Radiated heat problem - first grow

[stokesy]

DS120 tent
600 w HPS
6" fan blowing in fresh air
4" fans sucking out from the top
2 rotary fans moving air over the plants

3 x Arjans Strawberry Haze
3 x Barneys Honey B
Started in 3" pots

First two weeks - used 1 x 200w CFL in a reflector, switched to 600w HPS start of third week.

three weeks now since the beans sprouted, and I moved them up to 7" pots yesterday, and gave them a good soaking. The difference in the plants is very noticeable. For the better.

Problem I've had has been keeping the temperature below 85f. Even with the lamp 24" above the plants... with the fans playing over them, and the fresh air blowing UP into the lamp. Temperature everywhere else in the tent is around 65-68F

The roots have been steady at around 68-70F

Tonight.. I've discovered that if I position the fresh air (6" fan pushing in) onto the plants from about 18" above them, it brings the temp down to a handy 72-74.
Bit of a breeze on the plants, which will, I understand, force them to thicken their stems, rather than stretch.

I'm happy with the temperature situation now, but - is the breeze on such small seedlings ok? Or should I try and back it off?

It's my intention to LST them.

Being mainly Sativa, I understand that they're a bit more tolerant to higher temperatures - just trying to keep things right for the babies!

Was looking to add a cool tube -but opinion seems to be a bit divided. All advice welcome..

Thanks in advance...

 

[stokesy]

I think the problem has been solved... but I'd just like to check with you all. I've been using these digital thermometers - the little cream coloured ones you see all over the place, with a remote sensor on a wire. They can also record the max and min temps.
I'm using 3 of them to check the temps below the lamps.
Now I discover that they've been reading "high". I was using the remote sensor to check the soil temp, and that was steady at around 68/69F, while the readout for the air temp was in the high seventies to mid eighties Been trying all sorts to keep it around the 75F, but struggling. Now I discover that these digital thermometers, if placed flat on their backs below the light, read about 8-10F higher than the actual temp.
I've been all around the forums looking for info on temperatures, and never come across this fact before - although I know that loads of you out there must use these little digital thermometers.

Place the unit in a standing position and it reads more or less accurately.

So.. I was thinking that my plants were a bit slow coming on (just completed 3rd week from seed), compared to some of the illustrated journals on here... now I know why...
I've had two fans and cold air blowing onto the babies - now, all I need is the fresh air blowing...
Your comments are most welcome!

 

[Growing247]

I have a couple digital thermometer/hygrometers and the ones I have read about 10 degrees higher than analog depending on the situation and are also slow to change temp. What you are seeing is fairly typical and its why a lot of serious growers run multiple meters and use 3 or even 4 meters to calibrate. If at all possible I find it better to position the fans so that they are sucking the hot air out of the tent and sucking fresh air into the bottom. The colder air from the floor will force the hot stuff out the top. It is good to have a little airflow over the plants to remove the air from around the leaves and replace it with fresh air, but too much air can dry the plants out. Finding a balance between heat and airflow is usually the biggest problem I run into especially during winter.

 

[stokesy]

I'm also using a clinical thermometer. I'm going to stick with that for the time being till I cans sort this out. It's been a huge revelation to me.. there I was.. thinking everything was ok, apart from the fact that the lil' plants were a bit slow...when all the time, they've been sitting at around 65f while I've been considering cool tubes, bigger fans, cutting extra holes in the tent, etc.. When I didn't have a heat problem at all...!
OK. Any recommendations for a GOOD reliable thermometer?? Anyone?? !!

 

[Lusi]

Hey, I came across this while looking up tent info. Do you mind a few Qs? I'm new at this, so my advice wouldn't be worth a hill o beans.

1) Do you like this 120 tent? ANy gripes?
2) Do you have an air cooled reflector? If not, that alone could help you a lot with heat! I have a 600W HPS and hope to hell that if I vent it into the floor I can manage the temps into the 72-75 range. What do you have set up for heat control?

If you have an air cooled light and still that heat... I'll be reconsidering my new 600W light. I didn't get mine set up before finding the rip.

GL with your set up!!

 

[stokesy]

Lusi,

It's a DS120 - not a DR as originally specified...

The tent is ok from the point of quality, zip, etc. It's light-tight. The flaps at the bottom panel mesh vents would be better positioned on the outside of the tent, for ease of opening and closing. Maybe there's some good reason for them being on the inside - I don't know. I have a 600w HPS bulb in a standard reflector, not air cooled as such.
I've changed the fan setup, since I discovered the digital thermometers were reading about 10 degrees F too high!!!.. Thought I had a heat problem... way too hot, but in fact, the babies were sitting around 65f instead of 75 !!

First of all, I fixed them with little hoods, so they don't get the radiated light. The reading is pretty close to the actual temp.. within about one degree, I'd say.

Now, I have a 6" fan sucking warm air from the top, and two fans inside, controlled by a Primair fan controller. One blows across the plants, and the other circulates air within the tent. The exhaust fan is also controlled at the moment by the Primair.
I decided against the cool tube for non-technical reasons, and will try one when the opportunity presents itself.
No filter operating at the moment, as the babies are only 3&1/2 weeks from seed.
Bit of edge curl on the leaves now... could this be on account of the air blowing across the babies?

This is my second grow (first one was compromised and abandoned early on), so I'm very much a noob here. In short - I'm happy enough with the tent, but it's the only one I've had experience of. Took all my early advice from a close friend... Most of it was sound, and I've built on that advice.

I have a guest coming to stay this weekend, so we'll discover just how stealthy my setup is... !

Hope that helps

 

[stokesy]

Still not satisfied with the differences in temperatures here.
Let me explain.
If I place a thermometer in the centre of the grow, for example, with no shading, the temperature will read around 81/82. If I place a small shade over the thermometer bulb,so that it's not directly in the rays of the lamp, the temperature apparently drops to around 74/75.
So - what I'm seeing here, is the difference between the radiated heat directly on the plants, and the heat of the air surrounding the plants.
Clearly, the plants should be exposed to the radiated heat, so should I have the radiated heat temperature at 75, or the air temperature at 75? Clearly, they can't be both.
What this means is that the temperature above a leaf, for example, would read differently to the temperature below the leaf.
I have an alternating fan going, and using a fan controller. When the temp rises, the fans (fresh air in) and the rotating fan kick in. When the temperature falls, the fan speeds slow down. The exhaust fan is kept constant at full speed.
I'm still vegging and into the 7th week, due to the early setbacks.
I started to LST them about 2 weeks ago, and things are looking much better, but I WOULD like a definitive answer to the temp problem.

Which reading would be deemed to be correct? The radiated heat, or the air temp?

Top of the tent reads around 71F, and the air exhausts from the top.

Thanks all... for any comments..

 

[Marley15]

I believe the ambient temp (air temp) is what you are concerned about. In regards to the radiated heat, just make sure your hand can comfortably sit at the top of your plants for a minute. If it's not too hot for you hand it's ok for the plants.

I would still like to hear some other answers from the more experienced, but that is what I know from my limited experience.




Marley

 

[stokesy]

Common sense tells me you're correct. But I did run into trouble with them at the seedling stage with the "back of the hand" test. It was too much for them, and I lost the 2nd & 3rd sets of leaves.

I would like anyone else to chip in with their advice...