By using the HD-AL3 option (cover and printed circuit board) with HDH, HDH-RH, HDH-M and HDH-M-RH transmitters, different CO2 concentrations can be indicated by three leds as follows with the hysteresis of 50 ppm: Green = < 750 ppm (good), Yellow = > 750 < 1250 ppm (passable) and Red = > 1250 ppm (poor). Normal display is not available with the HD-AL3 option.