPD's over 12.5 (shaded area at the bottom right) will
    trigger humidification devices such as fog systems. Notice that relative humidity alone is not a good
    indicator of the vapour pressure stress on plants.
    Temp C 100% 95% 90% 85% 80% 75% 70% 65% 60% 55% 50%
    15 17.0 16.2 15.3 14.5 13.6 12.8 11.9 11.1 10.2 9.4 8.5
    16 18.2 17.3 16.4 15.4 14.5 13.6 12.7 11.8 10.9 10.0 9.1
    17 19.4 18.4 17.4 16.5 15.5 14.5 13.6 12.6 11.6 10.6 9.7
    18 20.6 19.6 18.6 17.5 16.5 15.5 14.4 13.4 12.4 11.3 10.3
    19 22.0 20.9 19.8 18.7 17.6 16.5 15.4 14.3 13.2 12.1 11.0
    20 23.4 22.2 21.0 19.9 18.7 17.5 16.4 15.2 14.0 12.8 11.7
    21 24.8 23.6 22.4 21.1 19.9 18.6 17.4 16.2 14.9 13.7 12.4
    22 26.4 25.1 23.8 22.5 21.1 19.8 18.5 17.2 15.9 14.5 13.2
    23 28.1 26.7 25.3 23.9 22.5 21.1 19.6 18.2 16.8 15.4 14.0
    24 29.8 28.3 26.8 25.3 23.9 22.4 20.9 19.4 17.9 16.4 14.9
    25 31.7 30.1 28.5 26.9 25.3 23.7 22.2 20.6 19.0 17.4 15.8
    26 33.6 31.9 30.2 28.5 26.9 25.2 23.5 21.8 20.2 18.5 16.8
    27 35.6 33.8 32.1 30.3 28.5 26.7 24.9 23.2 21.4 19.6 17.8
    28 37.8 35.9 34.0 32.1 30.2 28.3 26.4 24.5 22.7 20.8 18.9
    29 40.0 38.0 36.0 34.0 32.0 30.0 28.0 26.0 24.0 22.0 20.0
    30 42.4 40.3 38.2 36.0 33.9 31.8 29.7 27.6 25.4 23.3 21.2
    Table 2. Vapour Pressure Deficit in Millibars at Various Temperatures and Humidities
    Relative Humidity
    Relative Humidity

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    The above file is much better downloaded as a pdf.
    It has some very useful humidity tables in it.

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