6 SAMPLE PREPARATION WP4C
6 Sample Preparation
Your WP4C will continually provide accurate water potential mea-
surements as long as its internal sensors are not contaminated. Care-
ful preparation and loading of samples will lengthen time between
cleanings and will help you avoid downtime and repairs.
6.1 Choosing a Sample Cup
The WP4C comes with two types of samples cups: disposable plas-
tic cups and stainless steel cups. The disposable plastic cups are
adequate for most samples, but are not good for samples in the wet
end. If you are measuring samples with water potential wetter than
−1 MPa, you should use the stainless steel sample cups. You can
also oven dry soil samples directly in the stainless steel cups to de-
termine water content gravimetrically, which is convenient if you are
generating soil moisture characteristic curves. It is important to
note that you must thoroughly clean the stainless steel cups using
deionized water between uses to prevent solutes from contaminating
subsequent samples and causing artificially negative osmotic poten-
tial. Finally, if you calibrate the WP4C (see chapter 5), be sure to
calibrate using the same type of sample cup that you intend to use
for subsequent measurements.
6.2 Preparing the Sample
First, place the sample in a disposable sample cup, completely cov-
ering the bottom of the cup, if possible.WP4C may not be able to
accurately measure a sample that does not (or cannot) cover the
bottom of the cup. A larger sample surface area speeds up the read-
ing by shortening the time needed to reach vapor equilibrium. It
also increases instrument accuracy by providing more stable infrared
sample temperature measurements.
Do not fill the sample cup more than half full. Overfilled cups may
contaminate the sensors in the chamber, remember more is not nec-
essarily better.
22
To Next Page
Loading...
