I may need to stand corrected here, well at least just partly.
In the past I have had disagreements with other members here, about using an IR temperature gun to get leaf temperatures. I know these guns are not very accurate. I even conducted a test that proved my gun reads 4 degrees high. It seemed to me that everyone was shooting the leaf temperature, and then comparing this to the temperature device in the tent. After seeing this procedure online, I gave it a try.
Quote from source
“ The correct helps plants grow their best. To accurately track the VPD that plants feel, you need to know the temperature at the leaf. Pulse can take that into account when displaying VPD in the app to guide you to better results.
What you’ll need to get started
IR thermometer gun
Notepad
Pulse App
Calculator
Calculating the VPD leaf temperature offset
Make sure lights are on and have been for a good amount of time ( > 1 hr)
Get your IR thermometer gun
Collect 4-6 temperature readings of leaves in the canopy at various different spots: top, bottom, middle, sides, etc. Write these down.
Collect 4-6 temperature readings of non-plant material: supports, walls, pots, plastic, wood, etc. Write these down.
Get the average of the canopy temperatures.
Get the average of the non-plant temperatures.
Subtract non-plant temperatures from canopy temperatures: (T Canopy) - (T Other) = Leaf Temperature Offset
Ex: Canopy average temps 75, other average temps 80
75 - 80 = -5
Ex: Canopy average temps, 65, other average temps 60
65 - 60 = 5”
End of quote
Here is the reading I got.
Tent temperature by the AC Infinity controller was 76 degrees
Average tent temperature by the IR gun was 80 degrees
Average leaf temperature by the IR gun was 78 degrees
Shazam, by using the IR for both VPD temperatures the numbers add up. The IR is still reading high by 4 degrees, but now it being used for both inputs. I now feel somewhat more confident that I am running-2 degree offset. Before when using two devices it would have been a +2 offset. This is a big difference in the VPD calculation.
I hope this will help someone else!
I don’t know enough about VPD to know if it’s better to compare leaf temp to air temp, or other surface temps. Most seem to suggest air temps, but this method uses other surface temps.
The difference I see is in the effect the radiant heat given off by the light/s has on objects. For example, my tent air temp is 80 degrees. My leaves are around 78. The bottom of an idle circ fan is 80 degrees, same as the air, but the top, light-facing side, is around 83-85 degrees, just from the heat of the light. These are all expected readings, I’m just not sure which benchmark reading is right for VPD; the air or other objects facing the heat source.
Yes, some objects will pick up radiant heat from the light, and these readings should be ignored. The black AutoPots were reading about 83 degrees. My light is hanging at about 48 inches now. Everything at or slightly above this height was a constant 80 degrees, including all four tent walls and support post.
The problem with using IR leaf temperature with air temperature, is that you are using a surface temperature sensor against an air type sensor. This leaves one open to inaccurate readings, as the IR by designed is not accurate enough, to be compared to a good air sensor. But if you use the IR for both leaf and space by reading objects. At least chances are that both reading will be off by the same amount. I wouldn’t try to maintain VPD with an IR, but I think this is better way to determine leaf temperature offset.
You have quite a few degrees of variation to be in The Sweet spot for VPD so with your temperature reading device in the tent if you always shoot for the middle of the VPD range you’re going to be golden because whether you’re plus or minus a few you’re still in The Sweet spot a degree or two either way from The Sweet spot is not going to affect much
@JJ520, I agree completely with what you are saying about staying in the middle of the chart. I am not even recommending that anyone use VPD, or chase it. Just saying that if one choices to check VPD with leaf temperature offset. The above is a better method. What’s the old saying about computer data? Garage in equals garage out!
Taking accurate temperature measurements is kinda my thing. This is due to a 40 year career working on large water chillers. I am just trying to share some of what I have learned. I have seen people here going down a rabbit hole due to a 5 degree error. Helped them by recommending that they simply shade their temperature sensor from the light.
Maybe there’s just some garbage IR guns out there? I know mine’s been accurate to within tenths of a degree as long as it’s used right. I’ve watched maintenance guys at work try to read machinery temps through plexiglass , which is one of many reasons I never called maintenance, unless something needed taken back for welding or something…
I would recommend an ice water bath to check a temperature sensor, (provided it can be submerged), or an IR gun. Ice with just enough water to cover it, and this will pull the ice bath to 32 degrees. If your thermometer cannot be submerged you can use an old style glass tube one. Then place the glass one next to the non submergible one to compare.
Here are some of the pictures I have. It shows the IR reads about 3 degrees high on the ice bath. I also tested the IR on a leaf that was removed from any light influence, and it was 4 degrees high on the leaf surface. Mine is an Amprobe brand, and is not a cheap Amazon one, but it is about 15 years old. Also the IR toggles the display at a 1/2 degree increments, and it is for me to believe a 1/10 of a degree accuracy with a 1/2 degree increment meter.
@Cap_Ron, that’s pretty cool. My AC Infinity controller gives the VPD value over the phone app. Your chart agrees to 0.05 of a point, as compared to the app with the same values.
The recent purchase of the AC Infinity smart outlet controller, and their small humidifier, has made me circle back to looking at VPD again. Due to small plants and a cool lung room. I am now able to keep the intake and exhaust fans run time to a minimum. Even though this control doesn’t control VPD, but does display the value. This makes it easy to trigger the fans and a small heater with the controller, and the humidifier controls itself with a remote sensor. Here is a history chart from the controller’s app. The VPD is the very straight green line, dead on .86. I am amazed at how stable the conditions and VPD is after shutting the ventilation off from the lung room.
@Cap_Ron, I know co2 may not be very popular here on this forum, but I have the AC Infinity co2 controller and regulator kit due in today. Plan on going light with it, and start out at about 600 ppm and ramp up to 1200 during flower. Their controller has a nighttime lock out feature. Also I made a relay kit that will drop off power to the co2 solenoid valve whenever the exhaust fan runs. I guess I find out how long a 20 pound cylinder will last.
For the CO2 to work properly you need to have a sealed room you’re going to vent all of your CO2 out I have a CO2 monitor in my grow room we have two adults and one pet in the house and we both smoke I have a filtered vent that sucks air out of the house my grow room runs at 8:00 to 900 ppms just off of us breathing and living
@JJ520, yes the ACI co2 controller has what seems to have a sensitive monitor. I calibrated it in the outside air at 400, and was surprised to find when bringing it back inside the kitchen it when up to 650 - 700. Will find out what the grow tent reads.
My lung room is in my basement garage, and there is some fresh air infiltration around the big old garage door. So in the winter it is cool in there, if I don’t have any heat on. My drivers are remote mounted outside the tent. So I think it will be possible to hold the daytime temperatures in the low to mid 80s, without much if any ventilation.
The tent has speed controlled intake and exhaust fans. This will make it possible to maintain a negative or equal pressure in the tent insuring a minimum amount air leaking out. I am very surprised at how well the ACI 5x5 seals in the humidity. Have seen 75% inside and 30% outside the tent, and the humidifier is only using about a pint of water daily.
Anyway I have everything needed for the co2 injection. I guess, I will see where it goes. I have used co2 back in the 80s, and it did make a difference. But it was all guesswork back then, and was controlled by timers, and a double reducing regular set up. Since I had no idea what the co2 level was. This was very wasteful system.
I will post the results of the trial run with co2 on this tread.
I hear you, again I am not suggesting that anyone use VPD, or chase after it. I intended for this tread to be about taking accurate temperature readings. But as usual it seems to have swerved out of its lane. I am not complaining at all, as I find the versions of topics very interesting.
I use vpd to guide my heat and humidity… Especially after the heat of last summer… I realized sometimes it’s just impossible to keep it dialed in. In spring and fall/early winter… Using vpd is easier for me.
@Growmoredank, my IR must be very old, as it has nothing but the trigger and no other adjustments. I have used several during my career. But they all acted similar. When checking a rubber piping insulation it would be 30 - 40 degrees off, even when the insulation was painted green. On a bare copper pipe it would be within 10 - 15 off. If you wrapped the pipe with black electrical tape it would be within 2 degrees. When I would do a start up on a chiller that had the factory performance test done. It would have black tape wherever they were getting temperature readings. The IR wasn’t all that accurate, but was it very fast and easy.
, which is one of many reasons I never called maintenance, unless something needed taken back for welding or something…
