229-L Heat Dissipation Matric Potential Sensor

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1. Using a 229-L with a CR1000, can the system be programmed to read the equilibrium soil temperature before excitation for a matric potential measurement?

   Yes. Campbell Scientific recommends measuring and recording the temperature of the sensor before turning on the heater. 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.

2. How are readings obtained from the 229-L?

   For details and sample programs, see the "Example Programs" section of the 229-L manual.

3. If several 229-L probes begin returning very low positive and negative values for dT, what might be the cause?

   Very low values for dT, especially negative values, indicate that the 229-L sensor is not heating during the measurement time. 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 of the heater wire itself. With an ohmmeter, measure the resistance between the black and green wires on the 229-L. The resistance should read approximately 33 to 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 in the following locations:

   • 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
   • where the constant current interface connects to the data logger 12V, G, and control port

   Each of those wires should have approximately 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 12V and ground terminal screws to make sure that the constant current module is receiving power from the data logger. Next, temporarily move the wire connecting the data logger control port with the CTRL channel to a 5 V channel on the data logger to force the module to be on all the time, so that the current output can be measured. 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.

4. What is the 229-L heating element made of? In an acidic environment, such as copper mine tailings, would the sensor be expected to fail because of corrosion within five years?

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

5. Is calibration of the 229-L available?

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

6. Do heater resistance values of 37 to 48 ohm indicate a problem with the 229-L, or are those values within the normal range?

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

7. What is the stated accuracy for the thermocouple (temperature measurement) in a 229-L 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, refer to the "Thermocouple Measurements" section in the data logger manual.

8. What are the different cable length choices available for Campbell Scientific sensors?

   Most Campbell Scientific sensors are available as an –L, which indicates a user-specified cable length. If a sensor is listed as an –LX model (where “X” is some other character), that sensor’s cable has a user-specified length, but it terminates with a specific connector for a unique system:

   • An –LC model has a user-specified cable length for connection to an ET107, CS110, or retired Metdata1.
   • An –LQ model has a user-specified cable length for connection to a RAWS-P weather station.

   If a sensor does not have an –L or other –LX designation after the main model number, the sensor has a set cable length. The cable length is listed at the end of the Description field in the product’s Ordering information. For example, the 034B-ET model has a description of “Met One Wind Set for ET Station, 67 inch Cable.” Products with a set cable length terminate, as a default, with pigtails.

   If a cable terminates with a special connector for a unique system, the end of the model number designates which system. For example, the 034B-ET model designates the sensor as a 034B for an ET107 system.

   • –ET models terminate with the connector for an ET107 weather station.
   • –ETM models terminate with the connector for an ET107 weather station, but they also include a special system mounting, which is often convenient when purchasing a replacement part.
   • –QD models terminate with the connector for a RAWS-F Quick Deployment Station.
   • –PW models terminate with the connector for a PWENC or pre-wired system.
9. Where are the available cable termination options listed for a particular sensor?

   Not every sensor has different cable termination options. The options available for a particular sensor can be checked by looking in two places in the Ordering information area of the sensor product page:

   • Model number
   • Cable Termination Options list

   If a sensor is offered in an –ET, –ETM, –LC, –LQ, or –QD version, that option’s availability is reflected in the sensor model number. For example, the 034B is offered as the 034B-ET, 034B-ETM, 034B-LC, 034B-LQ, and 034B-QD.

   All of the other cable termination options, if available, are listed on the Ordering information area of the sensor product page under “Cable Termination Options.” For example, the 034B-L Wind Set is offered with the –CWS, –PT, and –PW options, as shown in the Ordering information area of the 034B-L product page.

   Note: As newer products are added to our inventory, typically, we will list multiple cable termination options under a single sensor model rather than creating multiple model numbers. For example, the HC2S3-L has a –C cable termination option for connecting it to a CS110 instead of offering an HC2S3-LC model. 

10. How can the 229-L be calibrated?

    For details, see the "Calibration" section of the 229-L manual.