Btu 4500 features include the following up to 4gas sensors 02,co,no,no2 or o2, co, n0 & s02 or 02,co,no,&cxhy(hydrocarboons)dillution pump for co auto range measurement upto 50,000ppm built in impact printer(non therminal) low nox capable with 0.1ppm resolution. All gas sensors are pre- calibrated and field reolaceable.Effiency, excess air & co2 calculation, stack gas and air temperature measurements. Draft& differential pressure measurement, external water trap for condensate removal rechargeable uthium ion battery pack. Unbreakeable metal hose connectors, internal memory (upto 900 tests pc) soft ware package, field upgradeable kits.