15
WP4C
CONTINUOUS MODE
Continuous mode measures the water potential of the sample continuously until the drawer
knob is turned back to the OPEN/LOAD position. The WP4C will measure the sample, display
the water potential and sample temperature, then begin another read cycle. WP4C will
signal the end of a reading with an LED flash and audible notification (if enabled).
Continuous mode is recommended for samples that take a long time (20 to 30 min) to come
to vapor equilibrium, such as plant samples and wet soil samples with water potential
>−0.5 MPa. METER recommends using Continuous mode and logging data to determine when
equilibrium conditions are reached. It may be helpful to connect the WP4C to a computer
while in Continuous mode in order to log and store data over time (Section3.2.6).
FAST MODE
Fast mode measures the sample once. Read time is typically 3 to 5 min. Readings are less
precise, particularly in the wet range.
Fast mode is recommended for dry soil samples with water potential <−40 MPa. For samples
with very little water holding capacity (i.e., dry sand samples), small leaks in the sample
chamber can cause water potential to drift down over time.
3.2.6 COMPUTER INTERFACE
The WP4C can connect to a computer to send water activity data to a computer for further
analysis and storage. An RS-232-to-USB serial cable is included for this purpose. The
interface is run through the AquaLink 4 Software or a terminal communication program.
NOTE: If the computer does not have a USB port, use a USB-to-RS-232 adapter.
There are several terminal program options. METER has its own terminal program (DecaTerm)
that can be downloaded from metergroup.com/wp4c-support. Two other options are
TeraTerm, a free program that can be found on the internet, and Hyperterminal, the standard
program with Microsoft
®
Windows
®
prior to Windows 7 software.
To integrate a terminal program with WP4C, use the following steps:
1. Power on the WP4C.
2. Connect the USB interface cable to the computer.
3. Follow the instructions for the program with the following settings:
c. Choose correct COM Port.
d. Set or verify COM Properties.
e. Bits per second (Baud rate): 9,600
Data: 8 bit
Parity: none
Stop: 1 bit
Flow control: none
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