PPFD v Distance from HPS lamp, which to follow?

[Need2Know]

Hello all. I got a question that I'm finding hard to find an answer to. Most encourage someone with a 250 watt HPS lamp to place it 16 inches maximum from the plant top during flowering. The problem is that if that is done, I'm finding my phone meter gives readings of over 2500 PPFD, which is much higher than the useful 900 PPFD. To get 900-1000 PPFD, I have to set my lamp to 30 inches. My question is what to follow, PPFD or distance? I'd like to hear from those that have a meter and measure PPFD. Thanks for any input. Wishing you all a most excellent day.

[Rootsruler]

If you had to stand next to a blowtorch and the people that told you to stand by it said you should stand 16 in from it but its too hot and 30 in is more comfortable would you still stand 16" away from the torch?

As long as your light meter is calibrated correctly follow it.

[Need2Know]

Thanks for the reply. Setting the distance from lamp to top by a meter's PPFD sounds better than the 'rule of thumb' 16 inches. I wonder why there is so much 'rule of thumb' for this important growing requirement rather than just saying follow the PPFD for the best results? Very strange.

[Rootsruler]

It's because most people don't bother measuring how much light their light is giving off. They just trust the company or person that told them to tell them the truth.

I will say that it does seem strange that you're getting those readings. You posted that your light is only 250w HPS lamp. 16" should be fine so I get why they use it as a rule of thumb. I use to hang 1000w HPS in Raptor hoods 30" from the canopy top.

[Orggrwr]

I would go by the reading on your phone to start and then lower the light a couple inches every few days until your plants start showing signs of light or heat stress then raise it back to the previous level. Then take a light reading there and you will know what ppfd your plants like. Mine usually seem happier at around 800 ppfd. I don't know if that's because the app is a little off or if that's just what they like in my set up.

[Mr.furley]

Can I ask why you are reading a PAR meter off a HID light and not using a LUX meter? You only got a High pressure sodium bulb? Got a Mh as well? You flowering? .....so many questions and yes I understand color of the marble vs. Numbers of the marble before I continue.

The first "rule of thumb" with HID lights is a LUX meter 26000-ish veg (Mh) 80000-ish flower (HPS) as a Baseline candle watts that fall on the surface. Second is the old back of the hand at canopy height for 30 seconds to check the heat, to hot for the hand, to hot for the plant. If you can control temperature at canopy height you can push LUX higher and add Co2 but that's a deeper rabbit hole.

Honestly I have never found a app on the phone to read Ppfd accurately and yes I own a actual PAR meter with the stick thingy, got a second one that controls dimming with the trolmaster for Ppfd and DLI that is all under LED. I have had good luck with LUX apps on the phone years ago with HID lighting and have been pretty successful.

In the end your plants will tell you if they are happily sitting in the sweet spot so depending on the stage you're at in the grow you can take or leave my rumblings.

[No3odiesShad0w]

First question.. what app are you using on your phone? I'm guessing the Photone app? If so, make sure you have the proper light source chosen at the bottom of the screen. That could skew the results if you have LED selected and using GPS bulb. Most apps have a selection for light source, at least the better apps do. It's important because it uses different math to calculate the light source. If all that's in order, we can move on.

The 16in maximum recommendation for the HPS is only that....a recommendation. It's for ppl who don't have any accurate way to test the PPFD and want to push their plants to the max. If 16in is giving you too high of a PPFD then raise the light. The tech is getting better at giving accurate measurements and there's science that backs the numbers. I would def go by the readings. I've been using the Photone app and I've been getting fantastic results. Though I have noticed that the light fixture distance does fall in line with the factories recommendation. 2500ppfd
At 16in is really high.... Maybe some cultivars would be fine but that would probably stress any plant.

[Need2Know]

Hello No3odiesShad0w, I checked my Photone App light settings as you suggested and found it on LED rather than the required HPS. That explains why I had to set my lamp 30 inches above the canopy to get 900 PPFD. The lamp is now set at 16 inches to get the required 900 PPFD. I had a lamp with burned LED's. I got a 250W HPS Dual Spectrum to replace it. I forgot to go back and change the settings on the Photone App. Thanks for the help.

Orggrwr, thanks for the step by step on how to set up the distance from lamp to canope. I'll use your method now and watch how the plant reacts over the next week.

Mr.furley, I thought LUX was used more in photography.

I've learned to follow the PPFD for distance, feel for lamp heat and watch the plant to see if it experiences any heat stress. Suggested distances like 12 to 16 inches is just a gereric rule of thumb. You guys are great.

[Mr.furley]

https://www.growweedeasy.com/lux-meter

[No3odiesShad0w]

Yeah super important to make sure you switch the light source. It's a different mathematical equation and it throws off the numbers by a lot. Glad that helped you 👍

[No3odiesShad0w]

Yeah super important to make sure you switch the light source. It's a different mathematical equation and it throws off the numbers by a lot. Glad that helped you 👍

[Need2Know]

Hey, this is a really good website that explains lighting well. It's the Photone Phone App website. https://growlightmeter.com/ I may be ignorant, but the PAR meter reading PPFD seems the best option. Signing off... Thanks for the input.