I bought a "Nighthawk 900-0089". I think it was from B&Q.
It has an LCD display that shows the actual and max CO ppm count as well as an alarm. That way you can tell if there is even a small leak, something the cards won't show. I'm very pleased with it, except for its size. So it just goes in the a/c somewhere & I look at it say every 1/2 hour for trend checks - or if I am familar with the a/c, for a "max reading" during the post flight review.