Stem Hygrometer FAQs

• What are the methods of measuring plant water potential?
• How long is the "useful life" for the Stem Hygrometer sensor, once set up in the tree?
• Maintaining Stem Hygrometer in the tree - Should I do something while the sensor is installed on the tree?
• How do I clean the Stem Hygrometer?
• Do you have a table for different species, when to clean and reinstall the Stem Hygrometers?
• How many sensors per stem or tree do you suggest to get representative data? Is one sensor enough to start on the field?
• Do you have any papers published about the Stem Hygrometer? I mean some references for the vineyards associations, research centres in Australia or in another country?

What are the methods of measuring plant water potential?

There are three main methods of measuring plant water potential:

1. The most common is using a portable but destructive device know as a Schollander Pressure bomb
2. The second is a portable and non destructive device called a Porometer which strictly is measuring stomatal conductance which can be used to infer plant water potential
3. The third is the Stem hygrometer. This is the newest device which is non-destructive and can be continuously logged in position.

Each method has its advantages and disadvantages. The choice of method depends on your application. However, the Stem Hygrometer has the fewest disadvantages as:
• it is non-destructive
• it does not change the boundary layer resistance of the stem
• the measurement is made in-situ on the plant stem, so this measurement is least susceptible to thermal variations
• it can be logged continuously to provide a complete diurnal cycle of the plant over several weeks which is very instructive about the plant's physiological response to environmental changes

How long is the "useful life" for the Stem Hygrometer sensor, once set up in the tree?

The Stem Hygrometers will last for many years if maintained well; we think 5 years or longer provided they are treated well, maintained and the heater does not blow. The heater is like a light bulb it can last for a very long time or suddenly just blow. At this early stage we have not had any blow!

Maintaining Stem Hygrometer in the tree - Should I do something while the sensor is installed on the tree?

This is a delicate task as the thermocouples are exposed and can easily be dirtied or broken. The Hygrometers have a 2 year return to manufacturer repair warranty as again there is nothing the customer can do to fix these very delicate welded thermocouples. However, they can clean the sensors when they get dirty.

How do I clean the Stem Hygrometer?

Cleaning routine:

1. Invert the instrument.
2. Fill the chamber well with a cleaning solvent (10% ammonium hydroxide, chloroform or acetone).
3. Let stand for a few seconds.
4. Rinse away with distilled water in a wash bottle.
5. Apply a mild detergent in distilled water solution with a wash bottle.
6. Again, rinse well with distilled water.
7. Finally, blow dry with a controlled stream of compressed gas (20-30 psi). The drying phase is important as residual water must be removed from the chamber well. Stubborn drops may reside around the copper binding posts and sustained streams of compressed gas may be required to remove all the water.

Do you have a table for different species, when to clean and reinstall the Stem Hygrometers?

The Stem Hygrometer has a much shorter measurement interval than the HRM, again it will depend on the species of plant being measured but it will range form a few days to perhaps a month before the sensor would need to be reinstalled. Once cleaned and reinstalled measurements would be accurate again.

How many sensors per stem or tree do you suggest to get representative data? Is one sensor enough to start on the field?

One Stem Hygrometer sensor would be OK per tree. However, consider an example of two tree discs: one has even growth rings all the way around the tree and the second has very irregular growth rings, some being very close together on one side of the tree but very wide apart on the other side. This was caused by non uniform planting densities and competition from other surrounding dominant and co-dominant trees. In this instance and for detailed scientific research I would recommend 4 sensors per tree and to average the results of these sensors to give an accurate representative total water use figure and also to identify the within tree variation of water use caused by competition.

Do you have any papers published about the Stem Hygrometer? I mean some references for the vineyards associations, research centres in Australia or in another country?

The Stem Hygrometers reference page has not yet been put up but this reference is very good and answers almost every question:
Dixon, M.A., & Tyree, M.T., 1984 "A new stem hygrometer, corrected for temperature gradients and calibrated against the pressure bomb" Plant, Cell and Environment 7, 693-697.

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