Frequently Asked Questions: 229-L
What is the stated accuracy for the thermocouple (temperature measurement) in a 229 Water Matric Potential Sensor?
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.
I have several 229-L probes installed at a site. They've been in operation for about 5 years. In the last 3 months, two of the sensors have begun returning very low positive and negative values for dT. The recorded temperature at the same depth seems OK, however. Is this an indication the heater is malfunctioning? Is there anything I can do to fix the probe without digging it back up? Is there possibly something else wrong that might be easier to fix? There have been no changes to the data logge
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.
Do you offer calibration of the 229L heat dissipation sensor for soil water matric potential?
At this time Campbell Scientific does not offer a calibration service for the 229 sensor. There are companies that offer this service. Contact Campbell Scientific for details.
How do I actually get my reading off of the device?
See Section 5 of the 229 manual (campbellsci.com/documents/manuals/229.pdf) for details and sample programs.
Can you explain, with as much detail as possible, how we calibrate each sensor individually?
See section 6 of the 229 manual (campbellsci.com/documents/manuals/229.pdf) for details.
I gather that the heater resistance should be around 33 ohms. Do resistance values of 37-48 ohms indicate a problem with the sensor, or is that within the normal range?
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.I am considering using the 229-L matric potential sensor with a CR-1000 datalogger. Can I program the system to read equilibrium soil temperature prior to excitation for matric potential measurement? That way I measure soil temperature and matric potential with on sensor.
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 soil
temperature reading. As long as matric potential measurements are not made
more frequently than once every 15 minutes, there will be no significant
heat buildup in the sensor and the sensor temperature will be the same as
the soil temperature.
What is the the 229-L heating element made of? In a acidic environment (copper mine tailings), would you expect it to fail due to corrosion in five years?
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.
