Tomorrow i am upgrading my light system to HPS. and i was just curious as to how far i keep the hps from the top of my plants??
depends , how old are they? if they are really babies i would suggest something less powerful at first, although keeping the light about 10-15 inches away should be fine.
The rule of thumb is that if your hand is not too hot niether are the plants. However I have found it better to use my face..............I cant really tell with my hand..........So I put my head down in the tops of the plants and turn my cheek to the light, then I can really feel the heat and make a better judgement. 15 to 24 inches is what works for me. Also if your "babies" have been growing under cfls or anything other than the sun or hid light you will need to condition them to the stronger light..........start the light out about twenty inches away........let them remain for four hours under the new hps then move them back to their org. light source to finnish thier cycle....add an hour a day utill they can handle it. Moving them stright into hps from cfl will burn them if not acclimated to the light. good luck peace B hope this helps.................
mine are prolly 18 inches with a fat box fan blowing into the front of the box and it seems to work. If you drop some cash on a cooltube you can go even lower though... oh, and be prepared for explosions of growth lol Check the difference in my journal if you want, from cfl's to HID, what a diff.
I believe with the cool tube water (cold) passes through the tube which helps keeps the bulb cooler i almost bought it myself but passed on a cheaper setup
This is how I'm running my light at night. My setup pulls outside air through ducting, the hood, and blows it back outside. At night we're down into the 30's so I'm pulling near freezing air through the hood at night, and lowering it down to within a few inches of the tops of the plants. Pull it back away in the morning as daytime temps rise. In another month or so when I start flowering the tent, it'll be so cold both day and night that I can keep the light close for the 12hrs it's on.
I don't like to keep my light more than 6" away from the tops of my highest nugs. The leaves get a little brown on the very tops, but those nugs are so thick you couldn't put a bullet through them! Right after my clones develop roots under a single 60w CFL, I move them to the 400W HPS and keep it about 20" above the tops until I start seeing growth and water consumption take off, then the light goes to 6-10" depending on how the plant responds. Once I start flowering, I keep a max distance of 6".
my closet stays so cool i can run my 400w HPS 4-8" above the plants. at 4" the tops of my canopy are 85* and at 6" they are 82* I have ain air cooled reflector with 6" ducting hooked up to a 440cfl inline fan. You can check out my journal. My nugs are DENSE!!