A digital registration system used with temperature-and humiditycontrolled cuvettes for net photosynthesis and transpiration measurements in the field is described. The associated errors of the measured parameters and calculated data are estimated. The digitalization is based on an analogue registration which is of primary importance in the control of experimental conditions in the cuvettes. The digital system is connected to the analogue registration in series. The error associated with digitalization is 0.1% across 70% of the scale. This error increases to 0.2% between 3 and 30% on the scale due to a minor lack of linearity. The reproducibility of the digitalization is =h 0.024 %.
The error associated with data transfer in the digita]ization and the errors of the analogue registration are estimated for temperature and humidity measurements (error of air and leaf temperature is • 0.1 ~ C; error of the dew point temperature is -4-1.i ~ C dew point). The effect of these errors on the calculation of relative humidity and the water vapour difference between the leaf and the air is determined using the progressive error law. At 30 ~ C and 50% relative humidity, the error in relative humidity is • 7.4 %, the error for the water vapour difference is ~= 6.6%. The dependence of these errors on temperature and humidity is shown.
The instrument error of the net photosynthesis measurement is calculated to be • 4.2 %. Transpiration measurements have an average inaccuracy of • 8.3 %. The total diffusion resistance which is calculated from values of transpiration and the water vapour difference has an average error of -4-10.9%. The sizeable influence of errors in humidity and temperature measurements on the calculated diffusion resistance is demonstrated. The additional influence of biological errors associated with field measurements is discussed.