What is VPD and how do I use it in a grow tent?

Typical ranges quoted are about 0.4 to 0.8 kPa for seedlings, 0.8 to 1.2 in growth and 1.0 to 1.5 in flowering. Use a VPD chart: find your temperature on one axis, your humidity on the other, and check which zone you're in. Too low and plants transpire little and stay damp. Too high and they close their stomata and stall.

The numbers vary somewhat by source and by leaf temperature, so treat them as a guide. You only need a reliable thermo-hygrometer or a climate controller.