Differences between the in situ water temperature and the instrument temperature can affect optical measurements in a few different ways. For example, ac-meter calibration tables rely on the instrument temperature properly reflecting the measurement temperature in order to apply the correct calibration coefficients. We have also found that ac-meters that have not been properly purged for humidity can develop condensation on the interior of the instrument windows further contaminating the measurements. To avoid these problems, immersing all or part of the instrument (detector canister) in a bucket or other enclosure with flowing water (e.g. coming out of the system) will alleviate the problem.