The H-Guard-D is a CO2 transmitter series designed to be installed in HVAC return air ducts. The CO2 sensing element uses the Non-Dispersive Infrared Technology (NDIR) with the patented self-calibration system to guarantee the long term stability and up to 15 years life time. H-Guard-D duct probe transmitter offers temperature and humidity option. The high accuracy temperature and humidity combined sensor provide a digital interface and outstanding full range linearity. The one or three analog 0-10V or 4~20mA outputs are selectable by jumpers. Modbus RS485 communication interface is provided. Optional backlit LCD can display all measurements and is usable easily in more applications. H-Guard-D offers simplicity in design and installation with the special demountable and replaceable filter to avoid pollution and influence of measurement.