Campbell Scientific CR3000 Micrologger - Manual (Page 318)

Section 8. Operation
318
greater than the extension-wire range. In any case, errors can arise if temperature
gradients exist within the junction box.
Figure Diagram of a Thermocouple Junction Box
(p. 318)
illustrates a typical
junction box wherein the reference junction is the CR3000. Terminal strips are a
different metal than the thermocouple wire. Thus, if a temperature gradient exists
between A and A' or B and B', the junction box will act as another thermocouple
in series, creating an error in the voltage measured by the CR3000. This
thermoelectric-offset voltage is also a factor when the junction box is used as the
reference junction. This offset can be minimized by making the thermal
conduction between the two points large and the distance small. The best solution
when extension-grade wire is being connected to thermocouple wire is to use
connectors which clamp the two wires in contact with each other.
When an external-junction box is also the reference junction, the points A, A', B,
and B' need to be very close in temperature (isothermal) to measure a valid
reference temperature, and to avoid thermoelectric-offset voltages. The box
should contain elements of high thermal conductivity, which will act to rapidly
equilibrate any thermal gradients to which the box is subjected. It is not necessary
to design a constant-temperature box. It is desirable that the box respond slowly to
external-temperature fluctuations. Radiation shielding must be provided when a
junction box is installed in the field. Care must also be taken that a thermal
gradient is not induced by conduction through the incoming wires. The CR3000
can be used to measure the temperature gradients within the junction box.
Figure 99: Diagram of a thermocouple junction box
8.1.5 Pulse
Figure Pulse-Sensor Output Signal Types
(p. 40)
illustrates pulse input types
measured by the CR3000. Figure Switch-Closure Pulse Sensor
(p. 319)
is a
generalized schematic showing connection of a pulse sensor to the CR3000. The
CR3000 features four dedicated pulse-input channels, P1 through P4, and eight
digital I/O channels, C1 through C8, for measuring frequency or pulse output
sensors.
As shown in table Pulse-Input Channels and Measurements
(p. 40),
all CR3000
pulse-input channels can be measured with CRBasic instruction PulseCount().
PulseCount() has various parameters to customize it to specific applications.
Digital I/O ports C1 through C8 can also be measured with the TimerIO()
instruction. PulseCount() instruction functions include returning counts or
frequency on frequency or switch-closure signals. TimerIO() instruction has
additional capabilities. Its primary function is to measure the time between state
transitions.
Note Consult CRBasic Editor Help for more information on PulseCount() and
TimerIO() instructions.