VPD calculations

GemGrows24

GemGrows24

Active Member
compassionateExotic said:
I’m only replying now cause Damn why even ask a question (make thread) honestly if u just want yes man or argue with no reason or logic, that’s not a discussion. I went to find ya a guide and chart with calculator to help ya and bam I saw many people do this for u and u obviously Don’t wanna learn and just want an answer if ur math is right? Put ur temps and rh and see what the tool tells ya or even look at graph . Simple if what temps what’s rh it needs to be for proper vpd.


pulsegrow.com

VPD Calculator

Solve For: Temp RH VPD Temp (F) Temp (C) RH (%) VPD Air (kPa) Leaf Offset (F) Leaf Offset (C) VPD Leaf (kPa)
pulsegrow.com pulsegrow.com
Click to expand...
The whole reason I posted in all honesty was to find like minded people that also enjoy doing the math. I found out shortly after I posted that my math was right and that was awesome to me. With the new understanding I won't be fucked without a VPD chart and can just calculate it out myself. What I haven't found was like minded folk that simple enjoy the math. Instead all I got was reply's on why what I was doing didn't need to be done and that I needed to stop. I didn't appreciate that . We're all different on how we do things I expected people to kinda respect that .Which I suppose I expected the backlash .but damn can't I just enjoy talking bout the math and how learning it might be beneficial.
 
Jimski

Jimski

Well-Known Member
Ok you want to understand the science? Giddeup

As you know vdp is the relationship between humidity and temp. If you really want to dial in vdp you will need to measure.

Most people running a vdp based system use electronic sensors that use a resistance change in a circuit to express the changes in temp. When you plug in this sensor the value is already suspect. Why? Cause on the box they say +- 2 degrees. 2 degrees off is another whole row on the vdp chart. Humidity sensors are better but still +-1%. Now that we have sensors we need to calibrate them.

For temp I use a temp gauge. A bulb style gauge that is slow and steady but when used for calibration it is very accurate and very precise.

For humidity you have the ole salt trick. Salt trick is not accurate enough for the miniscule accuracy we need to calculate these numbers. I use this

thermcoproducts.com

Pocket Slide-Rule Sling Psychrometer - Thermco Products

Learn more about the Sling-Type Psychrometer for Relative Humidity Measurement from Thermco Products.
thermcoproducts.com thermcoproducts.com

Using wet bulb dry bulb you can precisely measure humidity to baseline your sensors.

Now onto leaf temp variance. Outside of a lab I believe this is meaningless. Folks measure air temp with a resistance sensor and leaf temp with an IR radiation sensor. Again the formula is for measuring 2 objects to I degree. In order to measure the temps you need to baseline the sensors that gather the data. How can you calibrate the sensors to express a zero point?

Anyways there are 2 ways to figure vdp if the world blows up without the internet. You can do the 7th grade ap math or the freshman math for normal people or you could memorize the relevant targets for vdp based on your target temp.
 
compassionateExotic

compassionateExotic

Well-Known Member
GemGrows24 said:
The whole reason I posted in all honesty was to find like minded people that also enjoy doing the math. I found out shortly after I posted that my math was right and that was awesome to me. With the new understanding I won't be fucked without a VPD chart and can just calculate it out myself. What I haven't found was like minded folk that simple enjoy the math. Instead all I got was reply's on why what I was doing didn't need to be done and that I needed to stop. I didn't appreciate that . We're all different on how we do things I expected people to kinda respect that .Which I suppose I expected the backlash .but damn can't I just enjoy talking bout the math and how learning it might be beneficial.
Click to expand...
Words and exp online can easily be misguided or taken wrong way since no tone and if u don’t know whole story it can def misguided wrong way. wasn’t trying to be a jerk to ya or troll, i just know many people before me did try to give ya a great guide and graphs and I was about to do same 5hing till I saw u didn’t even want to hear or see it. Everyone’s got their own ways and wants. all good bro
 
Billy the Mountain

Billy the Mountain

Well-Known Member
GemGrows24 said:
Can someone thats done this before tell me if I calculated this right I'm pretty sure it's proofed but it's been a couple years since I've done any algebra View attachment 5447445View attachment 5447446View attachment 5447447
Ps. RH is 62 temp 25 c, leaf temp I estimated 2 degrees cooler (no IR gun yet) so 23 c.
Click to expand...
Charts are fine, but calculating vpd is trivial. I use this python function that's simple enough to not require explanation (hopefully):

def vpd( leaftemp, humidity) -> float:
# leaftemp a few degrees C below ambient air temp
vpsat = 610.7*10**((7.5*leaftemp)/(237.3+leaftemp)) / 1000
vpair = (610.7*10**((7.5*leaftemp)/(237.3+leaftemp)) / 1000) * (humidity/100)
return(round(float(vpsat - vpair), 2))

With that said, it seems you made a small error in your example calculation by using air temperature (25) in the first 'SVP=...' line instead of 23C for the leaf surface temperature. Your final result of 1.2 is correct for an LST of 25C and 62% RH, but if using an LST of 23C and 62% RH, VPD = 1.09.
 
GemGrows24

GemGrows24

Active Member
Jimski said:
Ok you want to understand the science? Giddeup

As you know vdp is the relationship between humidity and temp. If you really want to dial in vdp you will need to measure.

Most people running a vdp based system use electronic sensors that use a resistance change in a circuit to express the changes in temp. When you plug in this sensor the value is already suspect. Why? Cause on the box they say +- 2 degrees. 2 degrees off is another whole row on the vdp chart. Humidity sensors are better but still +-1%. Now that we have sensors we need to calibrate them.

For temp I use a temp gauge. A bulb style gauge that is slow and steady but when used for calibration it is very accurate and very precise.

For humidity you have the ole salt trick. Salt trick is not accurate enough for the miniscule accuracy we need to calculate these numbers. I use this

thermcoproducts.com

Pocket Slide-Rule Sling Psychrometer - Thermco Products

Learn more about the Sling-Type Psychrometer for Relative Humidity Measurement from Thermco Products.
thermcoproducts.com thermcoproducts.com

Using wet bulb dry bulb you can precisely measure humidity to baseline your sensors.

Now onto leaf temp variance. Outside of a lab I believe this is meaningless. Folks measure air temp with a resistance sensor and leaf temp with an IR radiation sensor. Again the formula is for measuring 2 objects to I degree. In order to measure the temps you need to baseline the sensors that gather the data. How can you calibrate the sensors to express a zero point?

Anyways there are 2 ways to figure vdp if the world blows up without the internet. You can do the 7th grade ap math or the freshman math for normal people or you could memorize the relevant targets for vdp based on your target temp.
Click to expand...
Yeah I prefere to do the math because leaf temp variance . I got an IR gun on the way .so I feel it'll be a more accurate way for me to calculate leaf transportation instead of trusting my equipments +/- 2 variance.
 
GemGrows24

GemGrows24

Active Member
Billy the Mountain said:
Charts are fine, but calculating vpd is trivial. I use this python function that's simple enough to not require explanation (hopefully):

def vpd( leaftemp, humidity) -> float:
# leaftemp a few degrees C below ambient air temp
vpsat = 610.7*10**((7.5*leaftemp)/(237.3+leaftemp)) / 1000
vpair = (610.7*10**((7.5*leaftemp)/(237.3+leaftemp)) / 1000) * (humidity/100)
return(round(float(vpsat - vpair), 2))

With that said, it seems you made a small error in your example calculation by using air temperature (25) in the first 'SVP=...' line instead of 23C for the leaf surface temperature. Your final result of 1.2 is correct for an LST of 25C and 62% RH, but if using an LST of 23C and 62% RH, VPD = 1.09.
Click to expand...
Thankyou !! And I didn't even notice I messed up LT thanks for pointing it out . This is pretty much what I was looking for since posting .
 
GemGrows24

GemGrows24

Active Member
Also I really just like doing nerdy shit this isn't all necessary and I really could just look at a chart as a guide. But I find the extra stuff fun personally . VPD goes further than plants too. Apparently u can use some of these same equations and plug ins to calculate air pressure . Which isn't needed knowledge but it's intriguing to me .
 
Jimski

Jimski

Well-Known Member
GemGrows24 said:
Also I really just like doing nerdy shit this isn't all necessary and I really could just look at a chart as a guide. But I find the extra stuff fun personally . VPD goes further than plants too. Apparently u can use some of these same equations and plug ins to calculate air pressure . Which isn't needed knowledge but it's intriguing to me .
Click to expand...
I was using vapor pressure differential 25 years ago building race cars. Mpfi race engines operate on speed density systems that calculate rpm and air density for fuel requirements. I still have my air chambers for multi carb motor cycles to balance fuel. I love technology but when I started growing I thought this would be all data points and data streams.

Boy was I wrong. No one can tell you how high yer light should be set or what the RH should be or how much water to put in your plant. Turns out the PLANT will tell you what it needs and my focus since that bit got through my thick skull is to learn how to recognize a wet and dry plant. Recognize stretch and canoing. Recognize wind burn from fans. Recognize a hot nute feeding and knowing what to do with THAT data. Complete change from what I had expected.
 
GemGrows24

GemGrows24

Active Member
Jimski said:
I was using vapor pressure differential 25 years ago building race cars. Mpfi race engines operate on speed density systems that calculate rpm and air density for fuel requirements. I still have my air chambers for multi carb motor cycles to balance fuel. I love technology but when I started growing I thought this would be all data points and data streams.

Boy was I wrong. No one can tell you how high yer light should be set or what the RH should be or how much water to put in your plant. Turns out the PLANT will tell you what it needs and my focus since that bit got through my thick skull is to learn how to recognize a wet and dry plant. Recognize stretch and canoing. Recognize wind burn from fans. Recognize a hot nute feeding and knowing what to do with THAT data. Complete change from what I had expected.
Click to expand...
But that's also the thing I know I probably come off similarly . there's a balance though and I understand that . That numbers are only reliable to a degree and that I need to pay most attention to the plant. Ig I just show more enthusiasm to the math side because I understand and relate more with numbers . I'll calculate things and do nothing about it when I get a number. weirdly I enjoy the process of calculating shit lmao . It soothes my ADHD .
 
CWF

CWF

Well-Known Member
I love math and am very good at it, but I'm not going to read that eye chart sideways stuff, or bother checking someone else's math. Sorry. I'm lazy, and disinterested. It's good you want to do it yourself, and understand it. That's way more that most growers bother with, so good on you for that.
 
GemGrows24

GemGrows24

Active Member
CWF said:
I love math and am very good at it, but I'm not going to read that eye chart sideways stuff, or bother checking someone else's math. Sorry. I'm lazy, and disinterested. It's good you want to do it yourself, and understand it. That's way more that most growers bother with, so good on you for that.
Click to expand...
Also I don't think any growers here are actually lazy it was just a sarcastic response to "some people like to make things complicated" . But they are right I love complicated shit . It oddly soothes my brain . If people don't wanna help me or give me an answer that's totally cool to. I honestly just wanted to know if I had the correct equation . And meet other people that like to make things complicated. I apologize if my comments came off judgemental . My humors rough.
 
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