Vapor pressure deficit
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I was surprised when it happened because the humidity was at 52%. I guess some plants just prefer higher humidity.
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There’s a chart, if you stay within range for your cycle you’ll do just fine. Plants need a balance of heat & humidity. This will help them transpire naturally and not just sweat all the water out then be left behind with just salt to cause burn holes.
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What is a vapor pressure deficit?
It does remind me of light burn.
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So if my room is 79/81 degrees I should shoot for 65-70% to be in a good range?
I know it sounds silly but I’ve never paid attention to humidity in there.
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How the heck do I check leaf surface temp?
Here is how I do it. Place hand at the top of the plant. If it feels hot, then raise the lights till its at a comfortable level. I hope that makes sense to you. 
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that’s how I’ve done it so far and it’s always worked. These just don’t like it.
I should have said a key piece of info. I’m growing Godfather OG. I’ve grown Super Skunk, Purple Kush and Northern Lights in this same setup with all parameters the same as now. Except the NL was grown in Coco Loco.
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In the first pic I posted you can also see canoeing of the leaves. Is that heat stress? Could this be because the humidity is too low?
I set up a humidifier just now so I wonder what that will do. I’ll also get runoff info later tonight.
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I got numbers:
Ph in is 6.6
Ph out is 6.3
Ppm in is 50
Ppm out is 243
I think that’s pretty good no?
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Looks good to me. Keep an eye on your ph as it’s dropping though. Your numbers coming out are higher than whats going in so there could be an issue there as well. If your numbers are good then your issue lies elsewhere. Just need to rule things out one at a time. Knowing your numbers are good let’s you confidently check other things. VPD chart is a great chart to have to hand. You don’t need to check leaf temps, if your temp and humidity are on point your leaf has to be as well. Nice result, just hope she turns around for you buddy 
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Here’s a chart for you. Gives you an idea of what you’re looking for at various stages. You have some really experienced growers here giving you great advice mate. You should get your problem sorted out.
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You get a temp gun, or move a thermometer to that position.
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I just listened to a podcast, Growcast from last Friday. They talked about Sulfur deficiencies and how it looks just like light burn.
Thought of this thread. Good luck!
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It would appear humidity had a lot to do with it. After I put the humidifier in everything looks fantastic. I’m really very surprised at how much difference there is. All new thick growth and no burns at all
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Awesome to hear! Thanks for the update.
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Humidity not lighting was the cause. Nice bit of detective work there man well done 
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