The temperature of the sample extracting system must be kept constant, as every change in temperature leads to a change in adsorption condition and therefore to a temporary change in the value likely to be measured. Given small moisture contents, low flow velocity, dirty piping, incorrect sample extraction or unsuitable tubing materials, the measured value changes will, in extreme cases, become subject to time constants of several days and form the reason for the so-called "24 hour effect". As soon as the difference between dewpoint temperature and ambient temperature becomes too small and therefore the relative humidity too high, condensation collects within the pores of the aluminium oxide sensor. This has the effect of making the measured values increasingly unreproducible. For this reason, the temperature difference between that of the dewpoint and that of the gas must be maintained to at least 15°C. However, the measurement sensor’s characteristics are not changed by condensed moisture.
