The Pump-Up Pressure Chamber is different from the conventional gas chamber in that it does not require a source of compressed gas. The instrument produces pressure in the chamber required to take water potential readings by pumping it as shown below. The relatively small chamber allows the user to achieve about one half Bar (7.25 psi) pressure per stroke. The instrument is limited to 20 Bar and is designed primarily for irrigation scheduling/monitoring, particularly for managing deficit irrigation.
How it works
Simply put, the pressure chamber is just a device for applying air pressure to a leaf (or small shoot), where most of the leaf is inside the chamber but a small part of the leaf stem (the petiole) is exposed to the outside of the chamber through a seal. The amount of pressure that it takes to cause water to appear at the cut surface of the petiole tells you how much hard the plant is working to pull water up through the roots. A high value of pressure means a high value of tension and a high degree of water stress. The unit of pressure most commonly used is Bar. (1 Bar = 14.5 PSI)
Principle of Operation
The pressure chamber can be thought of as measuring the “blood pressure” of a plant. The difference being that a plant uses water and not blood, and the water is not pumped by a heart using pressure, but rather pulled with a suction force as water evaporates from the leaves. Water within the plant mainly moves through very small inter-connected cells, collectively called xylem, which are essentially a network of pipes carrying water from the roots to the leaves. As water evaporates, a tension is created, pulling water up from the roots. As the soil dries, humidity, or wind, or heat load increases, it becomes increasingly difficult for the roots to keep pace with evaporation from the leaves. This causes the tension to increase. Under these conditions you could say that the plant begins to experience “high blood pressure.”
Since tension is measured, negative values are typically reported. An easy way to remember this is to think of water stress as a “deficit”. More stress means a higher deficit of water in the plant. The scientific name given to this deficit is the “water potential” of the plant. The actual physics of how the water moves from the leaf is more complex than just “squeezing” water out of a leaf, or just bringing water back to where it was when the leaf was cut. However, in practice, the only important factor is for the operator to recognize when water just begins to appear at the cut end of the petiole.
Description of the instrument
There are three different sealing lids available. When you order your instrument you must choose one with the instrument and can order a another one as an accessory. The small system (left) accepts samples up to .140 inches or just slightly larger than 1/8 inch in diameter. The stem must be at least 3/4 inch in length to pass through the lid. This system was designed for use with orchard tree leaves like prunes. The samples are sealed into a compression gland mounted in the chamber lid, which uses various sizes of O-rings. The large system (middle) accepts samples up to 1/4 inch. The stem must be at least 1 1/8 inch in length to pass through the lid. This system was designed to fit larger diameter stems. The samples are sealed into a compression gland mounted in the chamber lid. This system uses Compression Gland Gaskets to seal the sample. The last lid on the right is fitted to use our “Bladed Grass Compression Gland“ gasket and insert.