Specifications

Section 4. Sensor Support
4-29
K thermocouples, since the upper limit of the reference compensation
polynomial fit range is 100
o
C and the upper limit of the extension grade wire
is 200
o
C. With the other types of thermocouples the reference compensation
polynomial fit range equals or is greater than the extension wire range. In any
case, errors can arise if temperature gradients exist within the junction box.
FIGURE 4.4-4 illustrates a typical junction box wherein the reference junction
is th
e CR1000. Terminal strips will be 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 in the case where extension grade
wire is being connected to thermocouple wire is to use connectors which clamp
the two wires in contact with each other.
CR9000
H
L
A' A
B' B
Junction Box
TC
CR1000 Junction Box
FIGURE 4.4-4. Diagram of Junction Box
When an external junction box is also the reference junction, the points A, A',
B, and B' in FIGURE 4.4-4 all need to be very close in temperature
(i
sothermal) 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.
4.5 Pulse Count Measurement
FIGURE 4.5-1 shows a typical pulse sensor to CR1000 schematic. The
CR1000 features two dedicated pulse input channels, P1 and P2. It also
features eight digital I/O channels, C1 through C8, for measuring various pulse
output sensors. Activated by the PulseCount() instruction, dedicated 24-bit
counters on P1, P2 and C1 through C8 are used to accumulate all counts over
the user specified scan interval. PulseCount() instruction parameters specify
the pulse input type, channel used, and pulse output option.