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Easy VPD Calculator

Discussion in 'Hydroponic & Irrigation-based Discussion' started by Jimmy Joe James, Jun 9, 2018.

  1. Tony21

    Tony21 Smoke Cannabis Live with Friends on AFN

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    No I haven't tried. Im aware of the products you I've also looked at Mycostop, Actinovate, pretty much everything on https://www.planetnatural.com/product/mycostop/ I used great white/Orca for awhile, I got it for free haha but that did not convince me, double haha, I've looked at milk spraying and stuff. I'm not convinced the trouble is worth the effort.
    My conclusion is to not grow fat colas like that. It was just a couple a spot, and no way could it have been detected without destructive inspection. But it can happen, those plants were mauled every way known and the some to keep them open bounce the stems get woody there's only pruning left and I thought that a bit radical haha, still they just kept swelling into dense colas. It looks cool, but why grow to be cool when you can grow for yield and quality haha, pretty plants are just eye candy
    With luck sometimes it all comes together on the grow side for some individuals, those were just a few out of 6 and 8 plants for each strain. Not all ballooned like that, only a few.
    Would still have been interesting to have VPD data even if the value may be dubious. Measuring VPD with some accuracy is another challenge without spending mega bucks on sensors.
    The Eggnogs were a test strain, dunno if it will make it to market. Big plants, good smoke, stinky, who knows...
     
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  2. Boradan

    Boradan We don't need no stinking Badges!

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    VPD is calculated from temperature and humidity.
    No need for anything expensive.
    Here is the formula (I use it in my google spreadsheet to calculate VPD)

    Code:
    ((100-H)/100)*((610.7*10^((7.5*T)/(237.3+T))))/1000
    
    
    Where
    T = temperature in Centigrade/celcius
    H = relative Humidity (%)
     
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  3. Tony21

    Tony21 Smoke Cannabis Live with Friends on AFN

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    You are right if you only measure in one point and your sensors is calibrated.
    When I measure temps and hum in a 3 x 1,5 x 2,2 m tent, I can get pretty much any VPD by taking readings high and low and allowing for the +/- 5% accuracy of the hum sensors i have. How accurate is your hum sensor and is it calibrated, +/- 5% is a 10 % range, try going into the formula or table with 30 % rather than 40 %, and a few degrees up and down, play around, the difference from being in the green and red is not much.
    just like PAR readings, getting any meaningful info from a few spot readings is above my head, let alone real-time monitoring in a grow room with oscillating fans moving leaves around, and how to measure leaf surface temp on a moving leaf.
    It does sound very expensive to me to measure VPD but if there is a way to do it on the cheap, accurately I'm all ears, my middle name is Charlie haha
     
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  4. Boradan

    Boradan We don't need no stinking Badges!

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    I understand what you are saying here, but the same is true of every measurement we are taking.
    pH and EC are (almost) the exception as their sensors can be cheaply calibrated using solutions of known pH and EC - it still won't give +/-0% for accuracy, but it is as close as we can get.
    What you are chasing after though is a physical impossibility - you would need sensors covering every square inch of your canopy. The best we can do is get an estimate from a few spots and work from there.
    People have been doing this for thousands of years - 'It grew better last time when I had THESE conditions, so now I am going to try and replicate that'. The measurements for 'THESE' is never going to be 100% accurate.

    The best that we can hope for is a good ballpark figure in a (relatively) enclosed environment.

    Personally, I 'calibrate' my cheapo humidity/temp sensors by throwing about 10 of them into a sealed jar with boveda packs. Anything that is off by more than 1 or 2 %, I throw in the bin - they were about £1.50 each.
    That's the best I/we can hope for.

    As a side note, I am still in the 'not sure' category for VPD - similar to @pop22
    All I was saying is that the VPD is calculated easily from a set formula using Temp and Humidity. How accurate this figure is though depends entirely on the accuracy of your measurement equipment and how many sensors you take readings from over your entire canopy.
    :pass:
     
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  5. Tony21

    Tony21 Smoke Cannabis Live with Friends on AFN

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    Exactly, what you are saying, it is what I was trying to say, my English is not very good.

    Yeah, formulas are good, it's what goes into them that matters

    I'm not chasing anything, just trying to get a better understanding or perception is better, of what is relevant for canna. And what is relevant for small growers like me with no access to top quality gear and test environments.

    A tip, calibrating hum sensors, put them in a sealed plastic bag with moist salt. That is 75 %, and then write the correction factor on the meter. I do this in the tent at 25 c or so. Then you don't have to throw them away so much. It's still not completely accurate but better than nothing. The correction factor can be entered into the better and more expensive meters for a clean reading.
    Temp sensor you can calibrate in ice water slush, it will be a perfect 0 c when stirred, if it has a probe of course, then write the correction factor down.
     
    Last edited: Jun 19, 2018 at 3:29 AM
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