Hello, newbie here, how close can you place a 400w mh/hps light to the tops of the plant? I have access to a home made cooltube (pyrex bread/meatloaf tube) and I might use that if it produces any major benefits. I heard the closest to the tops of plants without a cooltube is 19in. Is this true? How close with a cooltube etc etc??????? PLease fill me in! Thx.
A new 400w HPS bulb can be 11" away from the plants without causing light burn. This doesn't account for temperature and only applies if you have enough lamp cooling to get it that close. Simply use a thermometer, your hand or the appearance of the plants' tops to gauge how close is too close.
I have some plants 3 inches with no problems. Just an air cooled hood with a 4 inch walmart fan in it. 5" is optimal for light intensity.
If the plants are 3" from a 400w HPS bulb and not frying, then either something in between them is blocking the light, the ballast isn't feeding the bulb enough power, or the bulb is old and not producing as much light. I think the average halflife on an HPS is about two years of use if I remember correctly; where it puts out half the light it used to. No marijuana plants can be closer than about ten inches from brand new, properly working, unobstructed, stationary 400w HPS bulb for twelve hours a day. edit: Please note that this does not refer to how far from the hood's glass the plants are, but from the bulb itself.
My plants are about 8" from the light without a cooling tube. In veg I also run 24/0. No cooked leaves, they love it. It's only 2 grows old, so I whould assume its "works like new" Perhaps older generation MH and HPS units were hotter? put your hand under it etc.
You want to place your HPS as close as possible without burning them. Thats the big negative on HPS lights you want your light close but to close it gets to hot overwhelming your vent system so you raise it and you get less light. Heres a light distance chart that shows you. Basically light follows the Inverse Square Law of Light (physics)....a object that is twice the distance from the point source of light will receive a quarter of the illumination. What that dictates to you is you want it as close as possible without burning your plants up. Commercial growers or large scale ones do not have this issue for the most part since they have dedicated AC units blowing in. http://forum.grasscity.com/attachme...-lights-plants-light_distance_chart_1__1_.jpg
I agree. I have a pretty good vent system in my tent also keep my place cold. Ive had my HPS very close for quite some time 24/7. Most people are unable to keep there HPS close because they can not overcome the heat issue..which is do inadequate fan set up, obstructed intakes, or they have there tent in some very small room like a closet where air exchange can occur. As long as you can keep up the vent and fans you can keep it cool. I was US Marine Corp for 10 years worked on Air traffic Control equipment. It was all tightly compacted..talking hundreds of circuit cards and solid state equipment all in boxes. We kept our boxes cool by hooking up 50,000 BTU ACs that ran cool air over them. If the AC's ever went off usually stuff would catch on fire and throw the breakers. Anyhow my point is if you can cool it you can do whatever you want. If you can't then you have it high above your plants.
I wouldn't say something is a fact if I wasn't certain. Here's a graph showing the light intensities of different wattage bulbs at various distances. I'm not sure of exactly what percentage of the sun's light intensity a 400w is at 3", but if the rate holds all the way to 3" then it should be about 600%. I don't get anything from these discussions. It's just in the hopes that you'll at least admit it could be possible that your light isn't working properly, look into it, end up getting twice what you get out of your plants now and become very happy. Best of luck.
Nothing to admit. My HPS bulb was new in March and I know for a fact that I have had it closer then 10 inches. My grow tent is only 64 inches tall. The last couple of harvests have had plants in there in excess of 30-40 inches. The assembly is X amount thick and I can 100% tell you my light has not nor has ever been at the top of the tent.
I've read that it's more than just the heat, that there is also a radiation concern if the lights are too close just what I've read in a couple places, don't shoot the messenger ... it sounds fishy to me I'd love to hear experience give some advice on this one instead of charts