Instead of relying solely on the heaters on the front of the tainter gate at Lock and Dam No. 2, an air diffuser line has been placed under the heater box to assist in distributing the heat more effectively. This diffuser line with eight orifices spaced 4 ft apart is shown in Figure 3 . The air diffuser is a 1-in. Air is delivered down the left-side riser pipe, which contains a check valve to prevent water from filling the delivery pipe and freezing in the winter. The diffuser pipe can fill with water when the compressor is off.
Electrical power is supplied to the heat cable down the right-side riser pipe.
About 30 min after the heaters in the heater box have been turned on and the steel box has been heated up, the air compressor is turned on. The combination of heat from the heater box and circulating water to melt the ice is far more efficient than just the heaters alone. Since the skinplate heaters for one gate use 20 kW, considerable energy savings can be realized by operating the heaters less when used in conjunction with the air bubbler. Details of the lower trunnion arm heater and the support channel heater are shown in Figure 4 . Cartridge heaters are enclosed in watertight cavities by steel plates or angles for mechanical protection.