You know - somehow I think this is part of the confusion. Most consumer sensing devices use the cheapest and least robust sensors. Some simply use parallel circuit traces to measure leakage current. I find the sensor on the dehumidifier to be most vexing - it's often off by 20 percentage points. Most of those simple sensing schemes are vulnerable to vapor deposition on the surface that changes the characteristics. The readings we get with most the ~$20 or so meters have numerous error sources and horrible linearity. Another case of getting what you pay for.

The most accurate devices I've found for humidity are the SHT31
https://www.adafruit.com/products/2857
and the BME280
https://www.adafruit.com/products/2652

Obviously, they are both silicon based integrated devices - used for higher end applications. Adafruit has a learning section where they give fundamental usage with Arduino - you could build a few loggers and get better data.

I've put these two, together right next to each other and gotten relatively similar outputs. You've probably noticed they aren't particularly cheap - but cheap enough. I recommend them highly - I don't think you would be disappointed.

Yow!
That sort of pricing for a device is what makes me build my own. I could build an equivalent with wifi or for $45 including 3d printed enclosure. BluetoothLE for a bit more. I may buy one of a device to reverse-engineer something that isn't obvious, or to take advantage of the ease in integrating with other devices, like I did with the CO2 meter. But after that $250 - I eventually found the same sensors used in that device that only cost ~$70. I can buy several of these for the cost of the meter - so I build my own now.

@Douglas.C check pm's bro....vpd is cool, yuh, I dis agree with one VPD fits all...just saying I really doubt one VPD is best for all cannabis, especially if you mind growing nanners...just saying I don't think anyone here mentioned stress induced nanners due to funky VPD or VPD swings...peace

It may have already been mentioned in the thread, but here's what I've gathered and think;

1. There is substantial research that suggests "Proper" VPD produce healthier greenhouse crops. Some cannabis farmers theorize that VPD stress may increase cannabis oil production, nutrient and water uptake. There is little official research to support this claim, to my knowledge.

2. Could it have more to do with genetic heritage? Perhaps strains with Thai, Indian, Colombian, Jamaica and Mexican heritage are better/more adapt to cultivation in a humid environment.
In the same respect, a strain with Kush/African/Dessert Mountain heritage may be better/more adapt to cultivation in a much drier environment:




In the middle of July with nearly 0 days of rainfall and temperatures hitting 104F/40C, we can assume a very high VPD is occurring; the standard VPD charts don't exceed 34C+, and a 80-90% RH would be necessary to maintain appropriate VPD levels at that temperature range.

Here's Panama:


We're now looking at 80% RH accompanied by 80F/28C. Landing itself on the lower end of the "optimum" VPD range(7.8-10.4).

Temperature is complicated, I'll have to do some digging but somewhere I read that a crop can sustain strong growth through high daytime temperatures as long as it had cool night time temperatures. Perhaps nighttime VPD is important too, I think many of the experienced growers may have noticed the surprising amount of evaporation/uptake occurring at night in their garden.

I like manipulating my plants, but I also try to remind myself that these strains were originally created and shaped over the course of hundreds and thousands of years in specific environments and climates. We've only been growing cannabis in our closets for ~70 years, and only in the last 15-20 years has there been an increased consciousness and effort towards such critical control and upkeep of information. Unless we have mutations occurring in the community, extreme variance in the plants original characteristics and needs due to our manipulation seems very unlikely IMO.

That's what I got right now! Hope it adds to the conversation.

X

This is perhaps the best description and demonstration of the train wreck that I often see in the "What happened?" Photos. This is why - often - LOWER nutrient concentrations seem to be 'the trick' for some growers. Also why a dialed in RDWC grow can rock the phenotype fairly consistently.