In the context of a differential pressure (DP) transmitter, the terms "dry leg" and "wet leg" refer to the two sides of the transmitter where pressure is measured. These terms are commonly used in applications where the DP transmitter is used to measure the level of a liquid in a vessel or tank.

1. Dry Leg:

   • The dry leg of a DP transmitter refers to the side of the transmitter where no direct contact with the process fluid occurs. Instead, it is exposed to the atmosphere or connected to a reference pressure such as a vent or a sealed chamber.
   • In level measurement applications, the dry leg is typically connected to the top of the tank or vessel, where it senses the pressure exerted by the vapor space above the liquid surface.
   • The dry leg provides a reference pressure against which the pressure exerted by the process fluid in the wet leg can be compared. This comparison allows the DP transmitter to measure the level of the liquid in the vessel.

2. Wet Leg:

   • The wet leg of a DP transmitter refers to the side of the transmitter where direct contact with the process fluid occurs. It is typically connected to a tapping point at the bottom of the tank or vessel, allowing it to sense the hydrostatic pressure exerted by the liquid.
   • As the level of the liquid in the tank changes, the hydrostatic pressure exerted on the wet leg varies. This pressure difference between the wet leg and the dry leg is proportional to the level of the liquid in the tank.
   • The wet leg transmits the process pressure to the sensing element (such as a diaphragm or Bourdon tube) inside the DP transmitter, which then converts the pressure difference into an electrical signal proportional to the liquid level.

By measuring the pressure difference between the dry leg and the wet leg, a DP transmitter can accurately determine the level of the liquid in the tank or vessel. The dry leg provides a stable reference pressure, while the wet leg senses the dynamic pressure exerted by the process fluid. This differential pressure measurement principle is widely used in level measurement applications across various industries, including chemical processing, oil and gas, water treatment, and manufacturing.