You'll need a PAR or Quantum sensor to read the amount of light at your plants location to figure out the DLI or Daily Light Integral. The optimal amount of light each plant needs per day.
You'll need two tools/meters to check your VPD:
A hygrometer and a laser temperature gun.
The hygrometers vary in size and provide relative humidity and temperature readings.
The laser temperature gun will measure the temperature of the leaves, which is usually, 2-3 degrees F lower than the room temperature.
These are the readings you'll enter into the app to figure out your VPD: the Air Temperature, the RH, and Leaf Temperature.
To get moisture readings you'll need a wood or hay moisture meter.
These devices are simple to use and give you the moisture content of a given material.
You enter these readings into the Dry charter in Hortus daily to properly dry your plants.