A potometer is a piece of apparatus designed to measure water uptake in a leafy shoot. There are 2 types of potometer, the bubble potometer and the weight potometer. As water is transpired from the leaves, the shoot absorbs water from the potometer. The distance the air bubble moves in a certain period of time can be used to calculate the rate of water uptake. The apparatus is set up under water to prevent the entry of any unwanted air bubbles. The junction between the shoot and potometer is sealed (usually with petroleum jelly) to prevent any air leaks. The apparatus is reset using the reservoir or syringe so that water uptake in the same leafy shoot can be measured making the investigation more reliable. It can also be reset in order to investigate the effect of a different environmental factor (e.g. light intensity, humidity, temperature or wind speed). When investigating the effect of an environmental factor it is important that all others factors are kept constant to ensure the experiment is valid. The apparatus and plant is weighed and then left for a period of time. When reweighed the amount of water lost by the plant can be calculated. A top pan balance that measures to two decimal points provides great accuracy. A potometer can measure the amount of water taken up by a leafy shoot but cannot provide an accurate value for the amount of water transpired, because some water will be used in photosynthesis and to provide turgor.