Anyone out there use a Testo 325M. I have been doing PM's on Ac & Furnaces and was wandering how long you leave the probe in the outdoor PVC flue to get a reading. I have read the book that came with the COMBUSTION ANALYSER and can not find a time frame to get a proper reading. Also what range can the ppm CO be, and what is the high limit for ppm CO. I am amazed at some of the reading I have been getting. As low as 2 ppm CO and some newer furnaces higher than 200 ppm CO. Any thoughts would be appericated.