
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:
1) Indirectly - by measuring the distance the water level drops in the graduated tube over a measured length of time. It is assumed that this is due to the cutting taking in water which in turn is necessary to replace an equal volume of water lost by transpiration.
2) Directly - by measuring the reduction in mass of the potometer over a period of time. Here it is assumed that any loss in mass is due to transpiration.
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 Limitations in using the potometer:
Introducing the air bubbles is not very easy.
The twig may not remain for a long time.
Any change in the outside air temperature may affect the position of the air bubble on the capillary tube.
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.
 