The 227 determines soil water potential by measuring electrical resistance. When the 227 is wet, electrical resistance is low. As the 227 dries, resistance increases. The 227 cylindrical block is composed of gypsum cast around two concentric electrodes, which confine current flow to the interior of the block, greatly reducing potential ground loops. Gypsum located between the outer electrode and the soil creates a buffer against salts that may affect the electrical conductivity.