Campbell CR3000 Micrologger, Руководство оператора

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Section 8. Operation
367
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 87: Diagram of a Thermocouple Junction Box
8.1.2.3 Resistance Measurements — Details
Related Topics:
• Resistance Measurements — Specifications
• Resistance Measurements — Overview
(p. 72)
• Resistance Measurements — Details
(p. 367)
• Measurement: RTD, PRT, PT100, PT1000
(p. 273)
By supplying a precise and known voltage to a resistive-bridge circuit and
measuring the returning voltage, resistance can be calculated.
CRBasic instructions for measuring resistance include:
BrHalf() — half-bridge
BrHalf3W() — three-wire half-bridge
BrHalf4W() — four-wire half-bridge
BrFull() — four-wire full-bridge
BrFull6W() — six-wire full-bridge
Resistance() — current-excitation resistance
Read More Available resistive-bridge completion modules are listed in
Signal Conditioners — List
(p. 605).
The CR3000 has six CRBasic bridge-measurement instructions. Table Resistive-
Bridge Circuits with Voltage Excitation
(p. 368)
and table Resistive-Bridge Circuits
with Current Excitation shows ideal circuits and related equations. In the
diagrams, resistors labeled R
s
are normally the sensors and those labeled R
f
are
normally precision fixed (static) resistors. CRBasic example Four-Wire Full-
Bridge Measurement
(p. 370)
lists CRBasic code that measures and processes four-
wire full-bridge circuits.