The gas leak detectors from Bacharach - the Leakator and the Leakator Jr. - are solid, good quality devices. The Tiff 8800 units are also good. Like many of these tools, it is almost always a matter of personal preference. The only issue I have with the Leakators, is that they do time-out and have to be recalibrated in fresh air. You can avoid this with the Leakator by push any of the buttons.
Two different issues with the CO detectors: 1 is ambient CO and, 2, is the undiluted CO. For ambient CO I carry a CO Experts household CO monitor. I trust it, and it reads down to 0 ppm. You can also carry one of the personal CO monitors.
For undiluted CO, the workhorse is the Bacharach Monoxor. You should contact Bill Spohn at Tru-tech tools about these things. The problem is that the CO sensor can be "fooled" by the NOx in the flue gases. So using one of the Monoxor devices, you should use a NOx filter to get accurate results. Bill says, "A CO-Undiluted (or CO-AirFree) measurement is a calculation that alters (usually raises, but never lowers) the CO reading correcting for the excess air that is diluting the CO volume concentration. To get a CO-Undiluted (or CO-AirFree) reading you MUST have an Oxygen sensor to correct for the excess air. Which usually means you use a combustion efficiency analyzer with built in CO sensor."
If the only element that you are interested in measuring (which is the case with BPI) is undiluted CO, you don't need to spend the big money that a combustion analyzer costs. But you can get efficiencies and other information with an analyzer.
The good news is that from my experience, it is hard to go wrong with the mainstream brand tools.
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