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1 <br />1 <br />II� <br />Water Matric Potential Sensor <br />Models 229, CE4, CE8 <br />Campbell Scientific's 229-L Water Matric Potential Sensor measures soil <br />water potential from -10 to 2500 kPa. The sensor consists of a heating <br />element and thermocouple emplaced in epoxy in a hypodermic needle, <br />which is encased in a porous ceramic matrix. To calculate soil water matric <br />potential, a Current Excitation Module applies a 50 mA current to the 229's <br />heating element, and the 229 thermocouple measures the temperature rise. <br />The magnitude of the temperature rise varies according to the amount of <br />water in the porous ceramic matrix, which changes as the surrounding soil <br />wets and dries. Soil water matric potential is determined by applying a secon <br />order polynomial equation to the temperature rise. Users must individuz <br />calibrate each of their 229 sensors in the soil type in which the sensors will m <br />A Campbell Scientific datalogger controls the Current Excitation Module, mei <br />thermocouple, and calculates soil water matric potential. The 229 is not compatible with our <br />CR200-series or CR510 dataloggers. <br />Current Excitation Modules <br />Two Current Excitation Modules are offered, the CE4 and CE8. These modules differ only in the number of 229 sensors <br />to which they source current. The CE4 sources current for up to four 229s, and the CE8 sources current for up to eight. <br />Both modules require a 12 Vdc power source. <br />You can connect a CE4 or CE8 to an AM16/32A multiplexer. Multiplexers increase the number of 229 sensors that can be <br />measured. Please note that both the CE4 and CE8 modules switch currents that are greater than 30 mA, which degrades <br />the contact surfaces of the mechanical relays. Therefore the multiplexer channels to which the CE4 or CE8 have been <br />connected may become unsuitable for other applications (refer to the multiplexer manuals for more information). <br />The CES allows up to eight 229 sensors to be <br />measured. More than eight 229 sensors can be <br />measured by using the AMI 6132A multiplexer. <br />The CE4 Current Excitation Module pimides a constant <br />t current to the heating elenunt of a 229-L Heat Dissipation <br />Water. Matric Potential sensor: The CE4 allows up to four <br />229 sensors to be measured. <br />NCAMPBELL SCIENTIFIC, INC0' WHEN MEASUREMENTS MATTER <br />