Tensiometers have been around about as long as gypsum blocks. Their advantage is simplicity. The ceramic cup inserted into the soil establishes an equilibrium with the soil water tension. A tensiometer reads what the plant root senses in terms of how much energy is needed to absorb water. Tensiometers are not affected by dissolved salts from fertilizer.
Tensiometers do have some limitations.
The third and final benchmark device over the years is the neutron probe. Dozens of irrigation consulting companies throughout the country still use this device as their standard irrigation tool. Neutron probes need to be calibrated for each type of soil. Good water balance studies and irrigation scheduling strategies can be performed using the neutron attenuation technique. Lugging around a neutron probe however is labor intensive.