Perspiration scales
The perspiration balance is a measuring device with the help of which the transpiration rate of plant material, usually individual leaves, can be determined.
This method is also known as Stocker's instantaneous method. The perspiration balance is a combination of a beam and a torsion balance. A freshly cut sheet is hung on one end and a corresponding counterweight on the opposite side. So that the weighing is not falsified by the wind, the balance is covered with a glass hood.
Measurement
First a representative, medium-sized and intact leaf of the plant to be examined is cut off. The subsequent stress reaction, the closing of the stomata , only occurs about three minutes after the cut off. The measurement must therefore be completed within this period. The freshly cut sheet is now weighed and then hung again near the cut in order to ensure the same parameters as before the sheet cut. The second weighing takes place after just under three minutes. The transpiration rate can be determined from the difference between the two weights determined. It is given in milligrams of water loss per gram of fresh weight per hour. Alternatively, the transpiration rate can also be related to the leaf area.