Thermal Conductivity Measurement; Principle Of Operation; Fig. 3-9: Wheatstone Bridge; Table 3-5: Examples Of Specific Thermal Conductivities - Emerson Rosemount Xstream Series Instruction Manual

X-STREAM
3-3

Thermal Conductivity Measurement

Thermal Conductivity Measurement primarily
is used for measuring concentrations of
hydrogen (H ) and helium (He). These gases
2
are characterized by their specific thermal
conducitivity, differing clearly from that of other
gases (see table 3-5).
Table 3-5: Examples of Specific
Thermal Conductivities
3-14
3-3 Thermal Conductivity Measurement
3-3-1

Principle of Operation

A Wheatstone bridge, made of 4 temperature
sensitive resistors (PT 100 sensors), is
surrounded by gas in a way that each 2 sensors
are located in the measuring gas stream (R
and in a reference gas stream (R
The bridge output signal (U
when in rest position (no gas flow).
By default the reference gas path is closed (not
flown through by gas). When sample gas is
supplied the sensors in the measuring gas path
are cooled due to the thermal conductivity
effect: The gas absorbs heat and carries it away
from the sensors. This tunes the Wheatstone
bridge and generates a signal proportional to
the thermal conductivity.
Additional electronics linearizes and conditions
this signal to provide usefull measuring values.
U
Br

Fig. 3-9: Wheatstone Bridge

Depending on application it is possible to
supply a reference gas to the bridge's reference
side. The output signal in this case is proporti-
onal to the difference of the thermal
conductivities of sample and reference gas.
Emerson Process Management GmbH & Co. OHG
Instruction Manual
HASAxE-IM-HS
05/2006
M
), see fig. 2-9.
R
) is adjusted to zero
br
)