Abstract. The apparent resistance to diffusion of water vapour presented by a set of holes in a calibration plate when measured by a commercial porometer depended on materials previously measured in the sensor head of the instrument. The data reported show the variation in apparent resistance of a set of holes with a specified value of 13.7 s cm−1 after a leaf of small resistance, a set of holes with a specified resistance of 3.75 s cm−1, or a hydrophobic membrane had been in the sensor head and the effect of covering the foam seal around the mouth of the cup with aluminium foil. It was concluded that the results were profoundly affected by the history of water vapour accumulation and release from the foam seal which could give rise to biased readings both during calibration and in measurements of leaf resistance and might also explain much of the scatter when calibration data obtained at different times were pooled for the calculation of a calibration equation. Reliable estimates of resistance over the effective range of the instrument cannot be expected unless this transfer of water vapour between the seal and the cup is prevented.
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