Measurement Method; Fig. 2-9: Measuring Principle Thermal Conductivity Sensor (Wheatstone Bridge) - Emerson ROSEMOUNT Analytical NGA 2000 Instruction Manual

NGA 2000 MLT Hardware
MEASURING PRINCIPLE
THERMAL CONDUCTIVITY
2.4.3

Measurement Method

The entire measurement cell is thermostated to a temperature of up to 75 °C. The four integral
temperature sensors are electrically heated to a higher temperature. Their temperatures, and thus
their electrical resistance, are dependent upon heat losses, which, in turn, result from heat
transport in the surrounding gas to colder chamber walls. For otherwise stable conditions, this heat
transport will be proportional to the thermal conductivity of the gas present between the sensor
and the chamber wall. Interconnecting the four sensors into Wheatstone bridge circuit (Fig. 2-9)
provides an electronic signal proportional to gas density. Electronic circuitry processes this signal
to obtain standardized signal amplitudes, and transmits these to both an indicator instrument and
to the signal output connector.
The internal gas paths are constructed to get a minimimized response time at minimized
dependence of the analytical signal upon sample gas flow rate.

Fig. 2-9: Measuring principle Thermal Conductivity Sensor (Wheatstone bridge)

2 - 16
R
reference
R
measure
DC voltage supply
R
measure
(Vol.-%)
R
reference
Instruction Manual
Display
Emerson Process Management GmbH & Co. OHG
90002929
07/2006