Potometer or transpirometer is used to measure the transpiration rate. It is a long glass tube, side tube at right angles. A plant shoot sample is included in the side tube via a cork, into the tube’s mouth. The instrument is supplied with water, joints sealed. Place this setup under sunlight. Air bubble moves into the tube. The lower end of the tube is then set up in the beaker which contains water.
The shoot absorbs water, transpired via leaves. Transpiration pull is produced, air bubbles start moving with transpiration pull. The amount of absorbed water is noted (readings for air bubble) and hence transpiration is calculated