Potometer
A potometer (from Greek ποτό = drunken, and μέτρο = measure), sometimes known as "transpirometer"', is a device used for measuring the rate of water uptake of a leafy shoot which is almost equal to the water lost through transpiration. The causes of water uptake are photosynthesis and transpiration. The rate of transpiration can be estimated in two ways: There are two main types of potometers: the bubble potometer (as detailed below), and the mass potometer. The mass potometer consists of a plant with its root submerged in a beaker. This beaker is then placed on a digital balance ; readings can be made to determine the amount of water lost by the plant.The uptake of water can be measured using a potometer. Under normal circumstances, the rate of water uptake gives a measure of the rate of transpiration . A simple potometer is a piece of capillary tubing to which a plant has been connected. The Limitations in using the potometer: Potometer do not measure the lost water during the transpiration, but measures the water uptaken by the cut shoot. Design. Potometers come in a variety of designs, but all follow the same basic principle. Use. Set up the  conditions of the experiment, such as alterations to lighting (placing the plant in bright light or shadow), wind (directing a fan at the plant), and humidity. Precautions. When a twig is cut from a plant, it should be immediately put under water (only the cut portion). Then, a small part is cut under water. This prevents entry of air into the xylem vessels. The conditions of the potometer, other than the alteration that is being tested, should not be changed during a test, as outside conditions (such as temperature) determine water uptake. Everything must be completely watertight so that no  leakage occurs. Limitations. The potometer does not measure the rate of transpiration accurately because not all of the water that is taken by the plant is used for transpiration (water taken might be used for photosynthesis or by the cells to maintain turgidity). The potometer measures the rate of uptake of water. To measure transpiration rate directly, rather than the rate of water uptake, utilize a scientific instrument which quantifies water transfer at the leaves. The water retained by the plant is so minute that it can be neglected. Introducing an air bubble may not be easy. A twig may  not be active for a long time. References.