The PMS-615 is the classic, highly portable pressure chamber, also called a pressure bomb, which has been a favourite for growers making routine water measurements and irrigation scheduling. The maximum operating pressure is 40 bar (4 MPa). Standard sealing option is a ¼ inch Compression Gland Gasket and Insert which is ideal for most woody crops. Other sealing options for bladed grass or other semi-round sizes are available.
PMS-615 Pressure Chamber Features
The Model 615 Pressure Chamber Instrument has quickly become a favorite for growers making routine water measurements and scheduling irrigation. It is the product of customer input. Our customers requested an instrument that was fully enclosed in a sealed case with an easily removable tank. They wanted an instrument that was easily hand carried or ready for mounting on an All-Terain-Vehicle (ATV). The results of these requests is the Model 615 Pressure Chamber. We have recently (June 2011) updated this instrument with some changes that most users will appreciate. We have upgraded the Control Valve and internal piping to now allow direct connection to Nitrogen Cylinders with 207 Bar/3000 PSI pressure. This stainless steel valve is more durable and has a better packing therefore giving more life.
Instrument comes fitted with our most popular sealing gasket size – 1/4 inch Compression Gland Gasket and Insert. This gasket will seal a sample that is 1/4 inch in diameter down to a completely closed position. If you need other sealing options such as bladed grass or other semi-round sizes check our other options.
This is an excellent choice for routine water measurements in crop plants and light research in plants with little to moderate water stress. For extensive research projects you might consider a Model 1000 or Model 1505D due to the large range of operating pressure.
The instrument has a bail-spring handle for carrying and comes complete with a shoulder strap to lighten the load in the field. It has 5 tie-down rings for mounting to ATV or other transportation. The instrument comes complete with light weight aluminum nitrogen cylinder (20 cubic foot) that is secured with 2 inch heavy duty velcro straps and can be quickly replaced with a full cylinder.
Simply put, the pressure chamber is just a device for applying pressure to a leaf or small shoot. Most of the leaf is inside the chamber, but the cut end of the stem (the petiole) is exposed outside the chamber (see illustration above). The amount of pressure it takes to cause water to appear at the cut surface of the petiole tells you how much tension the leaf is experiencing on its water supply. A high value of pressure means a high value of tension and a high degree of water stress. These stress levels vary within different species. 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 the plant — except that for plants it is water rather than 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. The water in the xylem is under tension. As the soil dries or humidity, 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.” The more the stress the more the plant is experiencing a deficit of water. 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.
The Plant Moisture Stress (PMS) reading at any given time reflects the plant’s interaction with the water supply and the demand for water placed upon the plant by its environment (see diagram on back cover). Since these factors are almost always changing, PMS is nearly always changing. The time of measurement therefore requires careful consideration — PMS is most at midday and least just before sunrise. Pre-sunrise PMS values will usually reflect average soil moisture tension, if the soil is uniformly irrigated. Midday PMS values reflect the tension experienced by the plant as it pulls water from the soil to satisfy the water demand of the atmosphere.
|Maximum Operating Pressure:||40 bar|
|Chamber Construction:||Hard Anodized Solid Stock Aluminum|
Bar – PSI Scale
|Size (L x W x H):||56 × 38 × 26 centimeters|
|Gauge:||4.5” Diameter Gauge
1/2 of 1% accuracy
40 bar (4 MPa) range
|Aluminum Tank:||207 Bar/ 3000 PSI Maximum Pressure 20 Cubic Foot|