User`s guide

275
Chapter 7 Tutorial
Measurement Fundamentals
4
7
Shunt Impedance The insulation used for thermocouple wire and
extension wire can be degraded by high temperatures or corrosive
atmospheres. These breakdowns appear as a resistance in parallel with
the thermocouple junction. This is especially apparent in systems using a
small gauge wire where the series resistance of the wire is high.
Shielding Shielding reduces the effect of common mode noise on a
thermocouple measurement. Common mode noise is generated by
sources such as power lines and electrical motors. The noise is coupled to
the unshielded thermocouple wires through distributed capacitance. As
the induced current flows to ground through the internal DMM, voltage
errors are generated along the distributed resistance of the thermocouple
wire. Adding a shield to the thermocouple wire will shunt the common
mode noise to earth ground and preserve the measurement.
Common mode noise can dramatically affect the internal DMM. A typical
thermocouple output is a few millivolts and a few millivolts of common
mode noise can overload the input to the internal DMM.
Calculation Error An error is inherent in the way a thermocouple
voltage is converted to a temperature. These calculation errors are
typically very small compared to the errors of the thermocouple, wiring
connections, and reference junction (see page 267).
HI
LO
DMM
WITH SHIELD
WITHOUT SHIELD
Distributed
Distributed
Capacitance
Resistance
Power Line
DMM
LO
HI
Power Line
C
R
CC
C
C
C
C
C
C
R
R
R
R
R
34970A Refresh UG.book Page 275 Wednesday, February 17, 2010 12:34 PM