In hydraulic systems, cold start-ups often cause pressure switching errors that disappear once the system reaches its normal operating temperature. The culprit behind these false high pressure signals is increased fluid viscosity, which temporarily increases the differential pressure across the diaphragm or pressure sensing device. A thermal lockout mechanism based on a snap-acting, temperature-sensitive bimetallic disc can prevent this problem.
PH = High Pressure Port | PL = Low Pressure Port | Tt = Threshold Temperature
The fluid is cold and viscous and the differential pressure across the diaphragm is high. The diaphragm reacts by moving the actuator away from the electrical switch. To prevent a false high pressure signal, the thermal lockout disc snaps into a convex shape and contacts the motion transfer mechanism, disabling it from swiveling and triggering the electrical switch.
The fluid is at or above the lockout’s threshold temperature, and the pressure differential is negligible across the diaphragm. In this condition both the diaphragm and bimetallic disc remain flat. The disc no longer contacts the motion transfer mechanism, enabling it to swivel freely.
The system is experiencing a high differential pressure condition across the diaphragm, forcing the diaphragm into a concave shape while the bimetallic disc remains flat. In this condition, the actuator and motion transfer mechanism follow the actuator away from the electrical switch. The switch then signals an undesirable pressure condition.