Using a pressure chamber



Louis Santiago


How to use a pressure chamber to determine water pressure in plant stems.


  • Pressure chamber
  • N2 gas tank
  • Plant stems (freshly cut)
  • Rubber stoppers – range of sizes
  • Razor blades
  • Light source
  • Magnifying glass or dissecting scope
  • Plastic bags
  • Wet paper towel
  • Safety glasses

Units, terms, definitions

Water pressure


  1. Ensure all valves are closed and all hoses are firmly attached.
  2. Turn the control valve (Chamber/Off/Exhaust) to off/closed.
  3. Turn rate valve to the right so that it is closed but not overtightened as overtightening can damage these valves.
  4. Open the N2 gas tank by turning the tank valve very slowly. Listen for any air leaks as you do so. If leaks are heard, reclose the valve and recheck all hoses for looseness or cracking. If no leaking is heard, continue to open valve and allow N2 gas to the machine. The gas is not flowing to the chamber at this point, but is being held in the hose due to closure of the rate valve. The instrument is now ready to use.
  5. Place a plastic bag around a leaf or leafy shoot from the measurement plant and cut from the plant.
  6. Remove the top of the chamber and and place a wet, folded paper towel into the chamber to maintain the leaf or shoot in humid air during measurement, as N2 gas is dry. Insert the cut end of the plant leaf or shoot into the hole in the lid such that the cut end sticks out of the chamber once the lip is closed. Depending on the thickness of the stem, you may need to change the stopper within the lid to ensure a good seal is formed around the stem. Take care not to damage the cut end of the stem during insertion.
  7. Place the lid back on the chamber with the plant gently placed within the chamber and stem sticking out the lid top. To close the chamber, push down and the lid and then turn it until it can no longer turn. IT IS VERY IMPORTANT TO MAKE SURE THE LID IS COMPLETELY SECURE!
  8. Whilst ensuring you are watching from the side, rather than bending over the top of the chamber, turn the rate valve gently to the left to pressurize.
  9. Very slowly, turn the rate valve to allow gas to flow into the chamber and increase internal pressure. Watch the dial on the machine next to the chamber to be sure the pressure is rising slowly (approximately 1 MPa per minute). If the pressure rises too fast, you might miss the exact instant when xylem sap emerges. If pressure is rising too fast, slowly turn the black pressure valve towards off until the rise in pressure slows.
  10. Watch the cut stem for emergence of liquid and bubbles using the flashlight and magnifying glass or dissecting scope. Take care to ensure that liquid and bubbles are actually emerging from the xylem in the center of the cut end of the stem, not from the phloem or resin ducts around the perimeter of the cut stem. When sap and bubbles emerge, turn the control valve to off and the rate valve to the left to stop pressurization. Record the pressure from the gauge – this is the water potential of the leaf or shoot. Note that although many instruments provide data in bars, the accepted units for water potential is MPa (1 bar = 0.1 MPa).
  11. Turn Control valve to exhaust to let the gas out of the chamber. Return the Control valve to the closed position.
  12. When you have completed all measurements, close the tank valve, then open the rate valve with the Control valve in the exhaust position. Finally, open the bleed line hose to be sure that the hose is equilibrated with ambient pressure. It is very important that all pressure be relieved from the hose before disconnecting, otherwise, the hose may whip and cause injury.

Notes and troubleshooting tips

  • Note: If your stems/leaves are small and you want to conserve gas, partially fill the chamber with rubber stoppers.

PMS Instrument Company
Soil Moisture Equipment Corporation
Skye Instruments Ltd

Literature references

Koide RT, Robichaux RH, Morse SR, Smith CM (1989) Plant water status, hydraulic resistance and capacitance. In: Pearcy RW, Ehleringer J, Mooney HA, Rundel PW (eds) Plant Physiological Ecology. Chapman & Hall, London, pp 255-280

Health, safety & hazardous waste disposal considerations

  • Ensure chamber lid is very secure (step 6) prior to pressurizing chamber
  • Plant tissue (herbaceous tissue in particular) will snap off it the bung is squeezed too tightly – so take care.
  • DO NOT EVER bend over the lid, watch from the side.