Thermal Conductivity Measurement; Principle Of Operation; Fig. 3-11: Wheatstone Bridge; Tab. 3-5: Examples Of Specific Thermal Conductivities - Emerson Rosemount Analytical X-STREAM X2 Series Instruction Manual

Instruction Manual
HASX2E-IM-HS
10/2012

3.3 Thermal Conductivity Measurement

Thermal conductivity is the property of a ma-
terial that indicates its ability to conduct heat.
Thermal conductivity measurement prima-
rily is used for measuring concentrations of
hydrogen (H ) and helium (He). These gases
2
are characterized by a specific thermal con-
ducitivity, differing clearly from that of other
gases ( table 3-5).
Gas
Air N /O
2
Ammonia NH
3
Argon Ar
Butane C H
4 10
Carbon Dioxide CO
2
Carbon Monoxide CO
Chlorine Cl
2
Helium He
Hydrochloric Acid HCl
Hydrogen H
2
Krypton Kr
Methane CH
4
Neon Ne
Nitrogen N
2
Oxygen O
2
Radon Rn
Sulfur Dioxide SO
2
Xenon Xe
Tab. 3-5: Examples of specific thermal conductivi-
ties
3.3.1

Principle of Operation

A Wheatstone bridge, made of 4 temperature
sensitive resistors (PT 100 sensors), is sur-
rounded by gas in a way, that each 2 sensors
are located in the sample gas stream (R
in a reference gas stream (R
Emerson Process Management GmbH & Co. OHG
3.3 Thermal Conductivity Measurement
l in mW / cm K
50 °C
276
2
270
189
102
184
267
371
1580
151
1910
185
96,8
516
277
283
26
113
60
) and
S
), Fig. 3-11.
R
The bridge output signal (U
zero 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 sample gas path
are cooled due to the thermal conductivity ef-
fect: 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 conditi-
ons this signal to provide usefull measuring
values.Depending on application, it is possib-
le to supply a reference gas to the bridge's
reference side. The output signal in this case
is proportional to the difference of the thermal
conductivities of sample and reference gas.
R
R
R
S
Sample

Fig. 3-11: Wheatstone Bridge

X-STREAM X2
) is adjusted to
Br
R
S
U
Br
R
R
Reference
3-13