Pump-Up Pressure Chamber

The Pump-Up Pressure Chamber is different from the conventional gas chamber in that it does not require a source of compressed gas such as nitrogen.  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 ½ 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 and for teaching students basic plant water relations.  The instrument was developed by Dr. Ken Shackel of the University of California - Davis.

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 tension the leaf is experiencing on its water: 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

In simplest terms, the pressure chamber can be thought of as measuring the "blood pressure" of a plant, except 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.

Description of the instrument

There are two different sealing lids available.  When you order your instrument you must choose one or the other or you can order a second 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 (right) 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.
 

Measuring Leaf Water Potential or Stem Water Potential

Leaf water potential readings are taken by simply cutting a leaf from the plant and then immediately testing it in the pressure chamber.  This measurement is called "leaf water potential", and is typically used for upper canopy, fully exposed leaves.  Some leaves may require protection from water loss to obtain valid measurements.  The protection is a plastic sandwich bag placed over the leaf prior to cutting it from the plant or tree.  Stem water potential is a reading of what is going on within the xylem of the plant.  To take this reading a reflective plastic bag is placed on a lower canopy, shaded leaf, and the bag is left on long enough (as little as 10 minutes may be sufficient) to allow the water tension in the leaf to come to equilibrium with the water tension in the stem.  The leaf is removed from the plant and tested in the pressure chamber while still enclosed by the bag.  For both leaf and stem water potential the steps of the pressure chamber test are described below.  For more information about Stem Water Potential click here.

1. Cut a leaf from plant to be tested.  Use a sharp blade to make a clean cut. Avoid breaking the sample.

2. Insert the leaf into the hole in the lid so that the end of the sample barely protrudes through the lid.  Twist the Compression Screw clock-wise to seal the sample.
3. Put leaf inside chamber and lock down the lid into chamber.  Place pins completely through holes so they are locked.
4. Ensure you are wearing eye protection in case sample slips out of chamber.  Place foot on foot-rest, swing eye lens in place and begin pumping instrument.  Instrument should increase pressure about 1/2 bar per stroke.  If instrument does not increase pressure, check sample seal and also check for obstructions between chamber lid O-ring and chamber wall.
5. While pumping on the down stroke watch sample through eye lens for a film of water to appear.  When water appears, stop pumping and record pressure indicated on gauge.
6. Use Pressure Relief Valve to release the pressure completely, remove pins and lid and you are now ready to measure another sample.
CAUTION: Ensure you have released all the pressure from the chamber before removing the Chamber Pins and the Chamber lid.  Failure to do this could cause injury. Always wear eye protection when using the instrument!

For more information on stress levels for specific crops, trees or plants click here. For other questions about the instrument feel free to e-mail us or call us directly.

MAINTENANCE

The instrument requires very little maintenance other than general cleaning and lubrication.  Be sure to keep chamber, O-rings and working parts free of dirt and debris.

Chamber Lid O-ring
Lubricate Chamber Lid O-ring daily when in regular use.  Wipe all dirt and grit away from O-ring with a rag.  Apply petroleum jelly to O-ring.

Small Sealing System O-rings and Washers
The small O-rings for sealing samples are Buna 40 O-rings.  Keep O-rings and washers clean of debris.  Use Buna 40 O-rings.  Buna 70 (normal O-rings) are not soft enough for this application.  If additional O-rings or washers are needed you can purchase them directly from PMS Instrument Company.

Large Sealing System
Occasionally remove the Compression Gland Screw by turning the screw counter-clockwise until it comes out of the lid.  Remove the washer and Compression Gland Insert and take out the Compression Gland Gasket.  Clean all parts and then re-assemble.  Ensure to lubricate the threads of the Compression Gland Screw with a durable lubricant.

Adjustable Eye Lens
The Eye Lens needs no maintenance.  However, ensure that the bolt stays firm in the aluminum chamber body.  Use the stainless steel nut to keep it firm in the body.  Focal point is adjusted by using the brass nut.  Loosen to raise the lens and tighten to press it closer to the sample.

Gauge
No maintenance required.  It is a 2.5 inch gauge with dual scales of 20 Bar/ 300 PSI.  It is accurate to 1%.

Compression Tube, Piston and Piston Rod
The Piston inside the Compression Tube should be lubricated occasionally for optimal operation.  To access the Piston, (pictures at right) loosen the stainless steel fitting at the base of the instrument.  This is accomplished by loosening the outer nut (not by unthreading the fitting from the chamber).  The piston should then be visible.  Push the rod up and lubricate piston and also squeeze a small amount of lubrication under piston to lubricate all of Compression Tube.  Cycle the piston through the Compression Tube and then repeat lubrication.  When re-assembling ensure Compression Tube is completely before tightening the fitting.  Recommended lubrication is a lithium based grease such as "White Grease".  Piston rod does not need to be lubricated
 
 


First, loosen the outer nut.

Second, push up piston and clean off old grease.

Next, apply lithium based grease.
Cycle the piston through the Compression Tube.
Apply more grease.

Finally, push piston down into Compression Tube.
Slide Compression Tube back into the fitting.
Ensure it is all the way in and tighten the outer nut.

Price: $1,545.00 U.S. (Includes one sealing lid - choose either size)
Additional Sealing Lid either large or small - $230.00
Foil Laminate bag (2 inches wide x 3.75 inches long) - $.40/each

Ordering Information
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