Campbell Scientific CR1000KD CR1000 Measurement and Control System - Page 312

Section 8. Operation 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. 312) illustrates a typical junction box wherein the reference junction is the CR1000. 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 CR1000. 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 CR1000 can be used to measure the temperature gradients within the junction box. 8.1.5 Pulse 312 Figure 96: Diagram of a thermocouple junction box Figure Pulse-Sensor Output Signal Types (p. 39) illustrates pulse input types measured by the CR1000. Figure Switch-Closure Pulse Sensor (p. 313) is a generalized schematic showing connection of a pulse sensor to the CR1000. The CR1000 features two dedicated pulse-input channels, P1 through P2, 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. 39), all CR1000 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.