I use this one: https://www.amazon.com/gp/product/B001AGS3CK It has worked well for me for the past five years.

Just FYI, the airflow of the exhaust usually is around 2-3 millibars of pressure or so (depending on your setup). That is a small fraction of one PSI (.043). If you are looking for something in-line versus a hand held to sample on demand, make sure that you get one rated for high heat. I'm pretty sure that most of those digital ones may not be rated for temperatures much over 150ºF.