229-L

If all the errors were summed in a worst case scenario, the thermocouple accuracy would be ±0.5° C. For an extended discussion about thermocouple measurements, please refer to the "Thermocouple Measurements" section in the datalogger manual.

Very low values for dT, especially negative values, do indicate that the 229 sensor is not heating during the measurement time. Because the sensors were working fine but suddenly stopped working without a change to the datalogger program, this indicates a physical problem. The most likely causes are damage to the heater wires, one or more loose wires, or a failure of the constant current excitation module. The first check to make is the heater wire itself. With an ohm-meter, measure the resistance between the black and green wires on the 229 sensor. The resistance should read about 33-36 ohm. An off-scale or infinite reading indicates a break in the heater wire. A low reading could indicate a short in the heater wire. In both of those cases, the fix would be to dig up the sensor wire and examine it for damage. The next check is to make sure that there are good electrical connections where the black and green sensor wires connect to the constant current interface or to a multiplexer, where the multiplexer common wires connect to the constant current interface, and where the constant current interface connects to the datalogger 12-V, G, and control port. Each of those wires should have about a quarter inch of bare copper securely connected to its terminal. Finally, the CE4 or CE8 constant-current module can be checked with a multimeter to make sure it is putting out 50 mA. Measure the voltage between the 12-V and ground terminal screws to make sure that the constant current module is receiving power from the datalogger. Next, temporarily move the wire connecting the datalogger control port to the CTRL channel to a 5-V channel on the datalogger to force the module to be on all the time so you can measure the current output. Set the multimeter to measure milliamps, and measure the current. It should be 50 mA ± 1 mA. If the current output is outside that range, the constant current module needs to be repaired or replaced.

See Section 5 of the 229 manual (campbellsci.com/documents/manuals/229.pdf) for details and sample programs.

See section 6 of the 229 manual (campbellsci.com/documents/manuals/229.pdf) for details.

At this time Campbell Scientific does not offer a calibration service for the 229 probe. There are companies that offer this service. Contact Campbell Scientific for details.

Yes, we recommend that you measure and record the temperature of the sensor prior to turning on the heater and that reading can be used as a soiltemperature reading. As long as matric potential measurements are not mademore frequently than once every 15 minutes, there will be no significantheat buildup in the sensor and the sensor temperature will be the same asthe soil temperature.

The heater wire itself should read around 33 ohms resistance, but the green and black wires connected to the heater wire will also add resistance to your reading. The longer the sensor cable, the greater your resistance reading will be, so readings of 37-48 ohms are within the normal range. A resistance reading of infinity or of less than 33 ohms would be caused by a break or short in the sensor cable and would be cause for concern.

The heating element is made of Evanohm wire. The element is encased in epoxy inside a stainless steel hypodermic needle and will not be exposed to the corrosive environment, so that part of the sensor should not fail in five years. However the sensor cable and the ceramic matrix might suffer damage after five years in a corrosive environment.