A steady state porometer measures stomatal conductance using a sensor head with a fixed diffusion path to the leaf. It measures the vapor concentration at two different locations in the diffusion path. You also need a light source
To accurately measure Stomatal conductance and other photosynthetic parameters sophisticated instruments such as LiCOR 6800, CI340 and CIRAS-3 are available which are portable and very useful in field measurements of grasses and crop plants. The obtained data is very reliable and accurate.
This task is somewhat ambitious other than in trees and large crops. You can measure stomatal conductance with help of available instruments as Arideep Mukherjee and vAndrew Paul McKenzie Pegman suggested.
This publication also describe your questions well:
Article Changes in stomatal conductance along grass blades reflect c...