Agilent Technologies 34970A manual Without Shield

Chapter 8 Tutorial

Measurement Fundamentals

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.

					Power Line
Distributed			C	C	C
		C	C	C
Capacitance					HI
			R	R
					R
			R	R	R
					LO
	Distributed
	Resistance		WITHOUT SHIELD		DMM
					Power Line
		C	C	C
					HI
					LO
			WITH SHIELD		DMM

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	8
typically very small compared to the errors of the thermocouple, wiring

connections, and reference junction (see page 345).

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