Instructions

UM-0085-B09 DT80 Range User Manual Page 315
RG
This configuration simulates a bridge by using the DT80's 3-wire compensation circuit to "compensate" for the voltage
drop across Rc. The end result is that, like a bridge, the DT80 measures the difference
between the voltages across the
two arms.
Figure 144: Wiring for 3 wire bridge input using internal current excitation
To measure
Use the command
bridge output
1BGI(R
a
)
Temperature – Thermocouples
Thermocouple Theory
Figure 145: Thermocouple principle of operation
A thermocouple consists of two wires of dissimilar metals that are
• electrically connected at one end (the measurement junction) and
• thermally connected at the other end (the reference, or "cold" junction).
• A small voltage is produced when the two junctions are at different temperatures. (The voltage is produced by
the temperature gradient along the wires, not by the junctions.)
 Making The Measurement Junction
The measurement junction can be made by welding, brazing, soldering or crimping the two wires together. Take care to
ensure that the wire material is not contaminated where the temperature gradient is to occur.
The junction can be insulated, or left bare for a more rapid response. If left bare, ensure that the junction does not make
intermittent contact with metal objects. This can introduce electrical noise.
Sometimes thermocouple measurement junctions are electrically connected (by welding, brazing, soldering or by
contact) to the object being measured. This is only possible if the object is grounded to the DT80’s analog ground
terminal AGND/EXT#, or if the voltage on the object relative to the DT80's analog ground is within the DT80's common
mode limits.

Reference Junction Compensation
Conventionally, the reference junction is held at 0°C, and thermocouple responses are determined with a 0°C reference.
This is inconvenient in most situations and so, in practice, the reference junction is allowed to follow to ambient
temperature. Then this non-zero reference junction temperature must be compensated for by measuring the reference
temperature with another temperature sensor. The DT80 does this compensation automatically when a thermocouple
channel type is selected.
 Isothermal Block
Generally the reference junctions and the associated temperature sensor are held at the same temperature by a physical
arrangement that ensures good thermal conductivity between the junctions. This structure is called an "isothermal block".
It is advisable to insulate the isothermal block from rapid ambient temperature changes.
Measurement
Junction
Reference Junction
(isothermal block)
To microvolt
meter
Reference
Junction
Temperature
Prime temperature
gradient
Metal 1
Metal 2
Copper
Copper
Temperature sensor