When refering to optimal temperature of 70-80F, is that refering to the temperature at the place of the plant or anywhere in the cab? Because I've seen setups where the thermometer is placed on the wall, but that doesn't accurately measure the temperature the plant, which is under light/heat, is feeling. There is a noticeable difference in temp. based on where I place my thermometer. I keep it sitting inside the pot, and its about 83-88F, but when I move it away from the light the temp goes down to a better level. So, I just wanted to clarify how to measure the temperature correctly.

i keep mine by the tops of the plant if i can...or as close to it as possiblke

[I2KanGrow]

I bought one of those wireless indoor/outdoor thermometers (Wally World - $12.00), which I place on an inverted beercup, right next to the pot, same height as the soil line.

Neat thing is - as I'm sitting in my office (on the main floor of my house), typing this post, I know that my microgrow temperature (in the basement) is @ 78.2 degrees F.

Another relevant question might be - should I shield my temperature probe w/ a piece of mylar, to take radiant temperature gain out of the equation???

I have mine sitting at soil level which is 17" from the 150w HPS at 83F. When I move it to the other pot which has about only an inch higher soil level, the temp jumps to 88F. And thats with 2, 65cfm ea. fans blowing air in directly beneath the lamp. I was planing to grow the plant to about 4" from the lamp (lowryder). The light is mounted on top so I can't raise it anymore and if its that hot 16-17" away its def. going to be too hot as the plant begins to grow. The lamp is centered above the two pots(1 on each side) so I thought as the plant gets bigger it could grow along the side of the lamp as well and not get too hot. Now I'm thinking maybe I should go with a smaller wattage bulb like a 70w or even a 50w. While it may reduce yield, if I can't get the temps down w/ the 150w I'm not going to yield anything. What do you think I should do?

[I2KanGrow]

Wow, 130cfm worth of air movement - that should be more then enough to keep things cool. 2 things to consider...

1) are you blowing air in, or exhausting air out? You may want to consider turning those fans around, so that they act as exhaust fans... I'm assuming that they are up high in your cab. I attempted to blow cool air in from the bottom - and I had real problems w/ maintaining a decent temperature also.

2) The area of your intake is critical for good air movement - 2X the area of your fan hole cutouts is a good place to start. You may want to draw air in from the bottom of your cab... cool air in from the bottom, warm air expelled from the top.

[VladTheInhaler]

Well, when i keep my thermometer shielded, it shows about 26- 28 Celsius. when I put it out in the open (so to speak) it goes off the scale, like 32-33 degrees. Bear in mind that I've been measuring temps without plants, hydro or circulation fan inside the flower room. I'm sure that with all of the above temps would be somewhat lower.

I2KanGrow,

I know I can't figure out why its so hot. I saw MrMicros pc grow with 50w hps and I think 25cfm fans and the temps in the 70's. I have my fans directly under the lamp going down the back wall because thats where all the heat is. I made a rectangular cut right behind the fans 9"H x 3.5"W (the size of the fans together). Right now they're blowing inward. I'm going to reverse them and see what happens. I'm really thinking I just bought too big of a lamp for a 26" x 26" x 14" cab. The 70 watter would probably cut the heat in half, so maybe I'll try that if reversing them doesn't help. Then I could add a few cfls later if I can keep the temps down.

[I2KanGrow]

Let us know how reversing the fans goes, awq...

[Makavellian]

i remember subcool mentioning that growers shouldnt measure their growroom temp but moreso the canopy temp

yeah, it makes sense to measure the actual canopy temp, cause thats the reason we cool the cabs/rooms for the plants sake, not the room. I went to reverse the fans and found the temp at 91F at soil level (16" from lamp) and just decided to try the 70w instead. I just got back and I got it installed already. Luckily, I found one thats built exactly like the 150w one so I could just screw it into the mount I already had and wire it. Now I'm gonna wait about an hour w/ the fan off and see what temps I get. Then try one fan and check the temp again, then if thats not cool enough try both. They had a 50w lamp there too, but it looked difficult to mount and since this one screwed right in to the mount I had I took it. If its still too hot, I'll try the 50w. Not sure of yield I'll get w/ 70w HPS for 2 plants, but the space is small so that should help. Plus, 1 of the plants is about a week ahead, as I planted 1 seed a week ago, then got the new cab and could fit another one, so planted the other yesterday. The 1st just popped its head above the soil, still covered by the seed shell. So, if I have to I can just scrap the other seed and put this one solely under the 70w.