Gee man, isn't this making a case for buying an IR gun first rather than the refractometer. I can build high brix soil, so for me, VPD monitoring might be a higher priority in order to get my lights right and within the DLI range? What do you think?
It's sort of your call Carmen. With your open room setup you will have a hard time getting VPD to work. So the need for an IR gun lessens.
The refractometer would tell you which light cycle was producing better photosynthesis by which cycle produced higher brix, as well as the state of your calcium, and the IR gun will tell you if your leaves are dialed for optimal transpiration.
If you were to make your day longer but reduce light intensity you would almost assuredly not have warm leaves because of the reduced ppfd. Transpiration would be slower but last longer to achieve the same amount in the end.
If you were pushing your ppfd to the limit I would say get the IR gun so you don't push too hard, but with lower light longer there's no pushing.
As for longer days at lower ppfd vs shorter days at higher ppfd, I'm not sure which is better, but your eye would tell you if it was producing a bigger plant.
But then there is the arguement that you could possibly be able to turn the light intensity higher while remaining on your 18/6 schedule, thus growing faster, and you would need the IR gun to do that.
So really you need both. I guess whichever is the most affordable at the time of purchase is probably your best route.
Your point of likely already being high brix is valid, which slants it in favor of the IR gun 1st.
The IR gun would tell you if you have room to push them harder at 18/6, but upping ppfd would change DLI.
Tough call. Your logic for the IR gun 1st is solid, so if that makes more sense to you then that's the route to go.
Myself, I see DLI as a byproduct, and would consider being able to give them maximum safe ppfd's more beneficial than following a DLI chart because every plant is different, but the DLI chart doesn't allow for that. It assumes all plants are the same.
So yeah, I agree with you, the gun is going to tell you more immediately because your brix is likely already solid.
You don't have calcium issues, but raising your light levels requires more calcium, so you will need to be extra dilligent watching for a calcium deficiency if the IR gun says you can raise light levels.
Dealers choice, but in your position I'm kinda leaning towards the gun. Just watch for calcium issues. Keep a close eye on your soil every drydown and if it starts to crust then give her a low dose of calmag.