Most of the early work with the pressure chamber was done with woody
plants because they are easiest to measure. Secondary lateral twigs
of large trees can be used, but the entire top of seedlings may be required.
In woody species the phloem (bark) should be stripped back from the cut
surface about 2 cm (about 3/4") to allow the xylem (woody portion) to protrude
through the gasket.
Re-cutting the stem of the sample to shape the cut end can be accomplished
safely in some species such as conifers, but species with large xylem elements
such as oak cannot be re-cut without introducing error. The practice
of trimming the sample after it is cut from the plant should be confined
to those species which are known to have only very small xylem elements.
A proper sized insert and gasket are all that is really needed, however
if the twig is not rigid enough an insertion tool will be of help.
For production purposes or multiple measurements a Compression Gland Cover
will be of benefit.
Herbaceous plants are only slightly more difficult to measure than woody
plants. Sealing the sample in the gasket without crushing or kinking
the stem is usually the biggest problem, and this problem can usually be
solved by using a gasket of the proper size and the insertion tool described
earlier. A Compression Gland Cover can also be helpful.
The endpoint may be difficult to determine, but a good light and perhaps
a good quality hand lens will be helpful. Some workers have used
a dissecting microscope to aid them in seeing the endpoint, but such measures
are usually not necessary.
With herbs it is especially important to measure the sample immediately
after cutting and to use the proper rate of pressure increase.
Measuring grass is similar to measuring herbaceous plants. Although some workers have tried to measure the round portion of the stem, most workers have been more successful using the flat portion of a leaf blade. A slotted cover insert for the chamber lid, a slotted gasket, and an insertion tool are required to seal the blade into the chamber. Measurements usually cannot be taken when stress levels exceed the wilting point by more than about 10 bar. For this reason most measurements on grass will be below 20 bar.
Measuring fruits such as apples, oranges, or tomatoes is not difficult, but the fruit must usually be supported in the chamber. A coil spring in the bottom of the chamber is helpful. Once the sample is in the chamber measurements can proceed as with herbs.