The LCD door monitor allows for displaying the PPM ammonia concentration from a single LBW-420/420-1 in up to two other locations. This option was designed utilizing the 4-20 mA output signal of these units. The 24 VDC loop-powered, 4-20 mA output circuit (a separate 24 VDC power supply is required for the LBW-420 units; provided by others) provides both the power for the LCD door monitor and backlight, and the input for the display (in PPM).