229 Heat Dissipation Matric Water Potential Sensor
6
A Campbell Scientific datalogger can read a single thermocouple junction directly
because the temperature at the wiring panel is measured with a thermistor and this
temperature is converted to a voltage which is then used as a thermocouple
reference. A thermocouple circuit voltage potential is affected by the temperature
of all dissimilar metal junctions.
When using a multiplexer with the 229 sensor, the temperature of the multiplexer
can be used as the reference temperature if a thermistor probe such as the 107 is
taped to the multiplexer panel near the 229 wires. Alternately, a CR10XTCR can
be used to get an accurate reading of the CR10X wiring panel temperature and
type T thermocouple wire (copper-constantan) can be used for the signal wires
between the differential voltage channel on the datalogger and the appropriate
common channels on the multiplexer (see program example #2 below). The
CR23X, CR800, CR850, CR1000, and CR3000 can use their own internal panel
temperature measurement instead of the CR10XTCR and type T thermocouple
wire to the multiplexer common channels as previously noted. The use of
insulation or an enclosure to keep the multiplexer and temperature sensor at the
same temperature will improve measurement quality.
5.2 Adjusting for Thermal Properties of Sensor During Early
Heating Times
The discussion presented at the beginning of the calibration section (Section 6)
describes how thermal properties can vary from sensor-to-sensor. The thermal
properties of the needle casing, wiring, and the amount of contact area between
the needle and the ceramic have a slight effect on the temperature response. Most
of the non ideal behaviour of the sensor is manifest in the first second of heating.
The measurement is improved if the temperature after 1 s is subtracted from some
final temperature. A typical
Δ
T would be T(30 s) - T(1 s).
5.3 Datalogger Program Structure and Multiplexers
The sequence of datalogger instructions for a 229 measurement is as follows:
1)
Measure sensor temperature prior to heating.
2)
Set a control port high to enable constant current excitation module and being
heating.
3)
Wait for one second of heating and measure sensor temperature.
4)
Wait for 29 more seconds of heating and measure sensor temperature.
5)
Set control port low to disable the constant current excitation module and end
heating.
6)
Calculate temperature rise by subtracting T(1 s) from T(30 s).
Since all of the output channels of the CE4 or CE8 are activated when the control
terminal is set high, power will be applied to all of the 229 sensors connected to
the current source. Inaccurate measurements can result if the temperature of
multiple sensors is simply read sequentially. The inaccuracy can occur because a
finite amount of time is required to execute each of the temperature measurement
instructions.
For example, a CR10X making multiple differential thermocouple readings with
60 Hz rejection takes 34.9 ms to read one thermocouple, and 30.9 ms more for
each additional thermocouple. In a configuration where six 229 sensors are
connected to a CE8 with their thermocouple wires connected sequentially to the
CR10X wiring panel, the sixth 229 sensor will heat for 154.5 ms longer than the
